==== Front J Funct Morphol Kinesiol J Funct Morphol Kinesiol jfmk Journal of Functional Morphology and Kinesiology 2411-5142 MDPI 10.3390/jfmk3030041 jfmk-03-00041 Review Dietary Manipulations Concurrent to Endurance Training https://orcid.org/0000-0003-0014-5878Rothschild Jeffrey 1* Earnest Conrad P. 2 1 TriFit Performance Center, Santa Monica, CA 90404, USA 2 Exercise and Sport Nutrition Laboratory, Texas A&M University, College Station, TX 77843, USA * Correspondence: rd@trifitla.com 25 7 2018 9 2018 3 3 4127 6 2018 23 7 2018 © 2018 by the authors.2018Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).The role of an athlete’s dietary intake (both timing and food type) goes beyond simply providing fuel to support the body’s vital processes. Nutritional choices also have an impact on the metabolic adaptations to training. Over the past 20 years, research has suggested that strategically reducing carbohydrate (CHO) availability during an athlete’s training can modify the metabolic responses in lieu of simply maintaining a high CHO diet. Several methods have been explored to manipulate CHO availability and include: Low-carb, high-fat (LCHF) diets, performing two-a-day training without glycogen restoration between sessions, and a “sleep-low” approach entailing a glycogen-depleting session in the evening without consuming CHO until after a morning training session performed in an overnight fasted state. Each of these methods can confer beneficial metabolic adaptations for the endurance athlete including increases in mitochondrial enzyme activity, mitochondrial content, and rates of fat oxidation, yet data showing a direct performance benefit is still unclear. endurancetrainingadaptationscarbohydrateLCHF ==== Body 1. Introduction The endurance athlete’s diet can impact the metabolic adaptations to training by enhancing or blunting cellular responses to exercise-induced perturbations. A high carbohydrate (CHO) diet has been traditionally promoted for these athletes in order to maximize muscle and liver glycogen stores, along with the use of exogenous CHO fuels during both training and competition [1]. Over the last 20 years, data suggesting benefits from purposely and strategically reducing the availability of CHO during some or all of an athlete’s training sessions has appeared with increased frequency in the literature. Such manipulations can modify the metabolic responses to training and result in more favorable responses to exercise stimuli [2]. To date, several methods have been used to strategically manipulate CHO availability surrounding training including: A low-carb, high-fat diet (LCHF), two-a-day training without glycogen restoration between training sessions, and a “sleep-low” approach where the athlete performs a glycogen-depleting session in the evening without consuming CHO until after a morning training session performed in an overnight fasted state. Each of these methods can confer beneficial metabolic adaptations to the endurance athlete including increases in mitochondrial enzyme activity, mitochondrial content, and rates of fat oxidation, though data showing direct performance benefits are less clear. To optimize training adaptations, the interplay of training volume, training intensity, and dietary CHO must be considered. For example, regulation of peroxisome proliferator-activated receptor c coactivator-1α (PGC-1α) protein content appears to be primarily dependent upon training intensity and is not affected by CHO availability [3,4,5,6]. Changes in citrate synthase (CS) activity are primarily affected by training volume [7] and largely unaffected by CHO intake [8], though some studies have shown an augmented training response with dietary manipulation [9,10]. A number of studies have shown that modulating CHO availability can enhance the training-induced responses of key signaling proteins involved in mitochondrial biogenesis such as p38 mitogen-activated protein kinase (MAPK) [3], 5′ AMP-activated protein kinase (AMPK) [11], and p53 [4]. Therefore, the objective of this review is to examine the impact of dietary manipulation on the adaptations to endurance training. 2. Low-Carb, High-Fat A LCHF dietary approach has been explored by endurance athletes due to its ability to increase rates of fat oxidation and reduce reliance on CHO during prolonged exercise [12]. Fat metabolism during exercise is a complex process involving many sites of regulation that may be affected by dietary intake including the release of free fatty acids (FFA) from adipose tissue and delivery to working muscle, transport of fat into the cell, binding and transport of fat in the cytoplasm, transport of fat into the mitochondria, and the regulation of intramuscular triglyceride (IMTG) synthesis and breakdown [13,14]. A LCHF diet increases the release of FFA from adipose tissue at rest and during exercise [15], which can be transported to the muscles and oxidized or re-esterified and stored into the IMTG droplets located next to the mitochondria. Adipose derived FFA increase rapidly during low to moderate intensity exercise with a greater proportion of FFA undergoing beta oxidation rather than re-esterification [16], while other fat sources, such as IMTG, are stimulated as exercise intensities increase [17]. A LCHF diet can increase resting IMTG levels by 50–123%, independent of glycogen availability [13,18,19,20], with just two days of LCHF being sufficient to increase levels by 36% in trained participants [21]. Training status appears to affect the extent of these adaptations, as untrained males on LCHF (55–60% fat) for 16 d showed no changes in IMTG levels [22]. When fuel availability is altered, intracellular metabolites regulating key cellular enzymes will also be altered. For example, five days of LCHF resulted in 12–17% increases in fatty acyl translocase (FAT/CD36) protein content, indicating an increased capacity for sarcolemmal and/or mitochondrial membrane fatty acid uptake [23,24]. In contrast, carnitine palmitoyltransferase-1 (CPT1) was not changed after 5 or 6 d of LCHF [23,24,25] but increased after 10 and 15 d [26]. The activity of β-hydroxyacyl-CoA dehydrogenase (β-HAD), which plays a key role in beta oxidation, was unchanged after 6, 15, and 28 d of LCHF [19,25,26] but increased by 120% after 7 weeks of LCHF [8]. Gene expression of β-HAD increased after 5 d of LCHF, suggesting a longer duration may be needed to see increased functional protein concentrations [24]. No changes have been observed in CS activity with LCHF [8,19,23,25,26], which is increased by training but largely unaffected by diet [8]. A LCHF dietary approach can dramatically increase rates of maximal fat oxidation, from ~0.59 g per minute on a mixed diet [27] to ~1.5 g per minute on LCHF with some athletes exceeding 1.9 g per minute after a three week intervention [28,29]. Some disagreement in the literature exists concerning the source of fat for the increased fat oxidation observed with LCHF diets. A number of studies have found that plasma VLDL-TG in combination with increased FFA uptake accounts for much of the increased fat oxidation observed during exercise after LCHF diet adaptation [30,31,32], while other research has shown the increases are the result of elevated IMTG concentration and not increased adipose tissue lipolysis or plasma FFA [21]. Reasons for divergent findings may be the length of adaptations to LCHF, CHO content of the meal consumed prior to the exercise testing, and fitness level, as trained endurance athletes have a substantially increased capacity to store and utilize IMTG during exercise [33]. Along with increased rates of fat oxidation on LCHF diets, decreased rates of CHO oxidation are observed due to reductions in muscle glycogen storage and breakdown, decreased activity of pyruvate dehydrogenase (PDH), reduced plasma glucose concentrations, and reduced intake of exogenous CHO during exercise [24,25,26,28,29,34,35,36,37,38]. The majority of research