Low-carbohydrate and ketogenic diets
Evidence: contested
Low-carbohydrate, high-fat (LCHF) and ketogenic diets reliably raise the rate at which you burn fat during exercise. Whether that helps performance is the contested part: the best-controlled trials show it impairs exercise economy and race results at intensity, while a vocal minority of researchers and athletes argue the benefits are real for some. The weight of evidence points one way.
Not medical advice
This is a general knowledge base, not medical or dietary advice. If you are injured, unwell or weighing up a supplement or a change to your diet, speak to a doctor, physiotherapist or registered dietitian who knows your situation.
The rationale
The appeal is simple arithmetic. Even a lean runner stores only a couple of hours of glycogen at race pace, but carries enough fat to fuel many days of running. If you could shift the body to burn fat rather than carbohydrate, the thinking goes, you would side-step the bonk and never need to fuel on the move. A very-low-carbohydrate, high-fat diet, under roughly 50 g of carbohydrate a day and often ketogenic, does drive exactly this shift, called “fat adaptation”.
The shift is real and large. The FASTER study compared elite ultra-runners habituated to a low-carbohydrate diet for around 20 months against carbohydrate-fed peers and found peak fat-oxidation rates 2.3 times higher in the keto-adapted group, the highest ever recorded (Volek et al. 2016). A later review put adapted oxidation at up to about 1.5 g of fat per minute, with the intensity of peak fat-burning shifting from roughly 45% to 70% of maximal aerobic capacity (Burke 2021). On the metabolic measure the diet is built to move, it works, and it works within days to weeks.
The decisive problem
A higher fat-oxidation rate is a means, not an end. The end is going faster, and here the controlled evidence turns against the diet.
The pivotal work is a series of three-week training studies in elite race walkers, an unusually clean model because race walking is a fixed, sustained, high-aerobic effort. Walkers were assigned to a high-carbohydrate, a periodised-carbohydrate, or a ketogenic LCHF diet during intensified training. The LCHF group more than doubled fat oxidation, exactly as predicted, and got slower. Their exercise economy worsened: the oxygen cost of moving at race speed rose, and they were the only group not to improve a 10 km race walk, despite three weeks of hard training. The carbohydrate groups improved markedly (Burke et al. 2017). A near-independent replication, with mostly new investigators and participants, reproduced the result: the same doubling of fat-burning, the same loss of economy, the same blunted performance (Burke et al. 2020).
Two mechanisms explain it. First, fat yields slightly less ATP per litre of oxygen than carbohydrate does, so a fat-burning runner needs more oxygen for the same pace, which is worse economy directly. Second, keto adaptation down-regulates the body’s carbohydrate-oxidation machinery (notably pyruvate dehydrogenase, PDH), so even when glycogen is present the muscle cannot deploy it quickly enough for the surges and high-intensity efforts that decide races. Crucially, the same studies found no carryover benefit from adapting to LCHF and then restoring carbohydrate before competition: the impairment did not convert into an advantage (Burke et al. 2020). This is also why fat adaptation sits at odds with the demands of in-race carbohydrate fuelling, which the high-intensity energy systems still rely on.
The verdict
For high-intensity work and race-day performance, keto tends to hurt, and the better-controlled the study, the clearer that becomes (Burke 2021). For easy, low-intensity aerobic running it neither obviously helps nor much harms, which is the kernel of truth defenders point to. The genuine disagreement is over the size of individual variation: the same review notes extremes of response at both ends, and a minority of athletes may tolerate the diet better than the group means suggest (Burke 2021).
Who might reasonably consider it: someone whose events are very long and slow, whose intensity rarely climbs, or who has a non-performance reason (a medical indication, body-composition goals) to eat this way. Most competitive distance runners do not, because their best races are run at intensities where economy and carbohydrate availability decide the result. As with any restrictive diet, watch energy availability so it does not tip into Relative Energy Deficiency in Sport. And keep it in proportion: the basics move the needle far more than the macronutrient ratio.