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Marconutrients: Carbohydrates

On today’s #ScienceFriday we are starting with a series of topics on macronutrients. The importance of carbohydrates for endurance athletes is the first topic. What exactly are carbohydrates and how do they influence performance? If you have any questions or comments, please do not hesitate to contact us!

What are carbohydrates?

Carbohydrates (CHO) are one of the three macronutrients found in foods alongside protein and fat. CHO are found in foods with varying ratios of sugars (e.g. honey, fruit juice, sports drinks, gels), starches (e.g. breads, pasta, potatoes, rice) and fibers (e.g. vegetables, fruit, legumes, grains, nuts and seeds).  

When digested, both sugars and starch are metabolized into glucose to be used as the body’s primary fuel source for the muscles and brain. Glucose is stored in the liver and muscles as glycogen to be used as energy when needed or immediately used as energy.

Sugars are short-chain CHO molecules, which come in various forms such as glucose, fructose, sucrose and maltose. Glucose and fructose are naturally found in most fruits, vegetables and grains, whereas sucrose is found mostly in sugar cane and some root vegetables such as carrots. Maltose is formed during the digestion of starch and is found in grains and brewed products such as beer. 

Starch consists of long chains of glucose molecules and is a slower release of energy. In vegetables, starch falls into two categories: starchy (e.g. potato, carrot, corn, butternut) and non-starchy (e.g. broccoli, spinach, zucchini, cucumber). The more starch a vegetable has, the higher its CHO content. 

Fibers, like starch, come in two forms, soluble and insoluble and are found in varying amounts in foods. Soluble fiber helps you feel fuller for longer whereas insoluble fiber helps with gastrointestinal emptying to relieve issues such as constipation. It is often suggested for endurance athletes to avoid high fiber foods in their pre-race meal, as the gastric emptying effects can cause stomach issues that may hinder performance.

Carbohydrates & endurance performance

From a performance side, CHO are needed to support intense bouts of exercise such as sprinting, hill climbs or sustaining a break away in a race. Full glycogen stores are important to sustain high intensity exercise for longer durations and delay the onset of fatigue (1). Due to the limited storage capacity, glycogen will begin to deplete as intensity prolongs. Exogenous CHO (e.g. gels, isotonic drinks and bars) are needed during exercise to help sustain optimal performance.

If refuelling using exogenous CHO sources cannot keep up with the demand, fat becomes the dominant fuel provider. Compared to CHO, fat is less efficient with a much slower energy turnover. Although fat can produce ATP, the speed at which it is achieved is not effective in meeting the demands of high intensity exercise. Therefore, when an endurance athlete is placed on a low-CHO diet, he/ she is very limited on their abilities to perform. Endurance athletes should aim to consume around 7-12g/ kg BW CHO per day (2).

If your goal of the training session or competition is to achieve optimal performance, it is important to consume adequate amounts of CHO during exercise. Research shows durations of 1-2 hours require ~ 30g CHO per hour, whereas longer durations (e.g. >3 hours) require ~ 60g CHO per hour. Elite endurance athletes can even increase this amount up to ~90g per hour depending on the type of race (3). The amount of CHO per hour when exercising is not required per grams per kilogram body weight.

To truly reap the benefits of CHO in the diet, it is important to think of quality. The majority of your CHO intake should come from whole foods and not an all-you-can-eat buffet of refined baked goods and processed foods. Due to the importance of an endurance athlete to maintain a healthy weight that optimises performance, ideally CHO intake should be altered daily according to the demands of an athlete’s training program (4). 

For example, more CHO should be consumed on a long or intense training day versus that of a rest day. This type of CHO periodisation may be a highly effective method for athletes to maintain a healthy weight during the off season. This is a healthier alternative to extreme dieting during race season, which may be detrimental to performance (4).

Carbohydrates for recovery

Beyond performance, CHO plays an essential role in recovery. In combination with protein, ~1-1.2 g per kg bodyweight of CHO should be consumed in 1-2 hours following the completion of a session (5). This has been shown to ensure maximal repletion of glycogen stores, as glycogen regeneration slows down significantly after the second hour post exercise (5). Rapid restorage is only needed if the athlete needs to compete at optimal performance the next day.

References

  1. Abbiss, C. R., & Laursen, P. B. (2005). Models to explain fatigue during prolonged endurance cycling. Sports Medicine, 35(10), 865–898. 
  2. Burke, L. M., Hawley, J. A., Wong, S. H. S., & Jeukendrup, A. E. (2011). Carbohydrates for training and competition. Journal of Sports Sciences, 29(sup1), S17–S27.
  3. Cermak, N. M., van Loon, L. J. (2013). The use of carbohydrates during exercise as an ergogenic aid. Sports Med, 43(11), 1139-1155.
  4. Impey, S. G., Hearris, M. A., Hammond, K. M., Bartlett, J. D., Louis, J., Close, G. L., Morton, J. P. (2018). Fuel for the Work Required: A Theoretical Framework for Carbohydrate Periodization and the Glycogen Threshold Hypothesis. Sports Medicine, 48, 1031–1048. 
  5. Ivy, J. L.; Katz, A. L.; Cutler, C. L.; Sherman, W. M.; Coyle, E. F. (1988): Muscle glycogen synthesis after exercise. Effect of time of carbohydrate ingestion. In: Journal of applied physiology (Bethesda, Md. : 1985) 64 (4), S. 1480–1485.