How Do Low Carb Diets Affect Performance?

In previous articles we have seen how crucial appropriate carbohydrate intake is for optimal performance. Another way of demonstrating this is by looking at the opposite situation. In other words, how low carbohydrate diets are linked to sub-optimal performance. Multiple studies show that low carbohydrate diets are linked to low levels of muscle fuel, and this, in turn, is linked to fatigue and decreases in performance1. Studies also show that low fuel levels may also cause ‘overtraining’: a situation where the athlete trains more than his or her body can recover from, to the point where performance declines.

Low carbohydrate intake and replenishment can be a serious issue for many high-performance athletes who may find it difficult to even keep up with carbohydrate requirements. This situation is made even worse if their knowledge of the importance of carbohydrates is lacking.

The Important Take Home Message

When fuel levels are depleted in this way, muscles start to use their own protein and amino acids to produce glucose. Since protein and amino acids are the building blocks of all muscle, by using them for energy purposes, the muscle is, essentially, ‘eating itself to feed itself’. This muscle breakdown process (called ‘catabolism’) is a kind of muscle damage that has been shown to interfere with glycogen storage, When this happens it is difficult for muscles to store and maintain glycogen, even with a high carbohydrate diet. Low fuel storage that triggers ‘catabolic’ breakdown of muscle in this way, has been proposed as the number one cause of overtraining in athletes2.


  • Costill DL, Hargreaves M. Carbohydrate nutrition and fatigue. Sports Medicine 1992 Feb;13(2):86-92.
  • San Millán, I. The Importance of Carbohydrates and Glycogen for Athletes. Training Peaks:, 2013.