Since the 1980s it has been known that carbohydrate intake during exercise can improve exercise performance lasting two hours or longer. Soon after this discovery, it was established that not all carbohydrates are equal and carbohydrates ingested during exercise may be utilised at different rates. Recently it was also demonstrated that there is a dose response relationship between the amount of carbohydrate ingested and oxidized and performance during prolonged exercise. Therefore it is important to identify carbohydrate sources that are oxidised rapidly. The ideal carbohydrate during exercise is rapidly emptied from the stomach, does not need digestion or is digested very rapidly, is absorbed quickly and can immediately be used by the muscle. This blog is based on a large number of studies and references and the reader interested in more detail is referred to the review articles listed at the bottom of this blog, that are downloadable in their entirety free of charge.
Carbohydrate use from a drink is limited to 60 g/h
Carbohydrates ingested during exercise can be oxidised at a rate no higher than 1g/min (60g/h), independent of the type of carbohydrate. So even when you would ingest large amounts of glucose for example, say 100 g/h, no more than 60 g/h would be utilised. This is reflected in guidelines published by the American College of Sports Medicine (ACSM) from 2007–2009 which recommended that, during exercise lasting more than one hour, athletes should consume carbohydrate at hourly rates of between 30 and 60g. In one study, carbohydrate was ingested at rates up to 3g/min (180g/h) (A “don’t try this at home” intake), the oxidation of the ingested carbohydrate did not exceed 1g/min (60g/h).
Differences between carbohydrates
Some carbohydrate are faster than others but no carbohydrates can be oxidized at rates of approximately 60 g/h. The faster carbohydrates include glucose (grape sugar), sucrose (table sugar), maltose (milk sugar), maltodextrins and some starches. Also some starches can be oxidized at very high rates. Some carbohydrates that are slower include fructose (fruitsugar), galactose, trehalose and some starches (those that are not very well soluble in water).
Apparently the digestion of maltodextrins (a chain of 10-20 glucose molecules) is not limiting and neither is the breakdown of the very large chain of glucose molecules in some starches. Fructose and galactose, however, are absorbed slower and need to be converted in the liver first before they can be used by the muscle. Fructose, for example, is converted to glucose or lactate, both of which are very good sources of energy for the muscle, but the conversions slows down the process.
If fructose is ingested at high rates (and it is not ingested with another carbohydrate), it is known to result in gastro-intestinal discomfort. The same has been observed for galactose. During exercise, it is recommended to choose a carbohydrate that is rapidly oxidized, so it does not accumulate in the intestine.
Most carbohydrate drinks on the market aimed at the endurance athlete use glucose and sucrose as their main energy sources, others have used maltodextrins or starches that are rapidly used by the muscle. All of these carbohydrates are equally beneficial in terms of carbohydrate delivery.
Taste
Carbohydrates also differ in their taste: glucose, fructose and sucrose are examples of very sweet carbohydrates (sugars). Maltodextrins and starches have very low levels of sweetness and in some cases no detectable sweetness at all. This can become important when preparing your own home made drink that is customized to your taste buds.
Practical recommendation
The practical recommendations that rolls out of these studies are simple:
Use a carbohydrate that can be used rapidly (glucose, sucrose, maltodextrin or soluble starches)
You can buy a drink with one of these carb sources or make your own
You can get carbohydrate sources from drinks or gels (and even some solid sources if carefully selected).
Ingest no more than 70 g/h if you use a carbohydrate source that contains mostly one type of carbohydrate (recommendation 30-60g/h)
Avoid drinks that contain only fructose or galactose or other carbohydrates that are slowly oxidized
In the next blog I will discuss how we can overcome the limitation of 60 grams per hour, and how it is actually possible to get more carbohydrate into the body using specific carbohydrate mixes.
References
Open Access (free) review articles on this topic: