Carb-free diet: good or bad for athletes?

High fat and protein diets are on the increase among endurance athletes, but at what cost to performance? Renee McGregor investigates…


Whether to eat or not to eat carbs is one of the most debated questions among sports scientists at present. Go back just a few years and there would have been no doubt that carbohydrates were a necessary fuel for exercise.


But today there’s a high proportion of endurance athletes discussing the performance benefits of ketogenic diets [for an example see last month’s Diet Analysis]. Ketogenic diets involve limiting your carb intake to 50g a day and making up the remaining energy requirements with foods high in fat and protein. So what’s the thinking behind this shift?

Finding a better mix of fuel

The continual search for performance gains, however small, has led sports scientists to investigate ways of getting our bodies to oxidise higher percentages of fat as fuel during endurance exercise. If it’s possible, it would mean an athlete could spare more of their glycogen reserves (the form carbs take when stored in the liver and muscles). In other words, teaching your body to become better at burning fat would allow you to go faster for longer during endurance events with less risk of hitting the wall.

We already know that our bodies use a mix of fat and carbohydrate for fuel, and rely more on fat during low-/moderate-intensity exercise. We also know from numerous studies over the past 20 years that if your exercise intensity increases above 65% of your maximum, your body needs carbohydrate in order to maintain the faster pace. But this doesn’t mean that it suddenly stops using fat for fuel, just that it shifts the balance to favour carbohydrates as they can be broken down into glucose and delivered to the muscles more quickly and efficiently.

Given that knowledge, simply removing carbs from your diet can’t be a good idea if you’re looking to improve your performance. If you’re looking to alter your body’s ability to burn fat the more sensible approach is to manipulate your carb intake.

Performance decline

Over the last 18 months I’ve encountered Olympic-, half- and full-Ironman-distance triathletes who’ve tried to improve their performances by adopting a low-carb/high-fat diet, and most haven’t achieved the results they hoped for. In many cases, their performances declined.

Professionally, I don’t advocate the use of ketogenic diets and strongly believe that such a regime should not be sustained for long periods. It’s a relatively new area of sports nutrition and the long-term effects are not fully understood. More specific studies and monitoring are needed before I start advocating this method of eating.

My observations of the athletes I’ve worked with seem to fall in line with the present scientific findings that insufficient carb intakes in athletes can lead to:

  •   A depressed immune system and an increased risk of infection.
  •   Poor recovery, increased fatigue and an increased  risk of overtraining syndrome.
  •   Hormonal changes, particularly related to low oestrogen levels in female athletes, resulting in many problems, including high cholesterol.
  •   Poor bone health and increased risk of stress fractures.

A better approach is to periodise your diet depending on your schedule. This involves eating high-carb foods around high-intensity training sessions and races to ensure that target paces can be maintained. Then, during long-endurance training sessions, at a low to moderate intensity, avoid carbs before and/or during so a higher percentage of fat stores are used. With time, training like this teaches your body to use a higher percentage of fat as fuel more efficiently. You body will still rely on carbs for races or fast sessions, but burning more fat allows its carb stores to last longer – a real bonus in events over three hours.

Don’t avoid carbs

Practically, this means doing low-/moderate-intensity training sessions in a fasted state or avoiding carbs for up to four hours prior. You should drink fluid during the session though and possibly electrolytes if it’s hot.

However, it’s important to stress that, firstly, these studies were done on well-trained athletes so this approach is not recommended for anyone new to endurance sport. Secondly, even for highly trained athletes, I’d recommend no more than three fasted/depleted sessions a week.


So the answer to the original question is that carbs should not be avoided as a low-carb intake can be particularly problematic for triathletes. Training for three disciplines with a mix of low-/moderate-/high-intensity sessions means the need for carbs is high to protect your immune system and decrease your risk of injury.