Published in Optimum Nutrition Spring 2018


Whether you're an elite athlete or go to the gym once a week, your body requires fuel when you exercise. We look at whether you need carbohydrates when you workout, if you can exercise on a low carb, high fat diet, and why protein is so important

Why carbohydrates are needed for exercise

Food is stored in the body as glycogen and broken down into glucose during exercise, feeding the muscles with energy.

We can get glycogen from most food groups, but carbs are thought to convert to glucose more easily than protein or fat. As a result, carbohydrates are considered to be the energy food; which is why we often see marathon runners eating bowls of porridge or bananas before their race. This is known as carb-loading; consuming large quantities of carbohydrates in the run up to an event to increase the amount of glycogen in the body.

The body naturally contains enough glycogen to run the course of an hour’s workout or 10 km run. But for distance runners, glycogen sources are often exhausted by the half marathon mark — hence the need for carb-loading.

Can people exercise on a low carb, high fat diet?

Professor Tim Noakes, Emeritus Professor at The University of Cape Town, argues that since the only function of carbohydrates is to produce energy, humans can exist on a zero-carb diet.

Noakes supports the consumption of a low-carb, high-fat diet (LCHF), which uses fat as the body’s primary fuel. “When you stop eating carbs, the body reduces the secretion of the hormone insulin — needed to store carbohydrates after we eat them — and for some reason becomes less hungry,” he says. “As a result, subjects eating this diet eat fewer calories and lose weight.”

A LCHF-diet contains plenty of animal produce including meat, fish, eggs, and dairy, along with nuts and non-starchy vegetables. It excludes grains and starchy vegetables like potatoes.

“The liver can produce all the glucose we need for exercise — we do not need to ingest any,” says Noakes. “A LCHF-diet teaches our bodies to burn the fat in our diet — not to store it as fat in our abdomens or under our skin.”

What is ketosis?

Despite countless studies suggesting that carb-loading helps endurance, Noakes believes that humans can exercise at high intensities without it.

The body naturally contains greater reserves of fat; and sustaining a high-fat diet forces the body into a state of ketosis. This is when there is not enough glucose for energy, so the body produces ketones from fat and burns these instead. Endurance athletes would, therefore, have enough energy from fat to cover greater distances, eliminating the need for carb-loading.

However, Graeme Close, Professor of Human Physiology at Liverpool John Moores University, disagrees. He points towards a study by Louise Burke from the Australian Institute of Sport. The three week study concluded that fat-loading was detrimental to the performance of elite athletes compared with traditional carb-loading. The increase in fat oxidation caused by the LCHF-diet meant that oxygen demand at a given speed increased, impairing the performance of athletes.

“I hear the fat-loading crowd screaming at me, saying three weeks of loading is not long enough,” says Close. “But I would ask how long an elite athlete would accept impaired performance when there is no proven gold at the end of rainbow.”

Close suggests those looking to boost performance in a high intensity sport should stick to a higher carb intake. “Consuming about 6-8 g of carbohydrate per kg of body weight the day before would be advisable and then a good carbohydrate breakfast,” he says.

Exercise for weight loss

According to US-based author and sports nutritionist Nancy Clark, the main appeal of a high fat diet is weight loss.

Ingesting carbs before exercise causes a rise in the hormone insulin which, when elevated during training, limits fat oxidisation — the reaction which causes our bodies to burn fat.

The Ohio State University found that endurance athletes who ate very few carbohydrates (10 per cent carbs versus 70 per cent fat) burned twice as much fat as athletes on a high-carb diet. On average, the low-carb runners’ fat-burning rate was 2.3-fold higher than the rate for the high-carb athletes.

However, Clark warns, as with any restrictive diet, cutting out carbs isn’t always sustainable. “My experience suggests that many people who seek out a keto-diet have a bad relationship with carbs, so instead of binge-eating the whole bag of cookies, say, they cut out carbs completely.”

Why is protein important for exercise?

Proteins are known as the building blocks of the body, maintaining and repairing cells and tissues. Even when we’re resting, they are used to replace proteins in tissues that are naturally turned over.

Proteins provide little energy during exercise but they’re important post-workout because they allow the muscles to repair and recover, preventing muscle-loss and building lean muscle.

Most people require 70-140 g of protein every day — around 1.5-2 g per kg of body-weight.

According to Professor Close, the key feeding time is breakfast, after the overnight fast. “If you’re training hard I also advise protein before bed to help with overnight recovery,” he adds.

The verdict

Ultimately, nutrition for sports performance comes down to your personal goal. Evidence suggests that a low-carb diet mainly supports fat-loss but can provide energy for moderate exercise, whilst a high-carb intake improves performance in high-intensity sport.

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