Low Carbohydrate Dieting for Sport: On its Last Legs?

Low carbohydrate dieting for sporting performance was popularised in the 1980s with the aim of enhancing fat usage during exercise, in return sparing muscle glycogen and improving exercise performance. Since then many studies have aimed to find out if low carbohydrate, high fat (LCHF) diets do further enhance performance when compared to their higher carbohydrate counterparts.
LCHF diets are promoted largely via social media, and would have you believe that it should be a favoured approach moving forward, not only for sedentary population, but for athletes striving to achieve maximal performance gains.

But first, let’s look at the need for carbohydrates

In the 1920’s it was found that carbohydrates were an important fuel for exercise. A few decades later, it was discovered that exercise performance and tolerance can be influenced by changing the carbohydrate composition of the diet. Following on from this, in the 1960’s it became clear that muscle glycogen was used during exercise. However, it wasn’t until the 1980’s where they discovered that consuming carbohydrates during exercise can improve exercise performance.
Although this is pretty straight forward to us now, it was ground-breaking stuff 30+ years ago… Since then, the use of carbohydrates to improve sport and exercise performance has become increasingly popular with both recreational and professional athletes across a variety of sports.

Since carbohydrates are king, why is there a need to ‘fat adapt’?

Depending on exercise intensity, both fat and carbohydrates are used to fuel exercise performance. Since limited amount of carbohydrate stores exist within the body ~500g, it has been theorised that if an athlete can ‘tap into’ a much larger source of fuel such as stored fat mass, then exercise performance is less dependent on carbohydrates and a lesser cause for concern of glycogen depletion which is associated with decrements in performance. Therefore, low carbohydrate/ketogenic diets as a strategy to up-regulate mechanisms for fat usage have emerged. Such types of diets range from;
  • Moderate carbohydrate restriction; 20% of calories from carbohydrates.
  • Extreme restriction; under 50g of carbohydrates per day.
To compensate for the carbohydrate and calorie restriction, a higher fat intake is advocated to improve substrate utilisation and energy balance. Fat adaptation has been observed in as little as 5 days following a LCHF approach, therefore from a physiological standpoint some merit is given. Such adaptations observed are:
  • Increase in muscle triglyceride stores.
  • Increased activity of hormone sensitive lipase – which mobilises triglycerides in muscle and adipose tissue (body fat).
  • Increases in key fat transport molecules.

Performance Vs Physiology

Despite some favourable physiological outcomes, LCHF diets fail to elicit performance benefits, especially at higher intensities that involved repeated sprint performances. One of the reasons to become ‘fat adapted’ is to spare muscle glycogen, however following a LCHF diet appears to impair the use of muscle glycogen through the down regulation of mechanisms responsible of utilising stored carbohydrates as fuel. . Furthermore, emerging evidence would suggest that high fat feeding during periods of carbohydrate restriction post exercise may be detrimental and impair the training response and adaptation.

Therefore, athletes competing in a range of sports looking for performance enhancement, LCHF diets should not become a tool in their tool box


A balancing act

Moving away from such a universal approach is advisable, where carbohydrate periodisation is a pragmatic approach to elicit enhanced training adaptations, whilst improving competition performances. This can be achieved by adjusting carbohydrate availability on a daily basis based on the type of training session and its desired outcome, i.e. train low.

Therefore, working off the motto ‘fuel the work required


Living with LCHF

Both LCHF and High carbohydrate, low fat diets are a compromise, of which one macronutrient is greatly reduced. A cornerstone for the adherence to any type of diet is acknowledging personal preference. Without accounting for diet enjoyability, schedule, convenience and preferences, rates of attrition will be high which will unlikely favour any positive enhancements in performance.
For example, If LCHF diets were shown to consistently demonstrate modest improvements in performance, but in order to facilitate this, the removal of your favourite carbohydrate rich foods is required – How long do you think you can adhere to it and benefit from LCHF? From experience, not very long as it will often lead to increased cravings, limit food choices during social events and cause poor relationship with foods as carbohydrates usually become demonised. This is often further exacerbated when a LCHF approach is adopted, but misunderstood regarding its execution – where failure to increase dietary fat leads to very low calorie dieting. In return compromises exercise performance, immune function, body composition and increase susceptibility to injury.
A trade off will always exist between different types of diets, both regards to its sustainability and its desired outcome. Use this information to rethink how you approach your nutrition to get the most out of your training and competitive performances.

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