In this study of elite ultra-marathoners and Ironman triathletes, Jeff Volek and colleagues compared athletes habitually consuming either a high-carbohydrate or long-term low-carbohydrate ketogenic diet and found that keto-adapted athletes exhibited more than double the peak fat oxidation rates during exercise, relied predominantly on fat even at higher intensities, and maintained similar resting and post-exercise muscle glycogen levels despite consuming far less carbohydrate. During a 3-hour run at ~64% VO₂max, the low-carb group derived ~88% of their energy from fat yet showed no impairment in glycogen depletion or repletion compared to high-carb athletes, suggesting that chronic keto-adaptation substantially enhances fat utilization while preserving glycogen dynamics. This challenges the belief that high carbohydrate intake is required for endurance performance.