What Should I Eat before Exercise? Pre-Exercise Nutrition and the Response to Endurance Exercise: Current Prospective and Future Directions

The primary variables in uencing the adaptive response to a bout of endurance training are exercise duration and exercise intensity. However, altering the availability of nutrients before and during exercise can also impact the training response by modulating the exercise stimulus and/or the physiological and molecular responses to the exercise-induced perturbations. The purpose of this review is to highlight the current knowledge of the in uence of pre-exercise nutrition ingestion on the metabolic, physiological, and performance responses to endurance training and suggest directions for future research. Acutely, carbohydrate ingestion reduces fat oxidation, but there is little evidence showing enhanced fat burning capacity following long-term fasted-state training. Performance is improved following pre-exercise carbohydrate ingestion for longer but not shorter duration exercise, while training-induced performance improvements following nutrition strategies that modulate carbohydrate availability vary based on the type of nutrition protocol used. Contrasting ndings related to the in uence of acute carbohydrate ingestion on mitochondrial signaling may be related to the amount of carbohydrate consumed and the intensity of exercise. This review can help to guide athletes, coaches, and nutritionists in personalizing pre-exercise nutrition strategies, and for designing research studies to further elucidate the role of nutrition in endurance training adaptations. Keywords:cycling; running; carbohydrate; adaptations; periodization; fasting 1. Introduction From Olympians to recreational exercisers, athletes of all levels face the same questions—what should I eat before exercise, and how does it a ect my training? Despite being relevant to anyone performing exercise, many questions relating to the e ects of nutritional intake on endurance training responses and adaptations remain unanswered. The duration and intensity of exercise are the most important factors in uencing the adaptive response to endurance training [1]. However, strategies altering

I eat before exercise, and how does it a ect my training? Despite being relevant to anyone performing exercise, many questions relating to the e ects of nutritional intake on endurance training responses and adaptations remain unanswered. The duration and intensity of exercise are the most important factors in uencing the adaptive response to endurance training [1]. However, strategies altering nutrient availability before and during exercise can also impact training adaptations by modulating the exercise stimulus and/or cellular responses to the exercise-induced perturbations [2]. Speci c strategies to alter nutrient availability can include exercising in the overnight-fasted state, restricting carbohydrate (CHO) ingestion between training sessions, and increasing CHO ingestion before or during exercise [3]. Although performance may be improved following pre-exercise CHO ingestion [4,5], exercise undertaken with reduced availability of CHO can increase the activation of key signaling proteins compared with exercise performed with high CHO availability [6], potentially in uencing longer-term training adaptations. Among the intracellular signals comprising the endurance training response are mechanical stretch, reactive oxygen and nitrogen species (RONS), calcium ux, AMP:ATP ratio, and the availability Nutrients2020,12, 3473; doi:10.3390 /nu12113473 /journal/nutrients

Nutrients2020,12, 3473 2 of 23 of endogenous CHO and free fatty acids (FFA) [7,8]. These signals are a ected by both the duration and intensity of an exercise session, and by the pre-exercise nutrition choices of an athlete (i.e., the size, type, and timing of the pre-exercise meal(s), Figure). Although some lines of evidence suggest ingesting CHO before exercise can negatively in uence endurance training adaptations, contrasting ndings have been reported. For example, ingesting CHO has decreased [9], increased [10], or had no e ect [11] on the activity of the 5 0 AMP-activated protein kinase (AMPK) following exercise. Similarly, training-induced improvements in maximal oxygen consumption (VO2max) have been reported to increase [12], decrease [13], or remain unchanged [14] following 4–6 weeks of CHO-fed compared with fasted-state training. These contrasting ndings can be a source of confusion and may explain why the beliefs and practices relating to the role and in uence of pre-exercise nutrition vary so widely among coaches and athletes [15,16]. Accordingly, the purpose of this review is to highlight the current knowledge of the in uence of pre-exercise nutrition ingestion on the metabolic, physiological, and performance responses to endurance training. We also highlight areas for practitioners where evidence is lacking, particularly regarding trained athletes, and suggest directions for future research.Nutrients 2020, 12, x FOR PEER REVIEW 2 of 24 Among the intracellular signals comprising the endurance training response are mechanical stretch, reactive oxygen and nitrogen species (RONS), calcium flux, AMP:ATP ratio, and the availability of endogenous CHO and free fatty acids (FFA) [7,8]. These signals are affected by both the duration and intensity of an exercise session, and by the pre-exercise nutrition choices of an athlete (i.e., the size, type, and timing of the pre-exercise meal(s), Figure 1). Although some lines of evidence suggest ingesting CHO before exercise can negatively influence endurance training adaptations, contrasting findings have been reported. For example, ingesting CHO has decreased [9], increased [10], or had no effect [11] on the activity of the 5′ AMP-activated protein kinase (AMPK) following exercise. Similarly, training-induced improvements in maximal oxygen consumption (VO 2max) have been reported