has shown that LCHF reduces levels of stored muscle glycogen and thus its contribution during exercise [34,37,39,40]. However, an observational study of ultra-endurance athletes on a long-term (>9 month) LCHF diet found no differences in muscle glycogen storage or utilization while running at 64% VO2max for 180 min [28]. The authors speculated this could be attributed to hepatic gluconeogenesis during exercise and that lactate and/or glycerol could have provided a source of carbons for glycogen synthesis during recovery as levels of both were higher at the end of exercise and decreased during recovery. Another long term (>8 months) LCHF study observed decreased breakdown of hepatic glycogen during exercise, though hepatic gluconeogenesis did not increase compared with high CHO athletes [37]. However, the gluconeogenic precursors were likely to be more dependent on glycerol in the LCHF group and lactate in the high CHO group. Using a combination of muscle biopsy and CHO tracers, there was a 4-fold reduction in muscle glycogen use and 3-fold reduction in blood glucose oxidation during moderate intensity cycling after 4-week LCHF, with glycerol from triglycerides providing up to 40% of the glucose consumed and gluconeogenesis from lactate, pyruvate, and glucogenic amino acids providing the additional substrate needed to maintain plasma glucose levels and allow glycogen restoration [34]. The effects of LCHF on performance have been equivocal, with some studies showing benefit [38,41,42] and the majority showing no changes [20,35,36,38,43,44] or decrements [15,44,45,46,47]. Fourteen days of LCHF was able to attenuate the decline in power during a 100 km time trial that was observed with high-CHO, though overall performance times were not significantly different [41]. However, performance decrements have been observed during shorter, higher-intensity efforts. These include 6% lower power output on a Wingate test [47], 15% less distance run during the yo-yo intermittent recovery test [47], and trends toward significantly lower 1 km time trial performance (p = 0.07) [44] and 15-min time trial (p = 0.11) performance [41]. Further research is needed on the individual responses to LCHF, as several studies that showed no mean differences have had participants with favorable performance improvements [34,38,45]. To counteract the negative effects on performance observed with reduced glycogen levels, a model of dietary periodization was established that included a 5–14 day LCHF fat adaptation phase followed by a 1–3 day CHO restoration phase [18,35,36,41,42,43,48]. The concept was that the CHO restoration phase would allow glycogen to be replenished while retaining the skeletal muscle responses to fat adaptation [35]. Indeed, multiple studies have shown elevated values of fat oxidation compared with baseline, though the levels are less than what is observed prior to CHO restoration [18,35]. Ten days of LCHF followed by three days of CHO-loading improved 20 km time trial performance by 4.3% following cycling for 2.5 h at 70% VO2max [42], though similar studies have not shown performance improvements [35,36,43,44]. LCHF diets with CHO restoration reduce rates of muscle glycogen utilization, even after replenishing glycogen stores [18,35,42], which can be explained by a reduction in PDH after five days of LCHF that remained after one day of CHO restoration [48]. These adaptations to LCHF can persist through at least 36 h of dietary strategies to restore muscle and liver glycogen and increase CHO availability during exercise, though the exact time course of this period is currently unknown [49]. Well-trained athletes are able to oxidize more fat for fuel during exercise compared with recreationally trained or untrained individuals [50,51], and correlations have been observed between maximal rate of fat oxidation and endurance performance [52]. At the same time, it is clear that LCHF dietary interventions are able to drastically increase rates of fat oxidation regardless of training status [28,29,53]. Arguments in favor of trying to increase fat-burning capacity focus around the ability to utilize the large stores of endogenous lipids found even in very lean athletes, while preserving the relatively limited supply of muscle and liver glycogen. Yet despite this theoretical advantage, measurable performance improvements from deliberately increasing fat burning capacity have been elusive. One reason may be the decreased oxygen efficiency with the oxidation of fat compared with CHO. It has long been known that CHO yields more ATP per liter of oxygen compared with fat [54,55], resulting in an increasing oxygen consumption for a given running pace/cycling power output as the respiratory exchange ratio shifts downward. A 3-week study in elite race-walkers following LCHF found that increased rates of fat oxidation were accompanied (and offset) by an increased oxygen cost of walking, resulting in a lack of improvement in a 10 km race while performance improvements were observed in high-CHO and periodized-CHO groups [29]. Similar findings of decreased economy have also been observed in recreational runners on LCHF [53], while three days of high-CHO diet improved cycling gross efficiency compared with low and moderate-CHO diets [56]. Throughout the literature, there is a challenge in determining whether the adaptations to LCHF are driven by high fat intake or reduced CHO availability. To separate the effects of low CHO from high fat on mitochondrial respiration, well-trained cyclists consumed 5 days of isoenergetic high fat or high-protein diets (~67% of daily energy intake, with CHO clamped at <20%) [23]. The high-fat diet was able to increase rates of whole-body fat oxidation to a greater degree than the high-protein diet. In addition, the high-fat diet decreased skeletal muscle mitochondrial respiration driven by both FFA and pyruvate, as well as uncoupled respiration during exercise compared with high-protein. One day of high CHO intake was sufficient to return mitochondrial respiration levels to baseline, while no changes in mitochondrial respiration were observed after 5 days of the high-protein diet. Rates of substrate oxidation in both groups also returned towards baseline after one day of high CHO. This study suggests that high dietary fat intake, rather than low-CHO intake, is the primary driver in the reductions in mitochondrial respiration and increases in whole-body rates of fat oxidation on a LCHF diet. These results are in line with research in untrained males where LCHF blunted the exercise-induced increase in uncoupled respiratory capacity rates [22]. Overall, LCHF diets can increase the amount of fat utilized during exercise, though performance at higher exercise intensities may be compromised (Table 1). Due to the complex nature of metabolic regulation it is still unknown what the exact limiters of fat oxidation in skeletal muscle during increased exercise intensities are. Future research should continue looking at mechanisms of action for the diet-induced differences in enzymes that regulate mitochondrial substrate flux as well as the time course of CHO washout after LCHF + CHO interventions. 3. CHO Manipulation Traditional approaches to endurance training diets have promoted high CHO availability before, during and after training sessions to allow the athlete to train longer and harder in order to maximize the adaptive response. However, research over the past 10–15 years has demonstrated an increased role for CHO both as a fuel source and in metabolic signaling [57,58]. Deliberately restricting CHO before, during, and/or after exercise can result in positive training adaptations beyond what would be seen with high-CHO including increased mitochondrial enzyme activity, mitochondrial content, and rates of fat oxidation, with some research showing improvements in exercise capacity (Table 1). It has become apparent that the nutritional strategies for optimal performance (e.g., maximizing CHO availability) may not be the same as the strategies for maximal training adaptations (e.g., strategic CHO reduction). For example, exercise undertaken in a glycogen-depleted state increases the phosphorylation of AMPK in the post-exercise window, which directly phosphorylates PGC-1α, a protein that can induce mitochondrial biogenesis, angiogenesis, and increases in fat oxidation [4,11,59,60,61]. This has led to the concept of ‘train low, compete high’ which features training sessions performed under conditions of reduced CHO availability while CHO reserves are restored prior to and during competition. 