Last updated: 3 October 2026
Reviewed by Jordan Morley, FA Level 2 Football Coach
The ketogenic diet restricts carbohydrates to under 50g per day (often 20-30g), forcing the liver to produce ketone bodies from fat as an alternative fuel. Originally developed in the 1920s for drug-resistant epilepsy (where it remains a clinical treatment), it has been widely adopted for weight loss. Initial weight loss is fast (partly water weight from glycogen depletion). Long-term data beyond 2 years is limited; concerns include LDL cholesterol elevation, kidney stress and nutrient deficiencies. Type 2 diabetes management under medical supervision is a well-supported use case.
- Under 50g carbohydrates per day
- Liver produces ketones from fat
- Medical use: epilepsy, diabetes management
- Limited long-term safety data beyond 2 years
In practice
A ketogenic diet (typically under 50g of carbohydrate per day) shifts the body into producing ketones from fat as its primary fuel source instead of glucose — some ultra-endurance athletes experiment with it to preserve glycogen during very long events, but most sports nutritionists caution it generally reduces performance at higher exercise intensities, where the body depends heavily on fast-access carbohydrate fuel.
FAQ
Does a ketogenic diet help athletic performance?
Evidence is mixed — it may offer some benefit for very long, lower-intensity ultra-endurance efforts by preserving glycogen, but most research shows it reduces performance at higher intensities where carbohydrate is the preferred fuel.
How long does it take to adapt to a ketogenic diet?
Most people need 2-4 weeks of consistent very-low-carbohydrate eating to become reasonably 'fat-adapted,' though full adaptation and any associated performance changes vary significantly between individuals.