1). Although some lines of evidence suggest ingesting CHO before exercise can negatively influence endurance training adaptations, contrasting findings have been reported. For example, ingesting CHO has decreased [9], increased [10], or had no effect [11] on the activity of the 5′ AMP-activated protein kinase (AMPK) following exercise. Similarly, training-induced improvements in maximal oxygen consumption (VO 2max) have been reported to increase [12], decrease [13], or remain unchanged [14] following 4–6 weeks of CHO-fed compared with fasted-state training. These contrasting findings can be a source of confusion and may explain why the beliefs and practices relating to the role and influence of pre- exercise nutrition vary so widely among coaches and athletes [15,16]. Accordingly, the purpose of this review is to highlight the current knowledge of the influence of pre-exercise nutrition ingestion on the metabolic, physiological, and performance responses to endurance training. We also highlight areas for practitioners where evidence is lacking, particularly regarding trained athletes, and suggest directions for future research. Figure 1. Schematic of areas where pre-exercise nutrition has the potential to impact the adaptive responses to endurance training. Green arrows suggest the potential to increase or augment specific signaling, and red dashed arrows suggest the potential to decrease or impair specific signaling. Abbreviations: AMPK, AMP-activated protein kinase; CaMK, calcium/calmodulin-stimulated protein kinase; CHO, carbohydrate; FFA, free fatty acids; LCHF, low-CHO high-fat; MAPK, mitogen- activated protein kinase; VO 2max, maximal oxygen consumption. 2. Acute Responses to Pre-Exercise Nutrition Intake The vast majority of pre-exercise nutrition interventions have been conducted in an acute context. Although acute responses to training do not always correspond with long-term adaptations [17,18], the accumulation over time of transient, exercise-induced changes in gene expression are thought to be the driving factor behind many adaptations to training [19]. Therefore, it is relevant to consider the acute effects of pre-exercise nutrition in addition to the longer-term adaptations. Figure 1. Schematic of areas where pre-exercise nutrition has the potential to impact the adaptive responses to endurance training. Green arrows suggest the potential to increase or augment speci c signaling, and red dashed arrows suggest the potential to decrease or

[19]. Therefore, it is relevant to consider the acute effects of pre-exercise nutrition in addition to the longer-term adaptations. Figure 1. Schematic of areas where pre-exercise nutrition has the potential to impact the adaptive responses to endurance training. Green arrows suggest the potential to increase or augment speci c signaling, and red dashed arrows suggest the potential to decrease or impair speci c signaling. Abbreviations: AMPK, AMP-activated protein kinase; CaMK, calcium/calmodulin-stimulated protein kinase; CHO, carbohydrate; FFA, free fatty acids; LCHF, low-CHO high-fat; MAPK, mitogen-activated protein kinase; VO 2max, maximal oxygen consumption. 2. Acute Responses to Pre-Exercise Nutrition Intake The vast majority of pre-exercise nutrition interventions have been conducted in an acute context. Although acute responses to training do not always correspond with long-term adaptations [17,18], the accumulation over time of transient, exercise-induced changes in gene expression are thought to be the driving factor behind many adaptations to training [19]. Therefore, it is relevant to consider the acute e ects of pre-exercise nutrition in addition to the longer-term adaptations. 2.1. Metabolism and Substrate Oxidation The liver plays a key role in metabolic regulation during extended exercise [20]. Despite the ~40% reduction in liver glycogen following an overnight fast [21], blood glucose concentration can