3.1. Twice Daily Training The idea that training with reduced CHO availability can beneficially impact training adaptations came from observations that an acute bout of exercise undertaken with reduced muscle glycogen led to enhanced expression of genes related to substrate utilization and mitochondrial biogenesis [5,62]. This was further explored using a single leg training protocol, with twice-daily training and CHO restriction between sessions every other day demonstrating greater increases in CS maximal activity, exercise time to fatigue, and resting muscle glycogen content compared with once daily training with high CHO availability [9]. Several three-week studies in trained cyclists looked at twice daily training with CHO intake restricted between sessions compared with participants training once daily with high CHO availability [63,64]. Although performance improvements were similar between groups, the twice daily training groups showed greater increases in CS maximal activity, β-HAD, cytochrome oxidase subunit IV (COX-IV), and whole body fat oxidation during steady state cycling which was due to increased oxidation of IMTG rather than plasma FFA [63,64]. Increases in the activity of mitochondrial enzyme succinate dehydrogenase (SDH) and reductions in glycogen utilization have also been observed during a similar 6-week protocol, though performance improvements were not affected by diet [65]. In contrast, a two-week study in healthy but untrained participants comparing twice daily training while either ingesting high-CHO (195 g) or low-CHO (17 g) during the three-hour period between workouts found that post-intervention time trial performance was greater in the group training with low-CHO though there were no differences between groups in mitochondrial enzymes CS or COX-IV [66]. Possible reasons for these contrasting results include differences in training status and training intensity. The two studies using well-trained cyclists showed no performance benefits from low-CHO training [63,64], while a study using untrained participants did find favorable improvements [66]. Total work completed during training sessions may also impact study outcomes. When participants were instructed to self-select training intensity (e.g., 8 × 5 min at maximum self-selected effort), power output was 8% lower with low-CHO training and no performance changes were seen [63,64]. When training intensities were prescribed to ensure both groups had trained at the same power output, additional performance improvements were seen with low-CHO training [66]. Also, though not directly measured in these studies performance improvements may have resulted from changes in mitochondrial respiratory function, which are not always associated with training-induced changes in mitochondrial content [67]. Finally, it is noteworthy for athletes and coaches that the declines in exercise capacity observed while training in a CHO depleted state [64] can be recovered in part by caffeine [68,69], CHO mouth rinse [70], or the combination of both [71]. 3.2. Sleep Low Although the training adaptations with reduced compared with high-CHO availability have shown favorable adaptations at the molecular and cellular levels, reliable improvements in real-world performance have been elusive. This may be because dietary strategies that reduce CHO availability also reduce the ability of the athletes to effectively complete high-intensity interval training (HIIT) sessions [64]. Because of this, and in light of favorable adaptations observed with post-workout CHO restriction [3], a “sleep-low” protocol was established. During this protocol, athletes consume a high-CHO diet prior to an evening session of HIIT, go to sleep without consuming any post-workout CHO and then complete a low-intensity workout the next morning in a fasted state to optimize fat-burning adaptations [72]. Compared with a CHO-fed control group, the sleep-low group increased fat oxidation during 2 h of steady state cycling along with elevated CPT-1 and greater increases in resting phosphorylation of AMPK and p38 MAPK, markers of mitochondrial adaptation [72]. In two follow-up studies lasting one and three weeks, respectively, participants completed a sleep-low protocol three days per week [73,74,75]. After three weeks the sleep-low group had improvements in submaximal cycling efficiency and supra-maximal cycling to exhaustion along with a 3% improvement in 10 km running performance and decreased fat mass, while no performance changes were seen in the control group [73]. The one week study saw a similar (~3%) improvement in 20 km cycling time trial performance [75]. It was also found that the 3-week protocol had minimal effects on markers of sleep quality and immune function [74]. While the initial sleep-low study had participants completely refrain from eating after the evening HIIT session [72], a key difference in follow-up studies [73,74,75] was the provision of a protein-containing, non-CHO meal after the evening HIIT. However, concern regarding the long-term impact of post-workout CHO restriction is warranted as levels of p70S6K (a key regulator of the skeletal muscle response to exercise) are suppressed following exercise and restored with intake of CHO but not from ingestion of protein-only [76]. Post-exercise fat and protein consumption also reduced p70S6K activity, in contrast with increased activity observed with high CHO [77]. This, along with elevated rates of post-exercise myofibrillar muscle protein synthesis and p70S6K phosphorylation with CHO and protein [78] suggests that athletes who withhold CHO prior to and/or during training sessions should consume both CHO and protein post-workout in order to maximize the skeletal muscle adaptive response. Thus, more research is needed to reconcile the observed short-term performance benefits from sleep-low protocols with any potential negative impact on the post-exercise muscle adaptive responses. This apparent contradiction may be explained by differences in the skeletal muscle response to acute vs. chronic HIIT [79], and the findings of a meta-analysis that total daily protein intake, rather than timing, was the strongest predictor of exercise-induced muscle hypertrophy [80]. For an in-depth review of molecular adaptations to exercise the reader can be directed to several recent reviews [81,82,83]. 