Can you follow a low-carb eating plan and still have the energy you need for marathons, ultra marathons, Ironman events and beyond? Nutritionist Christine Bailey reports

can low-carb diets work for endurance training? Men's Fitness UK

What is a low-carb, high-fat diet (LCHF)?

LCHF diets are those that typically propose a carbohydrate intake of less than 25 per cent of total daily caloric intake and a fat intake of greater than 60 per cent of total daily caloric intake.

Ketogenic diets tend to be lower in carbohydrate (five to ten per cent of calories) with around 60 to 75 per cent of calories from fat and 15 to 30 per cent of calories from protein.

Practically, this means limiting your daily carbohydrates to around 50g or less. In food terms, this means your diet is focused around fatty meats, fish, eggs, cheese, oils, nuts, seeds, butter, avocado, low-carb vegetables and limited low-carb fruit (e.g. citrus and berries).

This type of approach is in contrast to the more traditional high-carbohydrate, low-fat diet (HCLF), which typically consists of a carbohydrate intake of around 45 to 65 per cent or more of total daily caloric intake and a fat intake of 20 to 35 per cent of total daily caloric intake.

What happens when carbs are restricted?

When we severely limit our dietary carbohydrates, the body enters ketosis. This is a metabolic response of the body, which switches from using glucose as a primary fuel source to burning body fat and using ketones for fuel. This metabolic switch has a variety of known beneficial effects on the body, ranging from fat loss to improved brain function.

In terms of fuelling the body, a ketogenic diet differs dramatically from the carbohydrate-heavy diet. When you eat a carbohydrate-rich meal, the ingested carbs are broken down into glucose. Glucose is then shunted into cells by insulin, where it can be used for energy production.

Following a high-carbohydrate diet causes the body to rely on glucose for fuel, while rarely tapping into fat stores for energy.

A ketogenic diet does just the opposite: it forces the body to turn to fats for fuel. This process of switching from using carbohydrates as its main energy source to fats is known as ‘fat adaptation’ and it can take weeks or months to fully make this switch.

low-carb diets and endurance sport Men's fitness UK

Becoming ‘fat adapted’ allows endurance athletes to use their own fat stores for energy – rather than relying on a constant supply of glucose

Low-carb diets for endurance athletes

One of the primary reasons for the popularity of a ketogenic diet among endurance athletes is that a low-carb, high-fat diet avoids reliance on our limited carbohydrate stores. Instead, it makes use of our abundant stores of fat (even lean athletes have sufficient fat to draw upon during exercise) to keep us fuelled over long training sessions.

Research shows that well-trained athletes who follow an LCHF diet have higher rates of fat oxidation and use a greater percentage of fat as a fuel source. This may allow athletes to go further without the usual ‘bonking’ or ‘hitting the wall’ – that exhaustion associated with depletion of carbohydrate stores.

Such an approach is in contrast to athletes following a high-carbohydrate diet, who are reliant on a steady intake of carbohydrates while training to keep their fuel tank topped up.

Does a low-carbohydrate diet lead to improved performance?

While there are plenty of studies to suggest a low-carbo diet improves fat oxidation, there is limited evidence that it leads to enhanced performance. Fat metabolism is a much slower process than carbohydrate metabolism, making fat the ideal fuel choice when the exercise intensity is low (peak fat metabolism during exercise occurs around 45 to 65 per cent of VO2 max).

So for lower intensity exercise such as cycling, walking, running or swimming, fat utilisation will be highest. However, as the speed or pace of training increases, we become more reliant on carbohydrate metabolism.

Explosive movements (sprints, etc) draw on your muscles’ glycolytic capacity, powered by glucose from the carbohydrates we eat. When these are limited, your performance may suffer.

So while an LCHF can certainly fuel your endurance training, it may not make you a faster runner. In reality, endurance athletes – particularly during competitions – need ‘metabolic flexibility’, allowing them to efficiently use both fats and carbs during exercise.

low-carb diets for endurance athletes Men's Fitness UK

If you do want to make the switch to a more low-carb diet, consider a cyclical approach to avoid cutting out carbs altogether 

The cyclical approach

Following a very low-carbohydrate diet or ketogenic diet may not be suitable – or beneficial – for everyone. For example, it is not recommended for people with kidney or gallbladder disease, certain thyroid conditions, or if you are a hyper-responder to cholesterol or experiencing high stress.

Others find the diet difficult to adhere to and, if badly planned, you may miss out on valuable nutrients and electrolytes.

A more sustainable approach for endurance athletes is to adopt a cyclic ketogenic diet. This typically involves carb-loading one to two days of the week around higher intensity training, followed by a low-carbohydrate ketogenic diet for the rest of the week. This enables the body to enter ketosis to fuel long training sessions without compromising performance.

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