Nutrients2020,12, 3473 3 of 23 be maintained at normal levels during exercise due to increased gluconeogenesis and/or decreased utilization of glucose in skeletal muscle [22,23]. However, fatigue during extended exercise is often associated with reduced blood glucose concentrations [24], supporting a critical role for liver glycogen in achieving optimal performance during extended exercise. Exercising in the fasted-state generally allows higher levels of fat oxidation than exercise performed in the CHO-fed state during low-to-moderate intensity exercise [25] and can increase the relative intensity where maximal fat oxidation occurs [26]. Ingesting CHO before exercise increases plasma glucose and insulin levels, leading to a reduction in hepatic glucose output and an increase in skeletal muscle glucose uptake during exercise [27]. This can lower fat oxidation by decreasing plasma FFA availability via insulin-mediated inhibition of lipolysis [28], and also by inhibiting fat oxidation within the muscle due to an increased glycolytic ux [29]. Intramuscular triglycerides (IMTG) provide a key substrate for fat oxidation, primarily during exercise in the fasted state [30,31], although their use declines as the duration of exercise extends, while the oxidation of plasma FFA increases [32]. Up to 6 h may be required following a CHO-rich meal for substrate oxidation and glucose homeostasis to return to levels observed during fasted-state exercise [33]. In contrast with exercise performed in the overnight-fasted state, which lowers hepatic but not muscle glycogen [34], restricting CHO between training sessions allows exercise to be undertaken with reduced muscle glycogen concentrations [35]. During exercise with low muscle glycogen there is an increase in the oxidation of fat [36,37] and amino acids [38,39], and a reduction in muscle glycogen breakdown [36,40,41]. During exercise undertaken with normal muscle glycogen levels, muscle glycogen breakdown is similar between fed and fasted-state exercise [31,42–44] and may be reduced when ingesting CHO during exercise [45]. The majority of research looking at fat oxidation has compared CHO to a placebo, but the use of pre-exercise protein ingestion represents an interesting and under-researched area. Consumption of protein before and during steady-state exercise did not affect FFA availability or whole body fat oxidation compared with fasted-state exercise

[31,42–44] and may be reduced when ingesting CHO during exercise [45]. The majority of research looking at fat oxidation has compared CHO to a placebo, but the use of pre-exercise protein ingestion represents an interesting and under-researched area. Consumption of protein before and during steady-state exercise did not affect FFA availability or whole body fat oxidation compared with fasted-state exercise commenced with normal [46] or lowered [47] muscle glycogen concentration, despite elevated insulin levels. This may be related to the increases in catecholamine levels during exercise, which are an important determinant of the adipose tissue lipolytic rate and can override the inhibition by insulin [48]. Although protein ingestion before exercising in a low-glycogen state has no effect on rates of muscle protein synthesis, it is plausible that it could reduce muscle protein breakdown during exercise [49]. It also appears possible that pre-exercise protein ingestion increases amino acid oxidation during exercise [ To compare the in uence of pre-exercise CHO ingestion, muscle glycogen levels, and glycemic index on substrate oxidation and AMPK activity, we pooled the results of 125 studies (available as supplementary online les) that included the relevant intervention groups (Figures–7). Together, these studies included 1245 subjects (12.8% female), with an average age, BMI, and VO2maxof 25.4 3.1 years, 23.2 1.4 kg m 2 , and 56.7 8.2 mL kg 1 min 1 . Linear correlation analysis was used to calculate the correlation coe cient between variables, according to Pearson's product moment (r) using R statistical software. Pooled data are reported as mean SD, with the level of statistical signi cance set atp<0.05. 2.1.1. E ect of Exercise Duration The respiratory exchange ratio (RER—a measure of substrate oxidation) decreases with exercise duration, indicating an increasing reliance on fat oxidation as the duration of exercise extends [50]. Di erences in RER between exercising in the fed vs. fasted state and following low vs. high glycemic index CHO remain largely similar throughout exercise, while the di erences in RER between high and low starting muscle glycogen decrease as exercise duration extends (Figure). The latter could presumably be related to the greater utilization

