Endurance athletes of all levels face the same question — what should I eat before exercise? A surge in sport nutrition research and practitioners over the past 25 years has led to increased awareness around the role of nutrition in sports performance. However, limited data are available on the day-to-day practices of endurance athletes, how well they reflect evidence- based recommendations, or what dictates those choices. Furthermore, important questions relating to the effects of pre-exercise nutrition intake on performance, fat oxidation, hunger, and molecular signaling remain unanswered. Therefore, the purpose of this thesis was to better understand the pre-exercise nutrition beliefs and practices of endurance athletes by investigating: (i) stated beliefs and practices of athletes, (ii) scientific validity of common beliefs reported by athletes, and (iii) actual pre-exercise nutrition practices of athletes.

The first chapter of the thesis was an international survey of ~2,000 endurance athletes aimed at understanding the beliefs and self-reported pre-exercise nutrition practices of endurance athletes. Nearly two-thirds of athletes reported the use of fasted-state training, and it was observed that many athletes may not be following best-practice recommendations of varying dietary intake in relation to training sessions to optimize endurance training adaptations. Conflicting beliefs relating to the effects of fasted training on metabolism and exercise capacity were also found among athletes, often aligned with differences in competitive level, sex, and/or habitual dietary pattern.

In response to the discordant beliefs observed among athletes, the next three studies aimed to elucidate the metabolic, molecular, and performance effects of pre-exercise nutrition choices. In an acute cross-over study of 17 trained male cyclists (Chapter 5), a carbohydrate-rich breakfast reduced fat oxidation compared to exercising both in the overnight-fasted state and following pre-exercise protein ingestion, but there were no differences between the three trials for average power during high-intensity intervals, perceived exertion, oxidative stress, or hunger. These findings suggest that a low-carbohydrate breakfast can be a viable alternative to fasted-state training for athletes who wish to optimize fat oxidation during exercise, and that athletes can complete ~1-hr of exercise in the overnight-fasted state without compromising high-intensity training capacity or experiencing additional hunger. The next two chapters utilized multivariable regression techniques on data pooled from published studies to investigate factors influencing skeletal muscle AMP-activated protein kinase (AMPK) signaling (Chapter 6) and substrate oxidation during cycling exercise (Chapter 7). Pre- and peri-exercise carbohydrate intake had negligible influence on AMPK activation during exercise, whereas disrupting cellular energy charge had a large influence, suggesting high-intensity exercise may drive an adaptative response irrespective of nutritional factors. Chapter 7 reports factors commonly known to influence substrate oxidation during exercise such as exercise duration and intensity, age, sex, fitness level, muscle glycogen, and daily dietary intake explained only ~59% of the variation in respiratory exchange ratio (RER) during exercise. In addition, factors that could be easily modified by athletes such as exercise duration and intensity, daily macronutrient intake, and pre- and peri-exercise carbohydrate intake, only explained 36% of the variation in RER during exercise suggesting most of what dictates RER during exercise cannot be easily controlled on a daily basis.

The final study of the thesis monitored the daily diet, training, and sleep habits of 55 endurance athletes throughout 12 weeks of training with focus on the relationship between carbohydrate intake and training (Chapter 8), and the influence of prior-day carbohydrate intake on subjective recovery status (Chapter 9). It was found that many endurance athletes do not follow recommended practices of adjusting daily carbohydrate intake, or if they do, the magnitude of adjustment is small relative to changes in training volume and/or intensity (Chapter 8). Chapter 9 demonstrated total daily carbohydrate intake has minimal influence on subjective recovery scores the following day, after accounting for other factors related to training load and sleep. The data also displayed nonergodicity, an important phenomenon where group-level findings cannot be generalized to individuals, and a novel method was used to predict how an individual athlete would respond to carbohydrate intake.

Collectively, the studies in this thesis demonstrate that endurance athletes differ greatly in their beliefs and practices related to pre-exercise nutrition, and in many cases do not follow contemporary evidence-based recommendations. Novel findings include: 1) 63% endurance athletes report the use of fasted-state training, 2) beliefs and practices relating to pre-exercise nutrition differ based on sex, competitive level, and habitual dietary pattern, 3) the pre-exercise meal has minimal influence on work capacity, perceived exertion, oxidative stress, hunger, or AMPK activation during exercise, 4) although pre-exercise carbohydrate ingestion increases carbohydrate oxidation during exercise, most of what dictates substrate oxidation during exercise cannot be easily controlled on a daily basis, and 5) for most individuals, daily carbohydrate intake does not influence recovery status the following morning. These findings contribute to the sport nutrition literature and can be used by nutritionists, coaches, and athletes to make better-informed pre-exercise fueling choices.