Is there scientific evidence that low-carb diets enhance athletic and mental performance?

Question

Contrary to the recent past, when high carb - low fat diets were recommended, it appears that now the opposite type diet is often being recommended. Many doctors who have written books on the subject recommend diets high in fat and extremely low in carbohydrates, limiting them at 10% of caloric intake.

It is unclear how pragmatically one could live on just 10% carbohydrate. The latter represents the most efficient source of energy for the body. Athletic performance, endurance would suffer with only 10% carbohydrate.

I gather the idea of the high fat diet is to change the metabolic state of the body in a state of ketosis whereby given the absence of readily available carbohydrate fuel, the body learns how to use and burn fat for energy. Is this process all that healthy and sustainable over the long term? And, would it enhance cognitive capabilities over the long term as one ages.

Answer 1

The question is a quite broad since you don't state whether you mean a ketogenic diet, levels of protein and fats, or what you mean by "athletic and mental performance" so I will answer quite generally.

There is some anecdotal limited evidence that shows unhindered athletic performance on a Low Carb diet, such as observations made on Innuit people prior to dramatic changes in their diets. However the bulk of the literature shows impaired athletic performance, particularly in anaerobic events (eg sprints, lifting, field athletics), and no change in endurance events. Most studies also reported a 1-2 week diet adaptation time, during which most performance indicators suffered:

Burke, Louise M. "Re-examining high-fat diets for sports performance: did we call the ‘nail in the coffin’too soon?." Sports Medicine 45.1 (2015): 33-49.

Hawley, John A., and Jill J. Leckey. "Carbohydrate dependence during prolonged, intense endurance exercise." Sports Medicine 45.1 (2015): 5-12.

As for brain function, a recent review article by Koppel (2017) looked at over 50 studies and found that people suffering from neurological conditions such as epilepsy, and more recently, Alzheimer’s benefited from Low Carb, high fat diets (in particular improved cognition). Care should be taken if extrapolating these results to otherwise healthy individuals and/or athletes.

Koppel, Scott J., and Russell H. Swerdlow. "Neuroketotherapeutics: A modern review of a century-old therapy." Neurochemistry international (2017).