The low-carb debate: good or bad for athletes?

Does trying a low-carb diet, with the hope of making your body adapt to carb depletion, work? Renee McGregor and Alice Hector debate the issue

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THE NUTRITIONIST – Renee McGregor is a registered dietician and sports nutritionist


The debate around a low-carb diet has been around for a few years now, made very popular by the ultra running scene. The theory suggests that if you follow a low-carb diet then your body has to use fat as its preferred fuel source. Many individuals have claimed that switching to a LCHF diet, where carb intakes are no more than 50g a day, has had huge benefits to their performance. But, when you look closely at the actual scientific studies, it’s not quite as simple.

Scientifically, we know that our body actually uses a mix of both fat and carbs for fuel; relying more on fat stored in the muscle in low-to-moderate intensity training. We also know from numerous studies, that as exercise intensity increases, i.e. above 65% of your HRmax, the body needs carbs in order to maintain these faster paces. But this doesn’t mean that we suddenly stop using fat for fuel, but the body will rely on carbohydrate, which can be broken down into glucose and delivered to the working muscles quickly and efficiently. 

In actual fact, it’s endurance training and some manipulation of your diet that actually improves your ability to oxidise fat for fuel, not just removing carbs from your diet.

Professionally, I don’t advocate the use of HFLC diets and feel very strongly that such a regime should not be sustained for long periods of time. It’s still a relatively new area of sports nutrition and so the effects on the body are not fully understood. But we do know that insufficient carbohydrate intakes in athletes can lead to a depressed immune system, over-reaching and over-training syndrome, as well as changes to hormone and bone health. 

My advice would be to follow a more periodised approach: include carbs around high-intensity training and aim to do low-to-moderate sessions in a carb depleted or fasted state. This is both sustainable and helps keep training consistent, which results in progression.

THE ATHLETE – Alice Hector is a long-distance pro and ultramarathon runner

In 1879, explorer Frederick Schwatka and his team set off on a record-setting 2,709-mile sled journey across the Arctic. To fuel the arduous 11-month expedition, the men killed and ate reindeer, polar bears and seals, i.e. a lot of fat and practically no carbohydrate.

Schwatka initially noted in his journal “an apparent weakness and inability to perform severe exertive, fatiguing journeys.” But three weeks later the feeling disappeared. The reason? The low-carb diet had forced his body to adapt and use fat as its primary fuel.

As for triathlon, using fats as fuel brings a number of advantages: no more sugar spikes, a steady stream of energy, no endless gobbling in races and minimal risk of stomach upsets. It also has good long-term health implications through the control of blood sugar levels.  

The problem is that adaptation takes time. Fat is our protective survival blanket and your body will not let it go easily. Even when adaptation takes place, high-intensity sessions with low carb stores will make you feel as flat as the pancake you never had. Certainly in high performance sport, I believe carbs are absolutely necessary. They are the muscle’s fuel after all.

 That being said, getting yourself used to carb depletion is a vital part of training. We can’t possibly get all the fuel we need from carbs alone when we race. So we need to optimise availability of our endless fat stores. Here are some tricks of the trade: 


Do some fasted sessions before breakfast (nothing too long or intense); minimise fuelling on some longer bike rides and runs (but always carry an emergency something!) and kick-start fat-burning that way. When you get in from that session, have a decent carb/protein mix to recover properly and ensure that the next training session is of good quality. If training isn’t too intense the next day, have a low-carb evening meal.