Keto and exercise

You carry on training as normal on keto. How much your performance is affected depends on the sport: with weights, studies find no big difference, while in fast endurance races keto seems to cost you a little speed.

The basics of the diet are covered in the guide The keto diet for beginners.

What changes in the body

Muscles have two fuels: fat and glycogen (the carbohydrate they keep in store). On keto the body learns to burn far more fat; glycogen stays low.

Glycogen is stored together with water, three to four parts water for every part of glycogen. That is why the scales drop quickly in the first few days: some of the kilos you lose are simply water.

For walking, easy running or a gentle bike ride, fat is more than enough. Once you put your foot down things change, because the muscle wants energy fast and finds it more easily in glycogen.

The first few weeks

At the start training usually feels heavier. The body has not yet learned to burn fat and is losing water and salt at the same time; some people get keto flu on top, mostly tiredness and headaches.

For one to two weeks bring the intensity down a little, drink water and leave the salt in your food. If you get cramps or dizziness, look at the Keto flu checklist and the Electrolyte calculator.

Endurance: running, cycling, swimming

The best-known study of keto and endurance was done on world-class race walkers. Over three weeks of hard training one group ate less than 50 grams of carbohydrate a day and the other two ate plenty.

The keto walkers burned far more fat, but they also needed more oxygen for the same speed. In the 10 km race the two other groups improved their times by 6.6% and 5.3% respectively; the keto group stayed where it was.

These were elite athletes at race pace, of course. At an easier pace fat covers more of the energy and the gap narrows. If you are getting ready for a fast race, though, keto is probably not the diet for that stretch.

Weights and strength

With weights the news is better. A 2024 review pooled studies of trained men and women who lifted while on keto. In the one-rep max, for the squat (106 people) and the bench press (119 people), there was no significant difference compared with those who ate carbohydrate.

The researchers themselves say a one-rep max does not show everything; how you get on in a normal session with lots of reps, for instance, is not something it can tell you.

Building muscle also takes protein, so have something protein-rich at every meal. The Macro calculator works out your own amounts and the Most protein list has ideas.

Intense interval training

Sprints, HIIT, CrossFit, football, basketball: all of them mean lots of short, very hard efforts that rely mainly on glycogen. This is where keto struggles most.

If performance is what you care about, you can eat a few carbs before training; count them within your daily limit, though, or you may drop out of ketosis.

What to eat after training

After training you want a meal with protein. Two ideas, with figures from the food database:

What helps

Don't judge your performance by the first week. Drink water and salt your food as normal, since on keto you lose more water and sodium (mostly at the start). Make sure you eat enough, too; on very little food your training drops off, whatever diet you follow.

Frequently asked questions

Can I build muscle on keto?

The 2024 review suggests you do not lose strength. Muscle needs weight training, enough protein and enough calories; keto rules out none of them.

Do I need to eat carbs before training?

Not necessarily. With weights, studies show no loss of strength without them. If your training is very intense, try a few beforehand and count them within your daily limit.

Why have I had cramps since I started?

You may be short of electrolytes. On keto more sodium leaves the body with water, so keep an eye on salt, potassium and magnesium; the Electrolyte calculator works out the amounts.