Is keto or 'fat adaptation' good for endurance?
No. Keto diets raise fat burning, but in controlled trials they made elite athletes 5–8% less economical at race speeds, and most were still slower after going back to carbohydrate. Deliberately training low on carbohydrate showed no performance benefit and ketone drinks don't help, so fuel your hard sessions and your race with carbohydrate.
What to do
- Don't follow a keto or low-carbohydrate diet to improve marathon performance.
- Fuel your intervals, marathon-pace sessions and runs of 90 minutes or more with carbohydrate.
- Skip ketone drinks and supplements. They have not improved endurance performance.
- If you want to try an occasional run low on carbohydrate, keep it to one or two easy runs a week of up to about an hour. That is a cautious convention, not a performance tool.
- Don't restrict carbohydrate at all if you have had a bone stress injury, missed periods, or are eating too little.
The idea
A ketogenic (“keto”) diet is very low in carbohydrate and high in fat. The idea behind “fat adaptation” is that if your body learns to burn more fat, you will rely less on your limited carbohydrate stores and avoid hitting the wall. Older books by Noakes, Sisson and Maffetone helped popularize the approach for endurance athletes.
The first part is true: keto diets do make the body burn more fat. The question is whether that makes you faster. The evidence says it doesn’t, at least at race pace.
The Supernova trials
The best tests come from the Supernova trials in elite race walkers (Burke and colleagues, 2017; Burke and colleagues, 2021).
- The keto diet raised fat burning, as expected.
- But it worsened economy by 5–8% at race speeds. Economy is how much energy, measured as oxygen, it costs to move at a given speed. Worse economy means the same pace costs more effort.
- In one trial, athletes ate keto for just 5–6 days and then switched back to carbohydrate. Even after that restoration, 6 of 7 were still slower.
So fat adaptation is real, but at race pace it made these athletes less efficient, not faster.
Bones
A short period of carbohydrate restriction also impaired markers of bone formation. In that study it did so more than simply eating too few calories did (Fensham and colleagues, 2022).
Runners specifically
A review focused on runners found no benefit from keto diets or from ketone supplements (Sun and colleagues, 2025). Ketone esters did not help endurance performance in earlier work either (Valenzuela and colleagues, 2020). See caffeine for the one supplement that does hold up.
What about “train low”?
“Train low” is a milder idea: deliberately restricting carbohydrate around some training sessions rather than following a full keto diet. A meta-analysis found no performance benefit in trained athletes (effect size 0.17, not significant) (Gejl and Nybo, 2021).
If you still want to use it, our suggestion follows a cautious convention:
- At most one or two easy runs a week, of up to 60–75 minutes.
- Never on interval days, marathon-pace sessions, or runs where you are practising race fuelling.
- Eat a meal with carbohydrate and protein within 1–2 hours afterwards.
- Don’t use it at all if you have had a bone stress injury, have missed periods or other signs of low energy availability, are a teenager, or are trying to lose weight. See the menstrual cycle page for the warning signs.
Train high for quality
The flip side is simple. Fuel the sessions that matter: intervals, marathon-pace work and runs of 90 minutes or more (Gejl and Nybo, 2021; Burke and colleagues, 2017). That is also where you practise race fuelling, which is the nutrition strategy that has actually been tested in a marathon trial (see carbs per hour). Before the race, carb-load rather than restrict.
That contrast is worth spelling out. In the only randomized fuelling trial in recreational marathoners, runners who followed a planned strategy of about 60 g of carbohydrate an hour, with water and some caffeine, finished about 11 minutes faster than runners who fuelled as they liked (Hansen and colleagues, 2014). No keto or train-low study has shown anything close to that for performance.
If you already eat low-carbohydrate
Some runners eat low-carbohydrate for reasons that have nothing to do with racing. That is a personal choice, and this page is not dietary or medical advice. Our suggestion, based on the trials above, is to keep carbohydrate for the sessions and races where speed matters. If you have had bone stress injuries, missed periods or unexplained fatigue, see a doctor before restricting carbohydrate further.
How the book claims stack up
The verdict is about the claim, not the authors. The claim that keto or fat adaptation improves endurance performance, found in older books by Noakes, Sisson and Maffetone, is contradicted at race pace by the Supernova trials. Our evidence base did not cover any new editions of these books.
What we don’t know
- The key trials are in elite race walkers, not recreational marathoners. Slower runners work at lower intensities, and the effect at those speeds hasn’t been tested in the same way.
- The trials are small, as elite studies usually are.
- Long-term effects of years on a keto diet in runners are not covered by these trials.
- Women are under-represented in this research.
Sources
- Burke et al. (2017). Keto diet and race-walking performance (Supernova 1).
- Burke et al. (2021). Short-term keto diet and carbohydrate restoration (Supernova).
- Fensham et al. (2022). Carbohydrate restriction and bone markers.
- Sun et al. (2025). Keto diets and ketone supplements in runners.
- Valenzuela et al. (2020). Ketone supplements and endurance performance.
- Gejl and Nybo (2021). Periodized carbohydrate restriction: meta-analysis.
- Hansen et al. (2014). Planned vs self-chosen fuelling in the Copenhagen Marathon.
Last reviewed 13 November 2026.