3.3. Fasted Training Performing exercise after an overnight fast is another way to alter CHO availability and augment the adaptive response to endurance training. In contrast with twice daily training sessions performed without CHO restoration that result in depleted muscle glycogen levels for the second bout of exercise, overnight fasting reduces liver, but not reduced muscle glycogen, concentrations [84]. Blood glucose concentration may be maintained at normal levels during exercise after an overnight fast despite the depletion of liver glycogen, likely due to increased gluconeogenesis and decreased utilization of glucose in muscle as a result of lowered PDH activity [85]. A recent review and meta-analysis looking at the effects of fed vs. fasted aerobic training on substrate usage found that fasted exercise induces higher fat oxidation than exercise performed in the fed state, though no differences in plasma FFA were found [86]. However, muscle glycogen levels need to be determined when drawing conclusions on substrate oxidation between fasted and non-fasted training as no differences were observed in fat oxidation during the first 90–120 min of fasted compared with non-fasted exercise when muscle glycogen levels begin at the same level [87,88], despite reductions in lipolysis and plasma FFA concentration [88] as well as elevated insulin levels from CHO ingestion during moderate-intensity exercise [89]. Steady state cycling in a fasted state can also cause the breakdown of IMTG in type I muscle fibers, which was completely blunted in a CHO-fed state [90]. Broad conclusions from the available research are also challenged by variations among testing protocols. For example, IMTG usage was increased after 6 weeks of fasted (but not fed) training when tested in the fasted state [10] while another 6-week study resulted in no differences in fat oxidation rates or IMTG breakdown during exercise in a fed state while also providing additional CHO [91], suggesting a lack of carryover to real-world application. Mechanisms for increased fat utilization during fasted exercise can include changes in adipose tissue mRNA expression of PDK4, adipose triglyceride lipase, HSL, β-HAD, CPT-1, FAT/CD36, GLUT4, and insulin receptor substrate 2, all of which were lower in response to fed compared with fasted exercise [91,92]. A review and meta-analysis looking at the effects of fed vs. fasted exercise on performance and post-exercise metabolism found that exercise in the fed state enhanced prolonged, but not shorter duration aerobic exercise performance, while fasted exercise increased post-exercise circulating FFA compared to fed exercise and pre-exercise feeding blunted signaling related to mitochondrial adaptation such as β-HAD and CS [93]. Performance changes after 6-week endurance-training programs were not different between participants completing the training in the fed or fasted states [10,91], though only fasted training led to increases in maximal activity of CS and β-HAD while also preventing an exercise-induced drop in blood glucose [10]. In contrast, a 6-week study in overweight women performing HIIT in the fed or fasted state found no effect of fasting vs. fed state on training-induced increases in CS, β-HAD, or GLUT4 protein content [94]. It is possible that a different result would have been observed from higher-volume, lower-intensity training as CS activity is increased by volume and not intensity [7]. Sex differences may be seen with fasted training, as a 4 week study in untrained males and females found training-induced changes in CS and β-HAD were not different between fed and fasted training but women had greater increases with fed training while men had greater increases with fasted training [95]. That study also found the fasting group had greater training-induced increase in VO2max and resting muscle glycogen levels [95]. The majority of studies using sedentary and/or overweight/obese populations have found no changes in post-exercise glucose, insulin, or FFAs between fasted and fed conditions, further highlighting the potential role of training status on metabolic flexibility [93,96]. 3.4. Periodized Carbohydrate Perhaps the most pragmatic approach may be to use a periodized CHO intake, which refers to performing some training sessions with high CHO availability (high muscle glycogen, CHO feeding during session) and others with low CHO availability (low pre-exercise glycogen, avoidance of intra-workout CHO, overnight fasted, or delayed post-session refueling). This allows high-intensity workouts to be completed at the highest possible work output and low-intensity training to be performed without CHO in order to maximize mitochondrial adaptations. CHO-restricted training sessions may be critical for maximizing fat oxidation, as ingestion of CHO during exercise decreases the expression of genes involved in lipid metabolism (e.g., GLUT-4, PDK4, AMPK, CD36, CPT-1, and UCP3) rather than increasing genes involved in carbohydrate metabolism [97], as well as blunting the interleukin-6 response to exercise [98]. A study in elite endurance athletes that compared LCHF to both high-CHO and a high but periodized CHO intake found that LCHF and periodized-CHO had greater losses in body mass during the 3-week intervention, while the performance improvements and measures of oxygen consumption in high-CHO and periodized CHO groups were similar and both more favorable than LCHF [29]. Another 4-week study that compared high CHO to a periodized CHO intake found no significant differences in the training effects between dietary approaches [99]. While a periodized approach to CHO intake seems to intuitively make sense, longer-term studies are needed to determine if there are favorable performance benefits compared with high-CHO. In summary, deliberately restricting CHO before, during, or after endurance training can lead to greater training stimuli at the cellular and molecular levels but poses additional challenges for the athlete who wishes to maintain their desired training intensities. On a practical level it is critical for athletes practicing “train-low” approaches to also include workouts that are fully fueled to be sure they retain their capacity to absorb and oxidize CHO without gastrointestinal distress [100,101], and maintain activity of the PDH enzyme complex [48]. Consuming protein prior to exercise with low-CHO availability may be a sensible strategy as consumption of protein before and during exercise does not impair FFA availability or whole body fat oxidation despite elevated insulin levels [102], and exercising with reduced muscle glycogen increases both muscle protein breakdown and the contribution of protein to energy production [103,104]. Effects of training in the fasted state are inconsistent, likely due to differences in training status, sex, and testing conditions. Future work should attempt to further elucidate differences between CHO restriction surrounding low intensity and high intensity workouts and determine the effects of changing the order of low and high intensity workouts during twice daily training. Further examination into the role of training status on the effects of CHO restricted training as well as the time course needed to see real world performance effects would also be valuable for coaches and athletes. In addition, comparisons of the metabolic and performance responses of fasted (with high and low muscle glycogen) to LCHF interventions and protein-only feedings prior to exercise are warranted. 4. Personalization, Preference and Perception Though most athletes acknowledge the importance of diet in optimal sports performance [105], the make-up of an optimal diet may be less clear. In an observational study of athletes across 13 sports that were seeking nutritional advice from on-site dietitians at a major international competition, CHO intake (based on 24-h recall) ranged from 1.0–9.0 g/kg per day, with a median value of 3.8 g/kg [106]. Studies in elite and sub-elite male Australian football and soccer players scored an average of 57–62% in a questionnaire designed to test their nutrition knowledge, while intake of CHO was lower than recommended and nutrition knowledge was positively correlated with fat-free soft tissue mass [107,108]. In a survey of competitive tennis players, only ~50% of players reported opting for CHO dominant meals prior to a match, while 39% of players reported consuming “nothing specific” on the day after a match [109]. This suggests a large knowledge gap between sports nutrition research-based recommendations and application among athletes and practitioners in the field. Other research in elite endurance athletes has reported practices that more closely align with recommendations for athletes training at a high volume [110]. Professional male cyclists being observed during three consecutive days of high-intensity training consumed an average of 9.8–12.2 g/kg CHO per