Carbohydrates During Exercise

Carbohydrates (either in liquid or solid forms) are known to have a significant effect on endurance exercise performance during high intensity and long-duration aerobic exercise. During exercise at moderate intensities (~ 60-80% VO2max – 80% VO2max is approximately marathon intensity for well trained athletes) the consumption of carbohydrates can delay the point of fatigue by around 15-30minutes.  Carbohydrates delay fatigue by raising blood sugar levels, which can then be used to fuel aerobic metabolism and helps to preserve valuable muscle glycogen stores allowing exercise to continue for longer and at higher intensities before fatigue occurs.

Carbohydrates Improve Endurance Exercise Performance

During prolonged exercise the body starts to breakdown muscle and liver glycogen stores. As levels of muscle and liver glycogen start to decrease there are decreased rate of carbohydrate oxidation, increased rates of fatigue and reductions in endurance exercise performance (Coyle et al., 1986). Research even suggests that brain glycogen levels may deplete following prolonged exercise and this may be a factor in central fatigue mechanisms that inhibit exercise performance (Matsui et al., 2011). The ingestion of carbohydrates during exercise has been shown to be beneficial for  endurance athletes by preserving muscle and liver glycogen levels, delaying the point of fatigue, protecting against low blood sugar levels (hypoglycaemia) and improving endurance exercise performance (Coggan and Coyle, 1988; Coyle et al., 1986). A meta-analysis (Temesi et al., 2011) of the effects of carbohydrate ingestion on endurance exercise performance found that carbohydrate consumption (30-80g, 6-8% solution) led to improved time trial performance (~2%) and time to exhaustion (~15%).

Current research suggests that the consumption of between 30-80g/hour of carbohydrate (6-8% solution) can enhance endurance exercise in events of > 1 hour (Temesi et al., 2011; Rodriguez et al., 2009; Sawka et al., 2007). Carbohydrate consumption during exercise appears to be particularly important during prolonged endurance events lasting 2 or more hours.

Other Benefits of Carbohydrates During Exercise

Carbohydrates during exercise and immune health

Carbohydrate consumption during exercise can help to protect the immune system from the high levels of stress placed on it during long duration endurance races or prolonged training sessions. It’s known that endurance events like marathons, triathlons and cycling events place large amounts of stress on the immune system, increasing the risk of illness – particularly upper respiratory tract infections. In fact, after prolonged events like the marathon, triathlon or ironman competition your risk of illness can increase several fold (up to 6x greater risk of illness).

Why does prolonged or intense exercise negatively affect immune health?

The negative effects of endurance training on immune health are mainly due to a lowering of blood sugar levels. When blood sugar levels are low, the brain responds by increasing the levels of two key hormones (cortisol and epinephrine). While these two hormones help to elevate blood sugar levels they both have negative effects on the immune system. Cortisol, in particular has a very negative effect on immune health by lowering the numbers of lymphocytes – very important white blood cells that fight invading pathogens. The rise in both of these key hormones, and therefore the negative effects on the immune system, can be reduced, by keeping blood sugar levels elevated throughout training, or racing.

 Carbohydrate and protein breakdown during aerobic exercise

An additional problem during long duration races and training is that large amounts of damage occurs within muscles including: 1) muscle membranes can become damaged; 2) Individual muscle cells can become damaged and start to break down; 3) Muscle proteins are broken down for use as energy within aerobic metabolism. Again this problem is exacerbated by decreased blood sugar levels which cause a rise in cortisol levels.  The main role of cortisol during aerobic exercise is to preserve muscle glycogen stores and maintain blood sugar levels. It does this by increasing fatty acids and amino acid availability for aerobic metabolism. In the case of amino acids it does this by increasing the rates of muscle catabolism (muscle breakdown) which increases levels of amino acids in the blood for use in aerobic metabolism – the cortisol is actually encouraging your muscles to breakdown protein, and use it as an energy source in order to preserve blood sugar levels and muscle glycogen stores. Levels of cortisol are known to become particularly elevated during periods of glycogen depletion and low blood sugar levels.

The negative effects of cortisol can be reduced by maintaining blood glucose levels through adequate carbohydrate consumption during prolonged endurance events and training. Endurance athletes competing in events of >1hours duration can increase exercise performance, preserve immune function and reduce muscle breakdown by ingesting a carbohydrate drink containing 5-8% carbohydrate (ideally around 30-40g of carbohydrate in a 500ml drink) to the start line and to start to ingest this immediately on starting the race. Consume this over the first half hour of the race and then continue to drink around 500ml every half hour.

In order to delay fatigue, maintain immune function, and reduce muscle breakdown during your long continuous aerobic training you should consume around 150-300ml of fluid with around 5-8% carbohydrate (7.5-15g) every 15 minutes. This will help to maintain hydration levels, blood sugar levels, and delay the point of fatigue.

Additional consideration with carbohydrate ingestion during prolonged aerobic training

Although carbohydrate ingestion can enhance exercise performance, reduce muscle breakdown and preserve immune function, too heavy a reliance on carbohydrates during training may reduce the benefits of aerobic training in terms of enhancing fat metabolism. The main reason for this is that consuming carbohydrates can lead to increased levels of carbohydrate metabolism and this may compromise the training benefits of long endurance training sessions where the rpimary aim is to enhance fat metabolism. Click here to read more about the benefits of exercising in a fasted state.

Carbohydrate during exercise summary:

  • The ingestion of carbohydrates during exercise can have a significant effect on exercise performance
  • During prolonged or intense exercise the body starts to break down and deplete muscle glycogen stores. Eventually this will deplete glycogen levels to the point where carbohydrate oxidation rates decrease, this can negatively effect fat metabolism, increase rates of fatigue and lead to increased levels of muscle protein catabolism
  • The ingestion of carbohydrates during aerobic exercise can help to maintain blood sugar levels, preserve muscle and liver glycogen stores, reduce the rates of muscle breakdown, and enhance exercise performance
  • Current research suggests that athletes should consume 30-80g carbohydrate/hour in a 6-8% solution. This is beneficial in events of >1hour but is particularly beneficial during events of 2 hours or more.
  • Carbohydrate ingestion can help to preserve immune function and reduce the risk of illness (particularly upper respiratory tract infections) after prolonged endurance events.
  • Carbohydrate ingestion can also help to reduce the rates of muscle breakdown primarily by protecting against the effects of elevated cortisol levels.

Carbohydrates during exercise references:

Coggan A. R., and Coyle E. F. (1988) Effect of carbohydrate feedings during high-intensity exercise. J. Appl. Physiol. 65:1703–1709.

Coyle E. F., Coggan A. R., Hemmert M. K., Ivy J. L. (1986) Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate. J. Appl. Physiol. 61:165–172.

Matsui T, Soya S, Okamoto M, Ichitani Y, Kawanaka K, Soya H. (2011) Brain glycogen decreases during prolonged exercise. J Physiol. 2011 Jul 1;589(Pt 13):3383-93. Epub 2011 Apr 26.

Rodriguez NR, Di Marco NM, Langley S (2009) American College of Sports Medicine position stand. Nutrition and athletic performance. Med Sci Sports Exerc. 2009;41:709–31.

Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS (2007) American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007;39:377–90.

Temesi J, Johnson NA, Raymond J, Burdon CA, O’Connor HT. (2011) Carbohydrate ingestion during endurance exercise improves performance in adults. J Nutr. 2011 May;141(5):890-7. Epub 2011 Mar 16.

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