GOODBYE CARBS, HELLO SPEED?

Q Does the keto diet improve performance? Robert Williams

A A keto diet is effectively a high-fat, low-carbohydrate fuelling plan. What’s the theory? Well, the body relies on carbs as its primary source of energy, which is in the form of glucose. Without carbs, the body adapts by converting fat into ketones in the liver, which are used as an alternative energy source. This process is called ketosis, and for the body to adapt, fat intake needs to make up 80% of the diet, while carbs are restricted to less than 50g a day (equivalent to two slices of bread).

This is intriguing to endurance athletes because if you can rely more on fat for fuel than glycogen, you’ll tap into a huge store of energy. Glycogen – how the body stores carbs – maxes out around 1,600-2,000cals. Compare that to just 1kg of fat, which stores 7,700cals, and you can see the appeal. Beyond tapping into more energy, proponents say a keto diet improves body composition, too.

In theory, it’s tempting. But does the science stand up to endurance-boosting scrutiny? Yes and no. Research has shown that following a keto diet increases fat oxidation at higher percentages of VO 2 max compared to a higher carb diet

– which could mean working harder for longer – but there’s little evidence that this physiological adaptation transcends to performance improvements. That compares to a morerounded diet, featuring carbs, that does show performance improvements.

There are potential physical downsides, too. The diet relies on fat foods, including meat, poultry, cheese, eggs, cream, avocado, oils, nuts and seeds. All carb foods are excluded, including grains, cereals and starchy vegetables such as potatoes, as are legumes including beans, pulses, and lentils. Vegetables are generally limited to salad vegetables and greens, while acceptable fruits are limited to berries. It means you could miss out on valuable vitamins and minerals, so you should take a multivitamin if on keto. You can also feel sluggish in training.

A better approach might be a periodised one where you intermittently train with a low glycogen availability. This isn’t the same as ketosis because it’s a short-term depletion of carbohydrates done periodically. However, it will boost fat metabolism, which can spare glycogen stores for critical high-intensity race periods. Training before breakfast once a week is a good place to start. Rob Hobson