Carbohydrates During Exercise: How to Improve Performance

Consuming carbohydrates (either in liquid or solid forms) is proven to enhance endurance exercise performance.

In this article, we look at how carbohydrates improve exercise performance, reduce muscle breakdown and help to preserve immune function.

Key points:

When we exercise at moderate intensities (~60-80% VO2max), 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 can sustain an intensity that’s ~80% VO2 max during the marathon.

Carbohydrates help to delay fatigue by raising blood sugar levels and protecting valuable muscle glycogen stores. This allows exercise to continue for longer and at higher intensities, before fatigue occurs.

Carbohydrates can also help to preserve immune function, and reduce muscle breakdown, during prolonged exercise.

How Carbohydrates Improve Exercise Performance

During prolonged exercise your body starts to breakdown muscle and liver glycogen stores. We need this both to fuel your working muscles and to maintain normal blood glucose levels.

As exercise continues, the levels of muscle and liver glycogen deplete. And as glycogen depletion proceeds, this reduces carbohydrate oxidation, increases levels of fatigue and impairs endurance exercise performance (Coyle et al., 1986).

Interestingly, research indicates 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 several 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) identified that consuming 30-80g of carbohydrates (6-8% solution) increases time trial performance (~2%) and time to exhaustion (~15%).

How many carbs should you consume during endurance exercise?

Current research indicates that we should consume around 30-80g/hour of carbohydrate (6-8% solution) to increase endurance exercise in events of > 1 hour (Temesi et al., 2011; Rodriguez et al., 2009; Sawka et al., 2007).

This becomes increasingly significant as exercise duration increases. And is especially important during 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 energy gels and energy drinks, then you must account for the carbs in both the gels and the energy drink. My advice would be that if your using energy gels, then hydrate with water rather than using energy drinks.

For this to be effective, we should use it with an effective carbo-loading strategy.

Other benefits of carbohydrates during exercise

Besides the performance benefits, there are other reasons why carbohydrates are beneficial during training and competition.

Carbohydrates and immune health

It’s accepted that endurance events — like marathons, triathlons, duathlons, and cycling events — place a significant amount of stress on the immune system. This increases the risk of illness — particularly upper respiratory tract infections. And after prolonged events like ultra-marathons, ironman competitions, your risk of illness can increase significantly.

By ingesting carbohydrates, you reduce the level of stress placed on your immune system. Helping to reduce your risk of infection and improving your general health and wellbeing.

Why does endurance exercise affect immune health?

This is mainly because of the way it lowers blood sugar levels. When blood sugar levels drop, your brain responds by increasing the levels of two key hormones:

Cortisol
Epinephrine

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

When we experience elevated cortisol 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 exercise is muscle damage:

Muscle membranes can become damaged
Individual muscle cells can become damaged and break down
We also break muscle proteins down for use as energy within aerobic metabolism.

Importantly, the level of muscle breakdown increases when blood sugar levels drop and cortisol levels rise.

How does cortisol affect muscle breakdown?

First, levels of cortisol become elevated during periods of glycogen depletion and when blood sugar levels are low. This increase in cortisol helps to preserve muscle glycogen stores and maintain blood sugar levels.

Second, cortisol achieves this by increasing the availability of fatty acids and amino acids, for aerobic metabolism.

With amino acids, cortisol increases muscle catabolism (muscle breakdown). And this increases the amount of amino acids in the blood, which we can then use these during aerobic metabolism.

As mentioned, we can reduce the negative effects of cortisol by consuming carbohydrates to help maintain 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 enhance 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. We should use these in combination with adequate hydration.

Energy drinks: consume ~500-600ml/hour of 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.

That said, sometimes it’s beneficial to train in a fasted state:

Why limit carbohydrate ingestion during aerobic training?

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

Despite the obvious benefits, sometimes we choose not to consume carbohydrates during training.

So why is that?… Simply put, this may diminish some benefits of aerobic training, and in particular fat metabolism.

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.

Summary:

The ingestion of carbohydrates can have a significant effect on exercise performance
During prolonged or intense exercise, the body breaks down and depletes muscle glycogen stores. Eventually this reaches a point of reduced carbohydrate availability. And may negatively affect fat metabolism, increase rates of fatigue, and increase muscle protein breakdown.
By ingestion carbohydrates, we 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 especially during events of 2 hours or more.

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.