the duration of exercise extends [50]. Di erences in RER between exercising in the fed vs. fasted state and following low vs. high glycemic index CHO remain largely similar throughout exercise, while the di erences in RER between high and low starting muscle glycogen decrease as exercise duration extends (Figure). The latter could presumably be related to the greater utilization of muscle glycogen during exercise undertaken with higher levels of glycogen, leading to more similar levels during the later stages of exercise. This idea is supported by the pooled data, which show a strong correlation (r=0.89,p<0.001) between the di erences in pre-exercise glycogen levels and di erences in RER during exercise (Figure).

Nutrients2020,12, 3473 4 of 23Nutrients 2020, 12, x FOR PEER REVIEW 4 of 24 Figure 2. Substrate oxidation in relation to exercise duration for studies reporting respiratory exchange ratio (RER) at multiple time points comparing overnight-fasted and/or CHO-fed exercise with normal muscle glycogen levels (A), exercise undertaken with high (471 ± 208 mmol.kg −1 dry mass) and low (232 ± 112 mmol.kg −1 dry mass) muscle glycogen levels (B), and following high (82 ± 10) and low (36 ± 9) glycemic index meals (C). Shaded areas represent 95% confidence intervals. Data were obtained by pooling results from 60 studies (see supplementary files for references). Figure 3. Correlation between differences in respiratory exchange ratio (RER) during exercise and differences in pre-exercise glycogen levels. Shaded area represents 95% confidence intervals. Data were obtained by pooling results from 13 studies that manipulated glycogen levels and reported RER for high- and low-glycogen trials (see supplementary files for references). DM = dry mass. Figure 2. Substrate oxidation in relation to exercise duration for studies reporting respiratory exchange ratio (RER) at multiple time points comparing overnight-fasted and/or CHO-fed exercise with normal muscle glycogen levels (A), exercise undertaken with high (471 208 mmol kg 1 dry mass) and low (232 112 mmol kg 1 dry mass) muscle glycogen levels (B), and following high (82 10) and low (36 9) glycemic index meals (C). Shaded areas represent 95% con dence intervals. Data were obtained by pooling results from 60 studies (see supplementary les for references).Nutrients 2020, 12, x FOR PEER REVIEW 4 of 24 Figure 2. Substrate oxidation in relation to exercise duration for studies reporting respiratory exchange ratio (RER) at multiple time points comparing overnight-fasted and/or CHO-fed exercise with normal muscle glycogen levels (A), exercise undertaken with high (471 ± 208 mmol.kg −1 dry mass) and low (232 ± 112 mmol.kg −1 dry mass) muscle glycogen levels (B), and following high (82 ± 10) and low (36 ± 9) glycemic index meals (C). Shaded areas represent 95% confidence intervals. Data were obtained by pooling results from 60 studies (see supplementary files for references). Figure 3. Correlation between

