Sports Nutrition for Performance – Carbohydrate before and during Exercise

In the last post I described a current theory of fatigue during exercise that seems to best explain what occurs during exercise. In the next series of posts, I’ll examine how you can use nutrition to delay fatigue and improve performance. In this first post, I’ll look at carbohydrate and how maximising what you can get during endurance exercise will improve performance. I’ll assume at this point that you have already read the posts on carbohydrate and fatigue. If not I’d strongly suggest you read these first.

Carbohydrate stores – performance limiting in endurance sports

You’ll recall from the previous post on fatigue that the body has an ability to adjust pace to avoid damage occurring to any parts. This happens unconsciously and even before the athlete feels they are under any threat. For sports with a duration of longer than 2 hours such as road cycling and mountain biking, marathon & ultra-marathon running, Olympic, Half Ironman and Ironman triathlon and adventure racing, carbohydrate stores will be one (but not the only) of the limiting factors to performance.

The ultimate failure of the body to preserve carbohydrate storage is hypoglycaemia, or low blood glucose levels. Hypoglycaemia occurs in stages though. Initially it’s lack of energy, known to athletes as hitting the wall, bonking or hunger flatting. If blood glucose reduces further athletes can experience difficulty focussing, blurred vision, and trembling. Severe hypoglycaemia is rare except in people taking medication to reduce their blood glucose (ie. people with diabetes), but can cause loss of consciousness.

Our blood glucose levels are controlled within a very tight range in healthy people. Glucose levels that are too high (hyperglycaemia) generally only occur in people with diabetes who have lost some or all the ability to reduce levels and rely on medication to achieve this. For the rest of us, excess glucose in the blood (from eating carbs) is stored into muscles and the liver as glycogen, the storage form of carbohydrate.

You’ll recall from previous posts that carbohydrate is the body’s preferred source of fuel during moderate and intense exercise. The main source of carbs in the body is the muscle glycogen stores. If these are used up, the only other sources of available to the body is blood glucose, which is topped up by the liver’s glycogen stores to prevent hypoglycaemia. However the liver’s stores are not large, and once muscle carbohydrate stores are exhausted it’s only a matter of time until all stores are depleted.

However this isn’t really what happens in most sporting situations. Applying the anticipatory model of fatigue, your brain will begin to intervene long before hypoglycaemia occurs. In other words, your brain will attempt to pace you in a manner that evenly uses carbohydrate over the duration of the exercise. So if you’ve paced yourself correctly, you’ll just finish a race before hypoglycaemia sets in. If you run out too early you’ll hit the wall and drop off the pace. Leaving some carbohydrate left in the tank at the end means you could have gone faster (unless heat or other factors have limited performance).

Carbs to enhance performance

So using this model of fatigue, we can see that the body’s storage of carbohydrate is important as a fuel source, and to prevent hypoglycaemia. Provided heat or other factors aren’t limiting, the amount of carbohydrate available to the body will be one of the key factors in determining how quickly you can pace yourself during endurance exercise. The more carbohydrate, the greater the “reserve”.

There are three ways we can use this to our advantage. Firstly, beginning exercise with the maximum possible storage of carbohydrate will provide the greatest possible reserve. This is why athletes carbohydrate load before endurance events. Carbohydrate loading is essentially eating enough carbohydrate to maximise the body’s glycogen stores. Carb loading was first described back in the 1960’s, and preferred method has gradually evolved since. Initially it was thought that glycogen stores would be maximised only by completely depleting them first, allowing a “rebound” effect. Athletes in the 1970s would follow very low carb diets about a week prior to competition and do plenty of vigorous training, to deplete stores. Three days before a race they would taper their training and switch to a very high carb diet.

But research in more recent times has shown that the depletion phase of carbohydrate loading is not necessary. It is now acknowledged that athletes simply need to taper their training whilst consuming a high carb diet for 1-2 days before an event to adequately load. A study from the Western Australian Institute of Sport in 2003 showed that just one day of carb loading could be as effective as two, provided the athlete does an exercise session just prior to the carb load. Now this isn’t everyone’s cup of tea the day before a race, so the two day strategy may be the preferred option. I’ll cover carb loading in more detail in a future post.

The second way we can use carbs to our advantage in endurance sports is eating and drinking them during exercise. By doing this we can offset some of the stored carbs being used as a fuel, and either prolong the time we tap into carbs as a fuel source (in fixed intensity exercise) or pace ourselves faster as we have more “reserve” at our disposal (in self-paced exercise).

Theoretically anything we eat or drink during sport can be a potential source of carbs. This includes sports drinks or any other sugar or starch containing drink, sports foods and regular foods. I won’t go into detail about the optimal types of foods and fluids during exercise, that’s another post in itself. But needless to say that when carbs stores are a limiting factor (sessions generally longer than 2 hours), the more carbs you can get in during exercise the better (within limits of physical tolerance).


So by reading the blog posts to date you will now know about how the body experiences and regulates fatigue during exercise, and the various fuel sources available to the body and its preferences. In this post we’ve covered how we can use carbohydrate to our advantage to help minimise the impact of fatigue on performance in endurance sports. In the next couple of posts we’ll start to look more specifically at the two strategies mentioned here – how to maximise carbohydrate stores through carb loading, and how to maximise carbohydrate intake during endurance exercise.


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