day [111], while elite female cyclists observed during a nine day training block reported an intake of 7.5–10 g/kg per day [112]. Another study observing professional cyclists during preseason training camp reported a slightly lower self-selected CHO intake of 6.7 g/kg per day [113]. A case study of elite marathon runners reported training sessions performed with low-CHO availability 1–3 times per week; 90% of those sessions being after an overnight fast and 10% being reduced glycogen training from twice daily training without CHO restoration [114]. A survey looking at the dietary practices of 48 elite male and female middle- and long-distance runners/race-walkers found that while nearly all athletes (96%) focus on adequate intake around key training sessions, only 26% report ever training in an overnight-fasted state, 11% report periodically restricting CHO intake, and only 30% ingest CHO during training sessions [115]. When dietary intake during one week of high intensity training camp was analyzed for periodization between hard and easy training days, females had greater energy intake on hard training days (204 vs. 187 kJ/kg/day) while males did not significantly change intake, and females showed a pattern of periodization of post-exercise CHO for key vs. easy training sessions (0.9 vs. 0.5 g/kg, respectively) while males had only modest periodization (1.3 vs. 1.0 g/kg, respectively) [116]. Furthermore, only 73% of athletes who reported in the survey that they focused on adequate fueling before key workouts were observed to meet the recommended target intakes, while post-workout CHO and protein targets were only met by 56% and 26%, respectively, of the 87% of athletes that reported paying attention to nutrition recovery after key workout sessions [116]. These findings suggest a potential lack of awareness surrounding the benefits of strategic CHO manipulation as well as incomplete execution of the latest guidelines encouraging a periodized nutrition approach to support optimal performance, recovery, and training adaptation [110]. Future research should look at not just the beliefs and practices of athletes, but of coaches and nutritionists to determine if current, evidence-based recommendations are being provided to athletes. 5. Conclusions Dietary intake (both the timing and types of food) can impact the metabolic adaptations to training, though specific training responses can vary based on training status, pre-exercise glycogen levels, exercise intensity and duration, and post-workout feeding. Though direct performance benefits are challenging to measure, accumulating data suggests that partaking in some training sessions with low, and some training sessions with high, CHO availability may lead to optimal training outcomes. This can be accomplished by following a short-term low-carb, high-fat (LCHF) diet, performing twice daily training without glycogen restoration between sessions, a “sleep-low” approach where the athlete does a glycogen-depleting session in the evening and does not consume CHO until the following day, and undertaking AM training sessions in an overnight fasted state. Funding This research received no external funding. Conflicts of Interest The authors declare no conflicts of interest. jfmk-03-00041-t001_Table 1Table 1 Summary of adaptations to concurrent diet and endurance training strategies. Abbreviations and symbols: FAT/CD36: fatty acyl translocase; CPT-1: carnitine palmitoyltransferase-1; β-HAD: β-hydroxyacyl-CoA dehydrogenase; ↑: increased; ↓: decreased, Ø: nonsignificant change. Dietary Intervention Fat Oxidation Carb Oxidation/ Glycogen Utilization Glycogen Storage CPT-1/FAT/CD36 β-HAD Citrate Synthase Performance LCHF ↑ ↓ Ø ↓ Ø ↑ Ø ↑ Ø ↑ Ø ↓ Twice daily training ↑ ↓ ↑ ↑ ↑ Ø ↑ Ø ↑ Sleep Low Ø ↑ Ø ↓ ↑ ↑ Fasted Training Ø ↑ Ø ↓ ↑ ↑ Ø ↑ Ø ↑ Ø ↓ ==== Refs References 1. Coyle E.F. Timing and method of increased carbohydrate intake to cope with heavy training, competition and recovery J. Sports Sci. 1991 9 29 52 10.1080/02640419108729865 1895362 2. Bartlett J.D. Hawley J.A. Morton J.P. Carbohydrate availability and exercise training adaptation: Too much of a good thing? Eur. J. Sport Sci. 2015 15 3 12 10.1080/17461391.2014.920926 24942068 3. Cochran A.J. Little J.P. Tarnopolsky M.A. Gibala M.J. Carbohydrate feeding during recovery alters the skeletal muscle metabolic response to repeated sessions of high-intensity interval exercise in humans J. Appl. Physiol. 2010 108 628 636 10.1152/japplphysiol.00659.2009 20056852 4. Bartlett J.D. Louhelainen J. Iqbal Z. Cochran A.J. Gibala M.J. Gregson W. Close G.L. Drust B. Morton J.P. Reduced carbohydrate availability enhances exercise-induced p53 signaling in human skeletal muscle: Implications for mitochondrial biogenesis Am. J. Physiol.-Regul. Integr. Comp. Physiol. 2013 304 R450 R458 10.1152/ajpregu.00498.2012 23364526 5. Pilegaard H. Osada T. Andersen L.T. Helge J.W. Saltin B. Neufer P.D. Substrate availability and transcriptional regulation of metabolic genes in human skeletal muscle during recovery from exercise Metab.-Clin. Exp. 2005 54 1048 1055 10.1016/j.metabol.2005.03.008 16092055 6. Granata C. Jamnick N.A. Bishop D.J. Principles of exercise prescription, and how they influence exercise-induced changes of transcription factors and other regulators of mitochondrial biogenesis Sports Med. 2018 1 19 10.1007/s40279-018-0894-4 7. Bishop D.J. Granata C. Eynon N. Can we optimise the exercise training prescription to maximise improvements in mitochondria function and content? Biochim. Biophys. Acta (BBA)-Gen. Subj. 2014 1840 1266 1275 10.1016/j.bbagen.2013.10.012 24128929 8. Helge J.W. Kiens B. Muscle enzyme activity in humans: Role of substrate availability and training Am. J. Physiol.-Regul. Integr. Comp. Physiol. 1997 272 R1620 R1624 10.1152/ajpregu.1997.272.5.R1620 9176356 9. Hansen A.K. Fischer C.P. Plomgaard P. Andersen J.L. Saltin B. Pedersen B.K. Skeletal muscle adaptation: Training twice every second day vs. Training once daily J. Appl. Physiol. 2005 98 93 99 10.1152/japplphysiol.00163.2004 15361516 10. Van Proeyen K. Szlufcik K. Nielens H. Ramaekers M. Hespel P. Beneficial metabolic adaptations due to endurance exercise training in the fasted state J. Appl. Physiol. 2010 110 236 245 10.1152/japplphysiol.00907.2010 21051570 11. Wojtaszewski J.F. MacDonald C. Nielsen J.N. Hellsten Y. Hardie D.G. Kemp B.E. Kiens B. Richter E.A. Regulation of 5′ amp-activated protein kinase activity and substrate utilization in exercising human skeletal muscle Am. J. Physiol.-Endocrinol. Metab. 2003 284 E813 E822 10.1152/ajpendo.00436.2002 12488245 12. Volek J.S. Noakes T. Phinney S.D. Rethinking fat as a fuel for endurance exercise Eur. J. Sport Sci. 2015 15 13 20 10.1080/17461391.2014.959564 25275931 13. Spriet L.L. New insights into the interaction of carbohydrate and fat metabolism during exercise Sports Med. 2014 44 87 96 10.1007/s40279-014-0154-1 24791920 14. Van Hall G. The physiological regulation of skeletal muscle fatty acid supply and oxidation during moderate-intensity exercise Sports Med. 2015 45 23 32 10.1007/s40279-015-0394-8 25178498 15. Zajac A. Poprzecki S. Maszczyk A. Czuba M. Michalczyk M. Zydek G. The effects of a ketogenic diet on exercise metabolism and physical performance in off-road cyclists Nutrients 2014 6 2493 2508 10.3390/nu6072493 24979615 16. Frayn K. Fat as a fuel: Emerging understanding of the adipose tissue–skeletal muscle axis Acta Physiol. 2010 199 509 518 10.1111/j.1748-1716.2010.02128.x 20353493 17. Romijn J. Coyle E. Sidossis L. Gastaldelli A. Horowitz J. Endert E. Wolfe R. Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration Am. J. Physiol.-Endocrinol. Metab. 1993 265 E380 E391 10.1152/ajpendo.1993.265.3.E380 8214047 18. Yeo W.K. Lessard S.J. Chen Z.-P. Garnham A.P. Burke L.M. Rivas D.A. Kemp B.E. Hawley J.A. Fat adaptation followed by carbohydrate restoration increases ampk activity in skeletal muscle from trained humans J. Appl. Physiol. 