Carbohydrates to Fuel your Training

By Karlien Smit, registered dietitian with Shelly Meltzer & Associates (Gauteng)

There is a lot in the media on the impact of low carbohydrate (carb) diets on weight loss and health outcomes. Recently there have been a number of anecdotal reports of people claiming they have improved their endurance performance by changing their diets to a low carb and high fat diet.
This article specifically focuses on habitual low carb, high fat diets and endurance performance.

For the purpose of this article a low carb high fat diet refers to diets with an intake of less than 10% of total energy from carbohydrate and more than 70% of the total energy from fat. This should not be confused with high protein diets low carb diets.

There are essentially two reasons why chronic low carb high fat diets have been proposed for endurance performance:

  1. To enhance training adaptations so that fat is more effectively used as a fuel source during exercise;
  2. To reduce and maintain low levels of body fat – a goal commonly sought after by athletes.

At this stage there is not enough scientific evidence to convincingly show that a low carb diet will offer a performance advantage over a higher carb diet. Here are some of the reasons:

  • Adapting to a low carb diet takes time and is often accompanied by a period of underperformance and/or an increased perception of effort. The required duration of this adaptation period depends on the individual. This needs to be taken into consideration when planning important events i.e. it would not be advisable to try adapt to a low carb training diet close to an important event.
  • There seems to be large individual variation in performance after adapting to a low carb diet. Some claim to do much better, whilst many claim the opposite. The most common complaint is an increased rate of perceived exertion (RPE) in high intensity bouts.
  • In most cases, individuals who experienced an improved performance also lost a significant amount of weight. It can therefore be argued that the performance improvement is a result of the weight loss rather than metabolic adapations induced by the diet. If this is the case, there are also many other dietary strategies that can effectively help athletes manage their weight and body fat without having to resort to a low carb diet.
  • ‘In the long run’ training on chronically low carbohydrate stores may impair the functioning of the immune and central nervous systems. The brain primarily uses glucose as its energy source, and a lack of glucose can result in low blood glucose during exercise which may decrease performance and lead to mental as well as physical fatigue. This in turn can result in poor quality training sessions and/or in training sessions or competitions being missed thereby negatively affect performance over time.


  1. Periodize your carbohydrate intake around your training program.

It is best to plan weight loss (reduce body fat) when your training intensity is lower (typically in the “off season”). During this time lower your total carbohydrate intake, specifically decreasing your intake of nutrient-poor carbohydrates as well as high fat sources of carbohydrate (see table below).

  1. Manipulate the type, amount and timing of intake around training.

To enhance metabolic adaptations to utilize fat more effectively as a fuel source, train with low carbohydrate stores during one or two sessions in a week. E.g. do an easy 15km early morning run on empty stomach after the night’s fast. Monitor your progress and adjust accordingly.

  1. 3.       Incorporate carbohydrate before and during hard training sessions i.e. during high intensity sessions, in competition or races.

Consuming carbohydrate before and during exercise tops up your body’s carbohydrate fuel stores, therefore ensuring that during exercise it is available as a fuel and central nervous system support (for concentration, hand-eye coordination and to reduce perception of effort). However ensure that all these strategies have been tried out during training – do not try anything new on race day.

  1. Consume a pre-exercise meal that contains predominantly carbohydrate-rich foods 2-4 hours before exercise. It should ideally be low in fat and contain a moderate amount of lean protein e.g. grilled chicken served with rice or noodles, tomato-based pasta with lean meat porridge; cereal with low fat milk and a fruit or couple slices of toast with scrambled egg and fruit juice. If this is impractical either because of needing to wake up too early in the morning, or if appetite is limited, choose an easily digestible top up snack either 1-2 hours before or within 15min before you start exercising such as a large banana or energy bar or Liquid meal replacement.
  2. If the session is longer than 90min aim to consume between 30-60g of carbohydrate (plus fluid) per hour of exercise. Depending on the type of activity use a combination of sport drinks and energy bars, sweets, fruit or carbohydrate gels. (30g of carbohydrate = 500ml sports drink or 1 energy bar)
  3. For shorter sessions lasting 45-75min rinsing the mouth with a carbohydrate fluid can improve performance.
  4. When recovery time between training sessions is limited (especially < 8 hours) include carbohydrate as soon as possible after training.

Consuming carbohydrate after training improves the rate of recovery. Alternatively if you cannot consume large quantities of carbohydrate, incorporating 20-25g high quality protein together with 30-50g of carbohydrate as soon as possible after training will have similar results. Examples include low fat sweetened milk drink with an energy bar or some lean biltong and fruit juice.


The quality of carbohydrate also plays a role. Choose appropriate types of carbohydrate depending on your training and body composition goals.

Type of carbohydrate Types of food Examples Uses
Nutrient-rich carbohydrate Foods and fluids that provide additional nutrients such as vitamins, minerals, fibre, anti-oxidants and/or protein. Wholegrain Bread, cereals, grains, fruit, starchy vegetables (e.g. beetroot), legumes and low fat or fat free dairy Should form part of daily diet. Lower fibre options may be a better choice right before training to ensure gastro-intestinal comfort
Nutrient-poor carbohydrate Foods and fluids that are high in carbohydrate (often sugar, glucose, fructose, maltodextrin etc.) but provide little of no additional nutrients Sports drinks, cool drinks, cordials, sweets and gels Should only be used as a carbohydrate ‘top up’ in and around training sessions if needed
Foods rich in  carbohydrate and fat Foods that contain carbohydrate (often sugar, glucose, fructose, honey etc.) and are high in fat. Cakes, pastries, pies, hot chips, crisps, chocolate, some biscuits, desserts and ice cream Should be limited to occasional treats and not around training


Trackback from your site.

Karlien Smit

Karlien Smit has worked with various national sport teams, including Kaizer Chiefs, Nashua Cape Cobras, SA Canoeing, SA under-16 rugby team, Cycling SA and the SA Paralympic teams, as well as individuals training for ultra-endurance events such as Iron Man and the Cape Epic.

Leave a comment