Carbohydrates During Exercise

Consuming carbohydrates (either in liquid or solid forms) during high intensity or prolonged exercise, is known to improve endurance exercise.

During exercise at moderate intensities (~60-80% VO2max) the consumption of carbohydrates can delay the point of fatigue by around 15-30minutes.  You can appreciate the importance of this when you consider that well trained endurance runners are able to maintain an intensity that’s ~80% VO2 max during the marathon.

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 your body starts to breakdown muscle and liver glycogen stores. This is needed both to fuel your working muscles and to maintain normal blood glucose levels.

As exercise continues, the levels of muscle and liver glycogen begin to deplete. And as glycogen depletion continues, this causes a reduction in carbohydrate oxidation, increases levels of fatigue and starts to impair endurance exercise performance (Coyle et al., 1986).

Interestingly, research suggests that brain glycogen levels also deplete following prolonged endurance exercise. And this may be a factor in the central fatigue mechanisms that inhibit exercise performance (Matsui et al., 2011).

The good news is, we can protect against this by consuming carbs, directly before, and during exercise.

Carbohydrates preserve glycogen levels and delay fatigue

Ingesting carbohydrates during exercise has been shown to have a number of benefits for endurance athletes:

  • Preserves valuable muscle and liver glycogen levels,
  • Delays the point of fatigue,
  • Protects against low blood sugar levels (hypoglycaemia)
  • Improves endurance exercise performance (Coggan and Coyle, 1988; Coyle et al., 1986).

A meta-analysis (Temesi et al., 2011) found that consuming 30-80g of carbohydrates (6-8% solution) improves both time trial performance (~2%) and time to exhaustion (~15%).

How many carbs do you need to consume during endurance exercise?

  • Current research suggests that consuming around 30-80g/hour of carbohydrate (6-8% solution) improves endurance exercise in events of > 1 hour (Temesi et al., 2011; Rodriguez et al., 2009; Sawka et al., 2007).
  • The benefits of consuming carbohydrate becomes increasingly important as exercise duration increases. And is particularly important, during prolonged endurance events lasting 2 or more hours.
  • In practical terms this means consuming 2-3 energy gels/hour during endurance events lasting longer than 60minutes.
  • It’s important to remember that if you’re using both energy gels and energy drinks, then you’ll need to account for the carbs in both the gels and the energy drink. My recommendation would be that if your using energy gels, then hydrate with water rather than using energy drinks.

For this to be effective, it should be used in conjunction with an effective carbo-loading.

Other benefits of consuming carbohydrates during exercise

In addition to the performance benefits, there’s some other reasons why consuming carbs can be beneficial during training and competition.

Carbohydrates and immune health

It’s known that endurance events – like marathons, triathlons, duathlons and cycling events – place a large amount of stress on the immune system. This increases the risk of illness – particularly upper respiratory tract infections. And after prolonged events like the marathon, triathlon or ironman competition your risk of illness can increase significantly.

When you ingest carbohydrates during prolonged training sessions or competitions, you reduce the level of stress placed on your immune system. Helping to reduce the risk of infection and improving your general health and well being.

Why does endurance exercise affect immune health?

The negative effects of endurance training on immune health is mainly due to the way it lowers blood sugar levels. When blood sugar levels decrease, your brain responds by increasing the levels of two key hormones (cortisol and epinephrine).

Whilst, these two hormones play a role in elevating blood sugar levels, they can have negative effects on immune function. Especially when levels remain elevated for prolonged periods.

When Cortisol is elevated for sustained periods, it can have a negative effect on immune health, by lowering the numbers of lymphocytes – very important white blood cells that fight invading pathogens.

We can protect against this, by maintaining blood sugar levels during endurance training and competitions.

Carbohydrate and protein breakdown during aerobic exercise

Another problem associated with endurance training and racing is muscle damage:

  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.

Importantly, the level of muscle damage can increase when blood sugar levels drop and cortisol levels rise.

So, why does blood sugar and cortisol affect muscle breakdown? The main role of cortisol during aerobic exercise, is to preserve muscle glycogen stores and maintain blood sugar levels. Cortisol does this by increasing the availability of fatty acids and amino acids, for aerobic metabolism.

In the case of amino acids, cortisol increases the muscle catabolism (muscle breakdown), which increases the amount of amino acids in the blood. And these can then be used during aerobic metabolism. In fact, cortisol is known to become elevated during periods of glycogen depletion and when blood sugar levels are low.

As mentioned, the negative effects of cortisol can be reduced by maintaining blood glucose levels during prolonged endurance events and training.

How to improve endurance, protect the immune system and reduce muscle breakdown

Endurance athletes competing in events of >1hours duration can increase endurance exercise performance, preserve immune function and reduce muscle breakdown, by ingesting either energy gels or a carbohydrate energy drink:

Energy gels: aim to consume 2-3 energy gels per hour during prolonged endurance events. This should be used in combination with adequate hydration

Energy drinks: consume ~500-600ml/hour of an energy drink containing ~6% carbohydrate solution.

Carbohydrates during longer training sessions:

Consuming carbohydrates during longer endurance training sessions can help to reduce muscle breakdown. And may help to maintain normal immune function.

Having said that, sometimes it can be beneficial to train in a fasted state:

Why you might want to limit carbohydrate ingestion during aerobic training

As we’ve seen, carbohydrates can enhance exercise performance, reduce muscle breakdown and preserve immune function.

Despite the clear benefits, it’s not always beneficial to consume carbohydrates during training. So, why is that? Simply put, this may reduce some of the benefits of aerobic training, and in particular fat metabolism. The main reason for this, relates to how consuming carbohydrates increases carbohydrate metabolism. And this may compromise the training benefits of long endurance training sessions, where the primary aim is to develop muscular endurance, efficiency and fat metabolism.

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, which can negatively affect fat metabolism, increase rates of fatigue and increase muscle protein breakdown.
  • By ingestion carbohydrates during aerobic exercise, you can maintain blood sugar levels, preserve muscle and liver glycogen stores, reduce muscle breakdown, lower cortisol levels, maintain immune function and enhance exercise performance
  • Current research suggests that athletes should consume 30-80g carbohydrate/hour. This can either be in a 6-8% solution, or consumed as gels.
  • This is beneficial in events of >1hour but is particularly beneficial during events of 2 hours or more.

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.