2008 105 1519 1526 10.1152/japplphysiol.90540.2008 18801964 19. Kiens B. Essen-Gustavsson B. Gad P. Lithell H. Lipoprotein lipase activity and intramuscular triglyceride stores after long-term high-fat and high-carbohydrate diets in physically trained men Clin. Physiol. Funct. Imaging 1987 7 1 9 10.1111/j.1475-097X.1987.tb00628.x 20. Vogt M. Puntschart A. Howald H. Mueller B. Mannhart C. Gfeller-Tuescher L. Mullis P. Hoppeler H. Effects of dietary fat on muscle substrates, metabolism, and performance in athletes Med. Sci. Sports Exerc. 2003 35 952 960 10.1249/01.MSS.0000069336.30649.BD 12783043 21. Zderic T.W. Davidson C.J. Schenk S. Byerley L.O. Coyle E.F. High-fat diet elevates resting intramuscular triglyceride concentration and whole body lipolysis during exercise Am. J. Physiol.-Endocrinol. Metab. 2004 286 E217 E225 10.1152/ajpendo.00159.2003 14559721 22. Skovbro M. Boushel R. Hansen C.N. Helge J.W. Dela F. High-fat feeding inhibits exercise-induced increase in mitochondrial respiratory flux in skeletal muscle J. Appl. Physiol. 2011 110 1607 1614 10.1152/japplphysiol.01341.2010 21415171 23. Leckey J.J. Hoffman N.J. Parr E.B. Devlin B.L. Trewin A.J. Stepto N.K. Morton J.P. Burke L.M. Hawley J.A. High dietary fat intake increases fat oxidation and reduces skeletal muscle mitochondrial respiration in trained humans FASEB J. 2018 32 2979 2991 10.1096/fj.201700993R 29401600 24. Cameron-Smith D. Burke L.M. Angus D.J. Tunstall R.J. Cox G.R. Bonen A. Hawley J.A. Hargreaves M. A short-term, high-fat diet up-regulates lipid metabolism and gene expression in human skeletal muscle Am. J. Clin. Nutr. 2003 77 313 318 10.1093/ajcn/77.2.313 12540388 25. Peters S.J. St. Amand T.A. Howlett R.A. Heigenhauser G.J. Spriet L.L. Human skeletal muscle pyruvate dehydrogenase kinase activity increases after a low-carbohydrate diet Am. J. Physiol.-Endocrinol. Metab. 1998 275 E980 E986 10.1152/ajpendo.1998.275.6.E980 26. Goedecke J.H. Christie C. Wilson G. Dennis S.C. Noakes T.D. Hopkins W.G. Lambert E.V. Metabolic adaptations to a high-fat diet in endurance cyclists Metabolism 1999 48 1509 1517 10.1016/S0026-0495(99)90238-X 10599981 27. Randell R.K. Rollo I. Roberts T.J. Dalrymple K.J. Jeukendrup A.E. Carter J.M. Maximal fat oxidation rates in an athletic population Med. Sci. Sports Exerc. 2017 49 133 140 10.1249/MSS.0000000000001084 27580144 28. Volek J.S. Freidenreich D.J. Saenz C. Kunces L.J. Creighton B.C. Bartley J.M. Davitt P.M. Munoz C.X. Anderson J.M. Maresh C.M. Metabolic characteristics of keto-adapted ultra-endurance runners Metabolism 2016 65 100 110 10.1016/j.metabol.2015.10.028 26892521 29. Burke L.M. Ross M.L. Garvican-Lewis L.A. Welvaert M. Heikura I.A. Forbes S.G. Mirtschin J.G. Cato L.E. Strobel N. Sharma A.P. Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers J. Physiol. 2017 595 2785 2807 10.1113/JP273230 28012184 30. Helge J.W. Watt P.W. Richter E.A. Rennie M.J. Kiens B. Fat utilization during exercise: Adaptation to a fat-rich diet increases utilization of plasma fatty acids and very low density lipoprotein-triacylglycerol in humans J. Physiol. 2001 537 1009 1020 10.1113/jphysiol.2001.012933 11744773 31. Helge J.W. Long-term fat diet adaptation effects on performance, training capacity, and fat utilization Med. Sci. Sports Exerc. 2002 34 1499 1504 10.1097/00005768-200209000-00016 12218745 32. Schrauwen P. Wagenmakers A.J. van Marken Lichtenbelt W.D. Saris W.H. Westerterp K.R. Increase in fat oxidation on a high-fat diet is accompanied by an increase in triglyceride-derived fatty acid oxidation Diabetes 2000 49 640 646 10.2337/diabetes.49.4.640 10871203 33. Hurley B. Nemeth P. Martin W. 3rd Hagberg J. Dalsky G. Holloszy J. Muscle triglyceride utilization during exercise: Effect of training J. Appl. Physiol. 1986 60 562 567 10.1152/jappl.1986.60.2.562 3512511 34. Phinney S.D. Bistrian B.R. Evans W. Gervino E. Blackburn G. The human metabolic response to chronic ketosis without caloric restriction: Preservation of submaximal exercise capability with reduced carbohydrate oxidation Metabolism 1983 32 769 776 10.1016/0026-0495(83)90106-3 6865776 35. Burke L.M. Angus D.J. Cox G.R. Cummings N.K. Febbraio M.A. Gawthorn K. Hawley J.A. Minehan M. Martin D.T. Hargreaves M. Effect of fat adaptation and carbohydrate restoration on metabolism and performance during prolonged cycling J. Appl. Physiol. 2000 89 2413 2421 10.1152/jappl.2000.89.6.2413 11090597 36. Burke L.M. Hawley J.A. Angus D.J. Cox G.R. Clark S.A. Cummings N.K. Desbrow B. Hargreaves M. Adaptations to short-term high-fat diet persist during exercise despite high carbohydrate availability Med. Sci. Sports Exerc. 2002 34 83 91 10.1097/00005768-200201000-00014 11782652 37. Webster C.C. Noakes T.D. Chacko S.K. Swart J. Kohn T.A. Smith J.A. Gluconeogenesis during endurance exercise in cyclists habituated to a long-term low carbohydrate high-fat diet J. Physiol. 2016 594 4389 4405 10.1113/JP271934 26918583 38. McSwiney F.T. Wardrop B. Hyde P.N. Lafountain R.A. Volek J.S. Doyle L. Keto-adaptation enhances exercise performance and body composition responses to training in endurance athletes Metabolism 2018 81 25 34 10.1016/j.metabol.2017.10.010 29108901 39. Bergström J. Hermansen L. Hultman E. Saltin B. Diet, muscle glycogen and physical performance Acta Physiol. 1967 71 140 150 10.1111/j.1748-1716.1967.tb03720.x 5584523 40. Hultman E. Bergström J. Muscle glycogen synthesis in relation to diet studied in normal subjects J. Intern. Med. 1967 182 109 117 10.1111/j.0954-6820.1967.tb11504.x 41. Rowlands D.S. Hopkins W.G. Effects of high-fat and high-carbohydrate diets on metabolism and performance in cycling Metabolism 2002 51 678 690 10.1053/meta.2002.32723 12037719 42. Lambert E.V. Goedecke J.H. van Zyl C. Murphy K. Hawley J.A. Dennis S.C. Noakes T.D. High-fat diet versus habitual diet prior to carbohydrate loading: Effects on exercise metabolism and cycling performance Int. J. Sport Nutr. Exerc. Metab. 2001 11 209 225 10.1123/ijsnem.11.2.209 11402254 43. Carey A.L. Staudacher H.M. Cummings N.K. Stepto N.K. Nikolopoulos V. Burke L.M. Hawley J.A. Effects of fat adaptation and carbohydrate restoration on prolonged endurance exercise J. Appl. Physiol. 2001 91 115 122 10.1152/jappl.2001.91.1.115 11408421 44. Havemann L. West S.J. Goedecke J.H. Macdonald I.A. St Clair Gibson A. Noakes T. Lambert E.V. Fat adaptation followed by carbohydrate loading compromises high-intensity sprint performance J. Appl. Physiol. 2006 100 194 202 10.1152/japplphysiol.00813.2005 16141377 45. Zinn C. Wood M. Williden M. Chatterton S. Maunder E. Ketogenic diet benefits body composition and well-being but not performance in a pilot case study of new zealand endurance athletes J. Int. Soc. Sports Nutr. 2017 14 22 10.1186/s12970-017-0180-0 28706467 46. Helge J.W. Richter E.A. Kiens B. Interaction of training and diet on metabolism and endurance during exercise in man J. Physiol. 1996 492 293 306 10.1113/jphysiol.1996.sp021309 8730603 47. Wroble K. Trott M. Schweitzer G. Rahman R. Kelly P. Weiss E. Low-carbohydrate, ketogenic diet impairs anaerobic exercise performance in exercise-trained women and men: A randomized-sequence crossover trial J. Sports Med. Phys. Fit. 2018 10.23736/S0022-4707.18.08318-4 48. Stellingwerff T. Spriet L.L. Watt M.J. Kimber N.E. Hargreaves M. Hawley J.A. Burke L.M. Decreased pdh activation and glycogenolysis during exercise following fat adaptation with carbohydrate restoration Am. J. Physiol.-Endocrinol. Metab. 