high (471 ± 208 mmol.kg −1 dry mass) and low (232 ± 112 mmol.kg −1 dry mass) muscle glycogen levels (B), and following high (82 ± 10) and low (36 ± 9) glycemic index meals (C). Shaded areas represent 95% confidence intervals. Data were obtained by pooling results from 60 studies (see supplementary files for references). Figure 3. Correlation between differences in respiratory exchange ratio (RER) during exercise and differences in pre-exercise glycogen levels. Shaded area represents 95% confidence intervals. Data were obtained by pooling results from 13 studies that manipulated glycogen levels and reported RER for high- and low-glycogen trials (see supplementary files for references). DM = dry mass. Figure 3. Correlation between di erences in respiratory exchange ratio (RER) during exercise and di erences in pre-exercise glycogen levels. Shaded area represents 95% con dence intervals. Data were obtained by pooling results from 13 studies that manipulated glycogen levels and reported RER for high- and low-glycogen trials (see supplementary les for references). DM=dry mass. 2.1.2. E ect of Exercise Intensity Exercise intensity is well-established to in uence substrate oxidation during exercise, with RER increasing with intensity [51]. Di erences in RER between fed and fasted-state exercise are larger at lower intensities and decrease as intensity increases (FigureA,B). In contrast, exercise undertaken with low muscle glycogen maintains lower RER values compared with normal glycogen, despite increasing exercise intensity (FigureC,D). The glycemic index of the pre-exercise meal appears to have minimal e ects on the relationship between intensity and substrate oxidation (FigureE,F). 2.1.3. E ect of Carbohydrate Amount Several studies have directly compared varying amounts of CHO ingested before exercise, either showing no di erences in substrate oxidation with varying amounts of pre-exercise CHO [5,52–54], or di erences throughout all [55] or portions [56,57] of the exercise bout. When pooling a number of studies together, there is a weak positive relationship between the amount of CHO ingested and RER during subsequent exercise, while di erences in RER between CHO-fed and fasted-state exercise increase as the amount of CHO ingested is increased (Figure).

or portions [56,57] of the exercise bout. When pooling a number of studies together, there is a weak positive relationship between the amount of CHO ingested and RER during subsequent exercise, while di erences in RER between CHO-fed and fasted-state exercise increase as the amount of CHO ingested is increased (Figure).

Nutrients2020,12, 3473 5 of 23Nutrients 2020, 12, x FOR PEER REVIEW 5 of 24 Figure 4. Substrate oxidation in relation to exercise intensity for studies comparing overnight-fasted and CHO-fed exercise with normal muscle glycogen levels (A,B), exercise undertaken with high (471 ± 208 mmol.kg −1 dry mass) and low (232 ± 112 mmol.kg −1 dry mass) muscle glycogen levels (C,D), and following high (82 ± 10) and low (36 ± 9) glycemic index meals (E,F). Shaded areas represent 95% confidence intervals. Data were obtained by pooling results from 103 studies (see supplementary files for references). Figure 5. Substrate oxidation in relation to amount of carbohydrate (CHO) consumed before exercise, as absolute RER value during exercise (A) and difference in RER between fed and fasted-state exercise (B). Shaded areas represent 95% confidence intervals. Data were obtained by pooling results from 76 studies (see supplementary files for references). Figure 4. Substrate oxidation in relation to exercise intensity for studies comparing overnight-fasted and CHO-fed exercise with normal muscle glycogen levels (A,B), exercise undertaken with high (471 208 mmol kg 1 dry mass) and low (232 112 mmol kg 1 dry mass) muscle glycogen levels (C,D), and following high (82 10) and low (36 9) glycemic index meals (E,F). Shaded areas represent 95% con dence intervals. Data were obtained by pooling results from 103 studies (see supplementary les for references).Nutrients 2020, 12, x FOR PEER REVIEW 5 of 24 Figure 4. Substrate oxidation in relation to exercise intensity for studies comparing overnight-fasted and CHO-fed exercise with normal muscle glycogen levels (A,B), exercise undertaken with high (471 ± 208 mmol.kg −1 dry mass) and low (232 ± 112 mmol.kg −1 dry mass) muscle glycogen levels (C,D), and following high (82 ± 10) and low (36 ± 9) glycemic index meals (E,F). Shaded areas represent 95% confidence intervals. Data were obtained by pooling results from 103 studies (see supplementary files for references). Figure 5. Substrate oxidation in relation to amount of carbohydrate (CHO) consumed before exercise, as absolute RER value during exercise (A) and difference in RER between fed and fasted-state exercise (B). Shaded areas represent

This review discusses pre-exercise nutrition's impact on endurance training adaptations.