Keto has a devoted following among endurance athletes, and a matching crowd of sceptics, so this is one where the honest answer matters more than the hype from either side. The short version is that keto genuinely transforms how your body fuels exercise, but transforming the fuel is not the same as improving the performance, and the distinction is where most of the confusion lives.
What keto definitely does: fat adaptation
On this point the research is unanimous. Become keto-adapted and your body gets dramatically better at burning fat for fuel, reaching rates of fat oxidation during exercise far higher than a carb-fuelled athlete can manage. Studies of endurance athletes consistently show this, with some keto-adapted athletes burning fat at roughly double the usual peak rates during prolonged exercise. For very long, lower-intensity efforts, this is genuinely useful, because fat is a near-limitless fuel store and you become less dependent on topping up carbohydrate. It also tends to improve body composition, which helps in weight-sensitive sports.
What it does not reliably do: make you faster
Here is the catch the marketing skips. A systematic review of the research found that while fat oxidation always goes up, aerobic performance in trained endurance athletes is largely unchanged by switching to low-carb or ketogenic diets. Becoming a better fat-burner did not, on average, make people faster over their established aerobic capacity. So keto changes the fuel mix without delivering the performance gain that fat-adaptation enthusiasts hoped for.
Where it can actively hurt
For high-intensity efforts the news is worse. At intensities above roughly 70 per cent of maximum, burning fat is less efficient than burning carbohydrate; it costs more oxygen to produce the same power. A well-known study of elite race walkers found that a low-carb high-fat diet worsened their exercise economy and wiped out the performance gains they got from intensified training, while the carb-fuelled groups improved. The finding was striking enough that it was deliberately reproduced. The lesson is that anything relying on repeated hard, high-intensity efforts, sprinting, racing, team sports, lifting at the top end, depends on carbohydrate, and keto tends to blunt it.
So who does it suit?
It comes down to your sport and your goals. Keto can suit ultra-endurance athletes going for hours at a steady, moderate pace, recreational exercisers who care more about fat loss and health than competitive times, and anyone who values stable energy over peak power. It does not suit athletes chasing top-end speed, high-intensity intervals, or competitive performances where carbohydrate is the limiting fuel. Some athletes also use a middle path, eating low-carb generally but timing carbohydrate around their hardest sessions, to get body-composition benefits without sacrificing the top end.
What it means in practice
If you exercise mainly for health, weight and steady energy, keto is perfectly compatible and the fat-adaptation is a nice feature, with the usual advice to keep your electrolytes topped up since exercise increases the losses. If you compete, especially in anything fast or explosive, go in clear-eyed: keto will not make you quicker and may slow your hardest efforts, so it is a choice to make for other reasons, not a performance edge. And give any switch a long adaptation period of several weeks, because the early phase, before you are fat-adapted, feels especially flat.
The bottom line
Keto reliably turns athletes into excellent fat-burners, which helps body composition and very long, steady efforts, but the research shows it does not improve endurance performance overall and can measurably impair high-intensity work, where carbohydrate is the better fuel. It is a sensible choice for health-focused exercisers and some ultra-endurance athletes, and a poor one for anyone chasing speed. Match it to your sport, not to the hype.
This is general information about the ketogenic diet, not medical advice. Keto does not suit everyone; if you are an athlete with specific performance or medical needs, or are pregnant, on medication, or managing a condition, speak to a doctor or sports dietitian first.
Source: The Effects of Ketogenic Diet on Athletic Performance: A Systematic Review. 2025. Read it here.