The debate over the best diet for runners rages on: should endurance athletes embrace low-carb, high-fat strategies to become “fat-adapted” or stick to tried-and-tested carbohydrate-centred diets to fuel training and racing?
The Case for Low-Carb
Low-carbohydrate, high-fat (LCHF) diets aim to shift the body’s reliance from glycogen to fat as a primary fuel. Proponents argue that fat is virtually limitless, so “teaching” your body to burn it can improve metabolic efficiency and reduce mid-race bonks. Ultra-endurance athletes in particular sometimes use LCHF approaches to sustain energy at slower intensities over many hours.
However, going low-carb often reduces muscle glycogen, which becomes a problem when workouts require running at or above lactate threshold — the intensity where carbohydrates are king. Studies consistently show performance at high intensities suffers without sufficient glycogen, leading to slower times and impaired training quality.
The Case for Carb-Centred
In The Endurance Diet, renowned sports nutritionist Matt Fitzgerald studied the eating habits of elite endurance athletes worldwide. His findings? Across cultures and sports, the world’s best runners, cyclists, and triathletes overwhelmingly eat diets rich in carbohydrates — from Kenyan runners’ mountains of ugali, to Japanese marathoners’ bowls of rice. Fitzgerald argues it’s not coincidence but causation; high-carb diets give endurance athletes the glycogen they need to train hard and recover quickly.
Carbohydrates are stored as muscle glycogen, the fuel that powers fast running and high-intensity efforts. A carb-centred diet allows athletes to train consistently at the speeds required for performance improvements. Fitzgerald’s research shows elites typically get 55–65% of their daily calories from carbohydrates — far higher than any low-carb trend would suggest.
So, Who’s Right?
The answer may depend on your goals. If you’re aiming for personal bests in races at or above threshold intensity — like 5Ks, 10Ks, half marathons, marathons, or even hilly ultramarathons — you simply can’t afford to train or race depleted. Your legs and brain need glycogen.
On the other hand, if you’re preparing for a multi-day trek or a very low-intensity ultra where speed is irrelevant, experimenting with fat adaptation might make sense — but even then, most successful ultrarunners strategically reintroduce carbs before and during their events.
The Bottom Line
Carbs aren’t the enemy. For most endurance athletes, they’re the key to pushing hard, adapting to training, and racing your best. As Matt Fitzgerald famously summed up in The Endurance Diet:
“If you want to perform like the best endurance athletes in the world, you need to eat like them.”