2006 290 E380 E388 10.1152/ajpendo.00268.2005 16188909 49. Burke L.M. Re-examining high-fat diets for sports performance: Did we call the ‘nail in the coffin’ too soon? Sports Med. 2015 45 33 49 10.1007/s40279-015-0393-9 26553488 50. Coggan A.R. Raguso C.A. Gastaldelli A. Sidossis L.S. Yeckel C.W. Fat metabolism during high-intensity exercise in endurance-trained and untrained men Metabolism 2000 49 122 128 10.1016/S0026-0495(00)90963-6 10647075 51. Hetlelid K.J. Plews D.J. Herold E. Laursen P.B. Seiler S. Rethinking the role of fat oxidation: Substrate utilisation during high-intensity interval training in well-trained and recreationally trained runners BMJ Open Sport Exerc. Med. 2015 1 e000047 10.1136/bmjsem-2015-000047 27900134 52. Frandsen J. Vest S.D. Larsen S. Dela F. Helge J.W. Maximal fat oxidation is related to performance in an ironman triathlon Int. J. Sports Med. 2017 38 975 982 10.1055/s-0043-117178 29050040 53. Heatherly A.J. Killen L.G. Smith A.F. Waldman H.S. Hollingsworth A. Seltmann C.L. O’Neal E.K. Effects of ad libitum low carbohydrate high-fat dieting in middle-age male runners Med. Sci. Sports Exerc. 2017 10.1249/MSS.0000000000001477 29112626 54. Leverve X. Batandier C. Fontaine E. Choosing the right substrate Novartis Found. Symp. 2007 280 108 121 discussion 121–107, 160–104 17380791 55. Lusk G. Animal calorimetry twenty-fourth paper. Analysis of the oxidation of mixtures of carbohydrate and fat J. Biol. Chem. 1924 59 41 42 56. Cole M. Coleman D. Hopker J. Wiles J. Improved gross efficiency during long duration submaximal cycling following a short-term high carbohydrate diet Int. J. Sports Med. 2014 35 265 269 10.1055/s-0033-1348254 24022570 57. Baar K. McGee S. Optimizing training adaptations by manipulating glycogen Eur. J. Sport Sci. 2008 8 97 106 10.1080/17461390801919094 58. McBride A. Hardie D. Amp-activated protein kinase—A sensor of glycogen as well as amp and atp? Acta Physiol. 2009 196 99 113 10.1111/j.1748-1716.2009.01975.x 19245651 59. Yeo W.K. McGee S.L. Carey A.L. Paton C.D. Garnham A.P. Hargreaves M. Hawley J.A. Acute signalling responses to intense endurance training commenced with low or normal muscle glycogen Exp. Physiol. 2010 95 351 358 10.1113/expphysiol.2009.049353 19854796 60. Jäger S. Handschin C. Pierre J.S. Spiegelman B.M. Amp-activated protein kinase (ampk) action in skeletal muscle via direct phosphorylation of pgc-1α Proc. Natl. Acad. Sci. USA 2007 104 12017 12022 10.1073/pnas.0705070104 17609368 61. Baar K. Nutrition and the adaptation to endurance training Sports Med. 2014 44 5 12 10.1007/s40279-014-0146-1 24791912 62. Pilegaard H. Keller C. Steensberg A. Wulff Helge J. Klarlund Pedersen B. Saltin B. Neufer P.D. Influence of pre-exercise muscle glycogen content on exercise-induced transcriptional regulation of metabolic genes J. Physiol. 2002 541 261 271 10.1113/jphysiol.2002.016832 12015434 63. Yeo W.K. Paton C.D. Garnham A.P. Burke L.M. Carey A.L. Hawley J.A. Skeletal muscle adaptation and performance responses to once a day versus twice every second day endurance training regimens J. Appl. Physiol. 2008 105 1462 1470 10.1152/japplphysiol.90882.2008 18772325 64. Hulston C.J. Venables M.C. Mann C.H. Martin C. Philp A. Baar K. Jeukendrup A.E. Training with low muscle glycogen enhances fat metabolism in well-trained cyclists Med. Sci. Sports Exerc. 2010 42 2046 2055 10.1249/MSS.0b013e3181dd5070 20351596 65. Morton J.P. Croft L. Bartlett J.D. MacLaren D.P. Reilly T. Evans L. McArdle A. Drust B. Reduced carbohydrate availability does not modulate training-induced heat shock protein adaptations but does upregulate oxidative enzyme activity in human skeletal muscle J. Appl. Physiol. 2009 106 1513 1521 10.1152/japplphysiol.00003.2009 19265068 66. Cochran A.J. Myslik F. MacInnis M.J. Percival M.E. Bishop D. Tarnopolsky M.A. Gibala M.J. Manipulating carbohydrate availability between twice-daily sessions of high-intensity interval training over 2 weeks improves time-trial performance Int. J. Sport Nutr. Exerc. Metab. 2015 25 463 470 10.1123/ijsnem.2014-0263 25811132 67. Granata C. Jamnick N.A. Bishop D.J. Training-induced changes in mitochondrial content and respiratory function in human skeletal muscle Sports Med. 2018 48 1809 1828 10.1007/s40279-018-0936-y 29934848 68. Lane S.C. Areta J.L. Bird S.R. Coffey V.G. Burke L.M. Desbrow B. Karagounis L.G. Hawley J.A. Caffeine ingestion and cycling power output in a low or normal muscle glycogen state Med. Sci. Sports Exerc. 2013 45 1577 1584 10.1249/MSS.0b013e31828af183 23439421 69. Silva-Cavalcante M.D. Correia-Oliveira C.R. Santos R.A. Lopes-Silva J.P. Lima H.M. Bertuzzi R. Duarte M. Bishop D.J. Lima-Silva A.E. Caffeine increases anaerobic work and restores cycling performance following a protocol designed to lower endogenous carbohydrate availability PLoS ONE 2013 8 e72025 10.1371/journal.pone.0072025 23977198 70. Lane S.C. Bird S.R. Burke L.M. Hawley J.A. Effect of a carbohydrate mouth rinse on simulated cycling time-trial performance commenced in a fed or fasted state Appl. Physiol. Nutr. Metab. 2012 38 134 139 10.1139/apnm-2012-0300 23438223 71. Kasper A.M. Cocking S. Cockayne M. Barnard M. Tench J. Parker L. McAndrew J. Langan-Evans C. Close G.L. Morton J.P. Carbohydrate mouth rinse and caffeine improves high-intensity interval running capacity when carbohydrate restricted Eur. J. Sport Sci. 2016 16 560 568 10.1080/17461391.2015.1041063 26035740 72. Lane S.C. Camera D.M. Lassiter D.G. Areta J.L. Bird S.R. Yeo W.K. Jeacocke N.A. Krook A. Zierath J.R. Burke L.M. Effects of sleeping with reduced carbohydrate availability on acute training responses J. Appl. Physiol. 2015 119 643 655 10.1152/japplphysiol.00857.2014 26112242 73. Marquet L.-A. Brisswalter J. Louis J. Tiollier E. Burke L. Hawley J. Hausswirth C. Enhanced endurance performance by periodization of cho intake: “Sleep low” strategy Med. Sci. Sports Exerc. 2016 48 663 672 10.1249/MSS.0000000000000823 26741119 74. Louis J. Marquet L.-A. Tiollier E. Bermon S. Hausswirth C. Brisswalter J. The impact of sleeping with reduced glycogen stores on immunity and sleep in triathletes Eur. J. Appl. Physiol. 2016 116 1941 1954 10.1007/s00421-016-3446-3 27491620 75. Marquet L.-A. Hausswirth C. Molle O. Hawley J.A. Burke L.M. Tiollier E. Brisswalter J. Periodization of carbohydrate intake: Short-term effect on performance Nutrients 2016 8 755 10.3390/nu8120755 27897989 76. Impey S.G. Hammond K.M. Shepherd S.O. Sharples A.P. Stewart C. Limb M. Smith K. Philp A. Jeromson S. Hamilton D.L. Fuel for the work required: A practical approach to amalgamating train-low paradigms for endurance athletes Physiol. Rep. 2016 4 e12803 10.14814/phy2.12803 27225627 77. Hammond K.M. Impey S.G. Currell K. Mitchell N. Shepherd S.O. Jeromson S. Hawley J.A. Close G.L. Hamilton D.L. Sharples A.P. Postexercise high-fat feeding suppresses p70s6k1 activity in human skeletal muscle Med. Sci. Sports Exerc. 2016 48 2108 2117 10.1249/MSS.0000000000001009 27327024 78. Breen L. Philp A. Witard O.C. Jackman S.R. Selby A. Smith K. Baar K. Tipton K.D. The influence of carbohydrate–protein co-ingestion following endurance exercise on myofibrillar and mitochondrial protein synthesis J. Physiol. 2011 589 4011 4025 10.1113/jphysiol.2011.211888 21746787 79. Cochran A.J. Percival M.E. Tricarico S. Little J.P. Cermak N. Gillen J.B. Tarnopolsky M.A. Gibala M.J. Intermittent and continuous high-intensity exercise training induce similar acute but different chronic muscle adaptations Exp. Physiol. 2014 99 782 791 10.1113/expphysiol.2013.077453 24532598 80. Schoenfeld B.J. Aragon A.A. Krieger J.W. The effect of protein timing on muscle strength and hypertrophy: A meta-analysis J. Int. Soc. Sports Nutr. 2013 10 53 10.1186/1550-2783-10-53 24299050 81. Egan B. Zierath J.R. Exercise metabolism and the molecular regulation of skeletal muscle adaptation Cell Metab. 2013 17 162 184 10.1016/j.cmet.2012.12.012 23395166 82. Margolis L.M. Pasiakos S.M. Optimizing intramuscular adaptations to aerobic exercise: Effects of carbohydrate restriction and protein supplementation on mitochondrial biogenesis Adv. Nutr. 2013 4 657 664 10.3945/an.113.004572 24228194 83. Hawley J.A. Morton J.P. Ramping up the signal: Promoting endurance training adaptation in skeletal muscle by nutritional manipulation Clin. Exp. Pharmacol. Physiol. 2014 41 608 613 10.1111/1440-1681.12246 25142094 84. Knapik J.J. Meredith C.N. Jones B.H. Suek L. Young V.R. Evans W.J. Influence of fasting on carbohydrate and fat metabolism during rest and exercise in men J. Appl. Physiol. 