For decades, athletes have been told that carbohydrate loading is essential for optimal performance. The logic seems straightforward: muscles store carbohydrate as glycogen, and glycogen is used to fuel exercise; therefore, having more glycogen stored in the muscles should improve performance.
However, recent research demonstrates that human metabolism is more adaptable and more complex than this.
Fueling Performance: More Than Just Carbohydrates
The body can use both glucose and fat as fuel. In athletes who follow high-carbohydrate diets, performance at moderate to high intensities relies primarily on glucose, which is stored as glycogen in muscles. However, athletes adapted to low-carbohydrate or ketogenic diets develop a substantially greater capacity to oxidize fat.
Studies by Volek et al. have shown that fat-adapted endurance athletes can oxidize fat at rates approximately twice those observed in non–fat-adapted athletes, allowing them to rely less on glycogen during prolonged exercise. Importantly, this does not mean that glycogen stores are entirely depleted. Even on very low-carbohydrate diets, muscle glycogen remains at functional levels and can be replenished through gluconeogenesis from amino acids, lactate, and glycerol.
Does Low-Carb Hinder Performance?
Recent meta-analyses of controlled trials in trained athletes have found that well-formulated low-carbohydrate and ketogenic diets generally preserve maximal aerobic capacity and time-to-exhaustion performance after adaptation, while consistently increasing fat oxidation.
For very high-intensity, anaerobic efforts – such as sprinting or repeated maximal bursts – carbohydrate availability may play a larger role. However, the mechanisms underlying fatigue are more complex than glycogen depletion alone.
In their comprehensive 2026 analysis, Noakes et al. re-examined the long-standing assumption that endurance fatigue is primarily caused by muscle glycogen depletion or falling blood glucose, often referred to as exercise-induced hypoglycemia (EIH).
Their analysis indicates that:
- Blood glucose is tightly regulated during exercise through increased hepatic glucose production, even in low-carbohydrate–adapted athletes.
- Fatigue does not consistently coincide with low blood glucose concentrations.
- Performance limitation during prolonged exercise appears to be multifactorial and centrally regulated rather than determined solely by glycogen depletion.
When the authors analyzed carbohydrate supplementation trials, they found that performance often improved when carbohydrate was consumed compared with placebo. However, they found little evidence of a consistent dose-response relationship at higher intakes. In prolonged exercise lasting more than 2 to 3 hours, relatively small amounts of carbohydrate – approximately 10–30 grams per hour – appear sufficient in many cases to prevent or delay EIH in individuals who cannot adequately increase hepatic glucose production. Larger intakes, such as the commonly recommended 60–90 grams per hour, did not consistently produce additional performance benefits across studies.
Taken together, these findings suggest that one important physiological role of carbohydrate ingestion during prolonged exercise may be maintaining blood glucose stability rather than simply maximizing glycogen availability. Moreover, they challenge the assumption that high carbohydrate intake is universally required for athletic performance. Carbohydrates may still be useful in certain contexts, particularly during very prolonged events, but they are not physiologically mandatory for all athletes.
Metabolic Health and Individual Response
Metabolic health also plays an important role in determining optimal fuel intake during exercise. Some athletes may experience reactive hypoglycemia or energy crashes after consuming large carbohydrate loads, particularly if insulin sensitivity is impaired. In these cases, high carbohydrate intake may destabilize energy levels rather than improve performance.
Improving metabolic flexibility – the ability to efficiently switch between fat and glucose – may support both steady energy production and long-term health.
Some athletes may choose to strategically increase carbohydrate intake for specific high-intensity events, which could be appropriate. But that is very different from assuming all athletes must routinely carb-load.
An Individualized Approach to Fueling
Carbohydrate loading became standard practice when glycogen depletion was viewed as the primary cause of endurance fatigue. However, research now shows that exercise metabolism is far more adaptable than once believed. Indeed, carbohydrate needs during physical activity vary widely among athletes and situations. While strategic carbohydrate intake may benefit some athletes in specific contexts, others – particularly those well adapted to burning fat – may perform equally well without relying on traditional carb-loading strategies.
