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Carbohydrate Intake for Endurance Athletes

Written by Samantha Mare

Nutrition is an essential, and often overlooked, factor for any sportsperson who wants to fulfil their maximum potential. Yet, while the benefits from the right nutritional support have been proven in research and practice, ensuring you are getting the correct amount of nutrients for both performance and recovery can be overwhelming. Here are some guidelines to help you understand how much carbohydrate you should be consuming during exercise.

Mouth Rinse

For 30-75 mins of exercises is not necessary to ingest intra workout carbohydrates. The body should have enough stored glycogen if you are consuming a balanced diet and meeting calorie needs. However, some research has shown that including small amounts of carbohydrates or a carbohydrate mouth rinse could benefit performance.

It is well known that during exercise longer than 2 hours, carbohydrate feeding will prevent hypoglycaemia, maintain high rates of carbohydrate oxidation, and increase endurance capacity. However, it has now also become clear that carbohydrate ingestion during exercise can improve exercise performance even during shorter duration, higher intensity exercise (for example, approx. 1 hour @ 75% of maximal oxygen uptake; VO2max). The mechanism behind these performance improvements is completely different to those seen during longer exercise. When glucose (a subgroup of carbohydrates) is ingested, it is still taken up at high rates but does not add any performance effect. Therefore, increasing glucose availability, as a substrate to the working muscle, has no effect during shorter duration, higher intensity exercise.

However, performance improvements similar to those seen with carbohydrate ingestion at longer exercise durations, have been seen when individuals have rinsed their mouths with a carbohydrate solution. This method consists of rinsing your mouth with a carbohydrate rich drink for 5 to 10 seconds before spitting it out. There are now numerous studies that, on balance, demonstrate that this effect is real. As 30 minutes is not enough time to get food into the bloodstream and use it for energy, the positive performance effect has been attributed to oral sugar receptors in the mouth that activate neurological function. Therefore, using a mouth rinse could be advantageous for those athletes that do not have enough time to eat or feel discomfort when eating too close to exercise.

The most common mouth rinse formulas are made from glucose or hydrolysed maltodextrin (a glucose polymer with little sweetness which is rapidly digested, behaves identically to glucose and produces fewer GI problems than glucose). Drinks with sweeteners do not seem to have the same positive effects on performance.

Therefore, mouth rinsing with carbohydrates could be of benefit:

  • If you have experienced GI issues when digesting carbohydrates.

  • For fasting athletes or athletes with low glycogen levels (low carbohydrate diet).

  • During high intensity exercises of 75% maximum output lasting 30 - 75 min.

Rapidly Oxidised Carbohydrates

During exercise most carbohydrate oxidation takes place in the muscle, and nearly all of the ingested carbohydrate appears in the circulation and is used by muscle. When carbohydrates are ingested from the onset of exercise and at regular intervals thereafter, oxidation of the ingested carbohydrate increases and typically reaches a plateau after 60-90 minutes. However, some forms of carbohydrates need to be converted into different forms before they can be fully oxidised. Basically, there are many different types of carbohydrates, and these can be roughly divided into two categories: carbohydrates that are oxidised rapidly (up to ~60 g/h or 1 g/min) and carbohydrates oxidised relatively slowly (up to ~30 g/h or 0.5 g/min). Rapidly oxidised carbohydrates include glucose, maltose, sucrose, maltodextrin, and amylopectin starch, and it is recommended to consume these in moderate amounts during exercise lasting 2-3 hours.

Multiple Transportable Carbohydrates (MTCs)

Carbohydrates are composed of sugars. Glucose is the main source of energy for your body's cells, tissues, and organs. However, oxidation of ingested carbohydrate (exogeneous) is limited by the intestinal absorption of glucose, as the transporter used to absorb glucose becomes saturated at a carbohydrate intake of around 60 g/h. Studies have shown that various combinations of different carbohydrates (MTCs) such as glucose:sucrose:fructose, glucose:sucrose, and maltodextrin:fructose, ingested simultaneously can increase oxidation rates to >1 g/min as different forms use different transporters for absorption.

Training the Gut

It is vital to practise race nutrition strategies during training to both reduce the chances of gastrointestinal discomfort and to increase the absorptive capacity of the intestine. As the absorption of carbohydrate limits exogenous carbohydrate oxidation, and exogenous carbohydrate oxidation seems to be linked with exercise performance, an obvious potential strategy to improve performance would be to increase how much the gut can absorb. Evidence from studies on athletes suggests that the gut is trainable and that individuals who regularly consume carbohydrates or have a high daily carbohydrate intake may also have an increased capacity to absorb it. In addition, gastrointestinal symptoms such as bloating, cramping, diarrhoea, and vomiting are common in many sports, especially endurance sports. Targeted training of the intestinal tract, by consuming carbohydrates regularly, may improve the delivery of nutrients alleviate some (or all) of these symptoms.

Take Home Points

  • Events under 2 hours do not necessarily need carbohydrate ingestion and can be fuelled solely with stored glycogen from a balanced diet.

  • During exercise > 2.5 hours, carbohydrate intake will prevent hypoglycaemia, maintain high rates of carbohydrate oxidation, and improve performance capacity.

  • Consuming carbohydrates in the form of MTCs can increase oxidation and absorption rates while simultaneously reducing the risk of gastrointestinal distress during intense exercise.

  • If you experience gastrointestinal distress, consuming carbohydrates in the form of fructose could be advantageous.

  • If ingesting more than 30g/hr of carbohydrate mixing fructose and glucose is advised.

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