New study says fasted workouts don’t improve endurance performance

Most nutrition advice for distance runners encourages the consumption of a high-carbohydrate diet to improve performance, but since the 1980s, there has been some research suggesting that exercising in a low-carbohydrate or fasted state might provide some benefits to endurance athletes. Other studies, however, have shown that restricting carbohydrates does nothing but diminish performance — so what’s the right answer? Recently, a group of researchers reviewed the available literature on the topic and concluded that the drawbacks of fasted workouts outweigh any potential benefits.

LCHF diet may compromise performance in distance runners

The argument for fasted workouts

Proponents of training in a fasted (or carbohydrate-depleted) state argue that by running when your body’s glycogen stores are depleted, you can shift your body toward a higher reliance on fat as a fuel source during exercise. Since the amount of energy you have stored as fat is vastly greater than any other fuel source, teaching your body to use it more efficiently during exercise should, theoretically, improve performance.

Some studies have also suggested that exercising in a carbohydrate-depleted state could help increase mitochondrial biogenesis. Mitochondria are the small organelles found in your cells that are responsible for energy production, so having more of them in your cells should improve physical performance.

The cons outweigh the pros

The researchers examined all of the available literature studying the effects of fasted workouts and carbohydrate-depleted workouts, and concluded that periodized carbohydrate restriction does not enhance performance in endurance athletes. More specifically, restricting carbohydrates before or after workouts, even when only done for short periods of time, compromised athletes’ training quality and made them unable to train at high intensities during interval sessions.

“The physiological stimuli prompted by undertaking an acute exercise bout with low CHO availability does not translate into clear measurable enhancements of performance in already adapted endurance-trained athletes compared to training with high CHO availability,” the researchers concluded.

The researchers point out that including carbohydrate restriction during training will also likely affect other important parts of training, which detracts from any potential benefits an athlete could get from periodized carbohydrate restriction. In other words, prioritizing the quality of your training in order to see training adaptations is going to take you much further than implementing fasted workouts or periodized carbohydrate restriction.

Debunking the most common endurance sport nutrition myths

The bottom line

While there is some research suggesting periodized carbohydrate restriction can be beneficial to endurance athletes, the majority of studies say the cons outweigh the pros. For most runners, a sound nutrition strategy that includes plenty of carbohydrates is the best way to improve performance and prevent injuries.