1988 64 1923 1929 10.1152/jappl.1988.64.5.1923 3292504 85. Dohm G.L. Beeker R.T. Israel R.G. Tapscott E.B. Metabolic responses to exercise after fasting J. Appl. Physiol. 1986 61 1363 1368 10.1152/jappl.1986.61.4.1363 3536834 86. Vieira A.F. Costa R.R. Macedo R.C.O. Coconcelli L. Kruel L.F.M. Effects of aerobic exercise performed in fasted v. Fed state on fat and carbohydrate metabolism in adults: A systematic review and meta-analysis Br. J. Nutr. 2016 116 1153 1164 10.1017/S0007114516003160 27609363 87. Coyle E.F. Coggan A.R. Hemmert M. Ivy J.L. Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate J. Appl. Physiol. 1986 61 165 172 10.1152/jappl.1986.61.1.165 3525502 88. Horowitz J.F. Mora-Rodriguez R. Byerley L.O. Coyle E.F. Substrate metabolism when subjects are fed carbohydrate during exercise Am. J. Physiol.-Endocrinol. Metab. 1999 276 E828 E835 10.1152/ajpendo.1999.276.5.E828 89. Febbraio M.A. Chiu A. Angus D.J. Arkinstall M.J. Hawley J.A. Effects of carbohydrate ingestion before and during exercise on glucose kinetics and performance J. Appl. Physiol. 2000 89 2220 2226 10.1152/jappl.2000.89.6.2220 11090571 90. De Bock K. Richter E.A. Russell A. Eijnde B.O. Derave W. Ramaekers M. Koninckx E. Leger B. Verhaeghe J. Hespel P. Exercise in the fasted state facilitates fibre type-specific intramyocellular lipid breakdown and stimulates glycogen resynthesis in humans J. Physiol. 2005 564 649 660 10.1113/jphysiol.2005.083170 15705646 91. De Bock K. Derave W. Eijnde B.O. Hesselink M. Koninckx E. Rose A.J. Schrauwen P. Bonen A. Richter E.A. Hespel P. Effect of training in the fasted state on metabolic responses during exercise with carbohydrate intake J. Appl. Physiol. 2008 104 1045 1055 10.1152/japplphysiol.01195.2007 18276898 92. Chen Y.-C. Travers R.L. Walhin J.-P. Gonzalez J.T. Koumanov F. Betts J.A. Thompson D. Feeding influences adipose tissue responses to exercise in overweight men Am. J. Physiol.-Endocrinol. Metab. 2017 313 E84 E93 10.1152/ajpendo.00006.2017 28292758 93. Aird T.P. Davies R.W. Carson B.P. Effects of fasted vs. Fed state exercise on performance and post-exercise metabolism: A systematic review & meta-analysis Scand. J. Med. Sci. Sports 2018 28 1476 1493 29315892 94. Gillen J.B. Percival M.E. Ludzki A. Tarnopolsky M.A. Gibala M. Interval training in the fed or fasted state improves body composition and muscle oxidative capacity in overweight women Obesity 2013 21 2249 2255 10.1002/oby.20379 23723099 95. Stannard S.R. Buckley A.J. Edge J.A. Thompson M.W. Adaptations to skeletal muscle with endurance exercise training in the acutely fed versus overnight-fasted state J. Sci. Med. Sport 2010 13 465 469 10.1016/j.jsams.2010.03.002 20452283 96. Rynders C.A. Blanc S. DeJong N. Bessesen D.H. Bergouignan A. Sedentary behaviour is a key determinant of metabolic inflexibility J. Physiol. 2017 596 1319 1330 10.1113/JP273282 28543022 97. Civitarese A.E. Hesselink M.K. Russell A.P. Ravussin E. Schrauwen P. Glucose ingestion during exercise blunts exercise-induced gene expression of skeletal muscle fat oxidative genes Am. J. Physiol.-Endocrinol. Metab. 2005 289 E1023 E1029 10.1152/ajpendo.00193.2005 16030063 98. Akerstrom T.C. Krogh-Madsen R. Petersen A.M. Pedersen B.K. Glucose ingestion during endurance training in men attenuates expression of myokine receptor Exp. Physiol. 2009 94 1124 1131 10.1113/expphysiol.2009.048983 19592412 99. Gejl K.D. Thams L.B. Hansen M. Rokkedal-Lausch T. Plomgaard P. Nybo L. Larsen F.J. Cardinale D.A. Jensen K. Holmberg H.-C. No superior adaptations to carbohydrate periodization in elite endurance athletes Med. Sci. Sports Exerc. 2017 49 2486 2497 10.1249/MSS.0000000000001377 28723843 100. Costa R.J. Miall A. Khoo A. Rauch C. Snipe R. Camões-Costa V. Gibson P. Gut-training: The impact of two weeks repetitive gut-challenge during exercise on gastrointestinal status, glucose availability, fuel kinetics, and running performance Appl. Physiol. Nutr. Metab. 2017 42 547 557 10.1139/apnm-2016-0453 28177715 101. Cox G.R. Clark S.A. Cox A.J. Halson S.L. Hargreaves M. Hawley J.A. Jeacocke N. Snow R.J. Yeo W.K. Burke L.M. Daily training with high carbohydrate availability increases exogenous carbohydrate oxidation during endurance cycling J. Appl. Physiol. 2010 109 126 134 10.1152/japplphysiol.00950.2009 20466803 102. Impey S.G. Smith D. Robinson A.L. Owens D.J. Bartlett J.D. Smith K. Limb M. Tang J. Fraser W.D. Close G.L. Leucine-enriched protein feeding does not impair exercise-induced free fatty acid availability and lipid oxidation: Beneficial implications for training in carbohydrate-restricted states Amino Acids 2015 47 407 416 10.1007/s00726-014-1876-y 25471599 103. Howarth K.R. Phillips S.M. MacDonald M.J. Richards D. Moreau N.A. Gibala M.J. Effect of glycogen availability on human skeletal muscle protein turnover during exercise and recovery J. Appl. Physiol. 2010 109 431 438 10.1152/japplphysiol.00108.2009 20489032 104. Wagenmakers A. Beckers E. Brouns F. Kuipers H. Soeters P.B. Van Der Vusse G. Saris W. Carbohydrate supplementation, glycogen depletion, and amino acid metabolism during exercise Am. J. Physiol.-Endocrinol. Metab. 1991 260 E883 E890 10.1152/ajpendo.1991.260.6.E883 2058665 105. Khan S. Khan A. Khan S. Khan M. Khan S. Perception of athletes about diet and its role in maintenance of sports performance J. Nutr. Food Sci. 2017 7 592 10.4172/2155-9600.1000592 106. Burkhart S.J. Pelly F.E. Dietary intake of athletes seeking nutrition advice at a major international competition Nutrients 2016 8 638 10.3390/nu8100638 27754418 107. Devlin B.L. Belski R. Exploring general and sports nutrition and food knowledge in elite male australian athletes Int. J. Sport Nutr. Exerc. Metab. 2015 25 225 232 10.1123/ijsnem.2013-0259 25387042 108. Devlin B.L. Leveritt M.D. Kingsley M. Belski R. Dietary intake, body composition, and nutrition knowledge of australian football and soccer players: Implications for sports nutrition professionals in practice Int. J. Sport Nutr. Exerc. Metab. 2017 27 130 138 10.1123/ijsnem.2016-0191 27710165 109. Fleming J.A. Naughton R.J. Harper L.D. Investigating the nutritional and recovery habits of tennis players Nutrients 2018 10 443 10.3390/nu10040443 29614035 110. Thomas D.T. Erdman K.A. Burke L.M. American college of sports medicine joint position statement. Nutrition and athletic performance Med. Sci. Sports Exerc. 2016 48 543 568 26891166 111. Garcia-Roves P.M. Terrados N. Fernandez S. Patterson A.M. Comparison of dietary intake and eating behavior of professional road cyclists during training and competition Int. J. Sport Nutr. Exerc. Metab. 2000 10 82 98 10.1123/ijsnem.10.1.82 10939878 112. Martin M.K. Martin D.T. Collier G.R. Burke L.M. Voluntary food intake by elite female cyclists during training and racing: Influence of daily energy expenditure and body composition Int. J. Sport Nutr. Exerc. Metab. 2002 12 249 267 10.1123/ijsnem.12.3.249 12432171 113. Vogt S. Heinrich L. Schumacher Y.O. Grosshauser M. Blum A. Konig D. Berg A. Schmid A. Energy intake and energy expenditure of elite cyclists during preseason training Int. J. Sports Med. 2005 26 701 706 10.1055/s-2004-830438 16158378 114. Stellingwerff T. Case study: Nutrition and training periodization in three elite marathon runners Int. J. Sport Nutr. Exerc. Metab. 2012 22 392 400 10.1123/ijsnem.22.5.392 23011657 115. Heikura I.A. Stellingwerff T. Mero A.A. Uusitalo A.L.T. Burke L.M. A mismatch between athlete practice and current sports nutrition guidelines among elite female and male middle-and long-distance athletes Int. J. Sport Nutr. Exerc. Metab. 2017 27 351 360 10.1123/ijsnem.2016-0316 28338358 116. Heikura I.A. Burke L.M. Mero A.A. Uusitalo A.L.T. Stellingwerff T. Dietary microperiodization in elite female and male runners and race walkers during a block of high intensity precompetition training Int. J. Sport Nutr. Exerc. Metab. 2017 27 297 304 10.1123/ijsnem.2016-0317 28387576