Not All Carbs Are Equal
In this article we explore the differences between different types of carbohydrates including fast and slow carbs.
There are however many studies that compared the use of carbohydrates ingested during exercise. Knowing this may is important because it was demonstrated that there is a dose response relationship between the amount of carbohydrate ingested and oxidised and performance during prolonged exercise. Therefore, it is important to identify carbohydrate sources that are used rapidly (we refer to this as exogenous (ingested) carbohydrate oxidation). 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.
Carbohydrate use from a drink is limited to 60 grams per hour
Many studies have shown that carbohydrates ingested during exercise can be used at a rate no higher than 1gram per minute (60 grams per hour). This is independent of the type of carbohydrate: some are oxidised much less than 60 grams per hour, but no carbohydrate is oxidised at rates that exceed this. So even when you would ingest large amounts of glucose for example, say 100 grams per hour, no more than 60 grams per hour would be utilised. This is why many guidelines such as those published by the American College of Sports Medicine (ACSM) recommended athletes should consume carbohydrate up to 60 grams per hour. In one study, carbohydrate was ingested at rates up to 3g/min (180 grams per hour) (A “don’t try this at home” intake), the oxidation of the ingested carbohydrate did not exceed 1 gram per minute /min (60 grams per hour).
Differences Between Carbohydrates
Some carbohydrate are faster than others. The faster carbohydrates include glucose (grape sugar), sucrose (table sugar), maltose (milk sugar), maltodextrins and some starches.
Misconceptions about complex carbs and sugar
Some complex carbohydrates (starches) can be oxidised at very high rates. So, it is a general misconception that complex carbohydrates are slow and simple carbohydrates are fast. Some carbohydrates, including simple sugars are slow. This includes fructose (fruit sugar), galactose and trehalose. There are also starches (those that are not very well soluble in water) that are slow. Thus, in general, we can divide carbohydrates in two categories: those oxidised at high rates and those used at low rates. 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. Some slower carbohydrates are limited because their digestion is slow, some because their absorption is slow and others because their conversion to a fuel the muscle can use is slow.
Fructose
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 oxidised, so it does not accumulate in the intestine.
Rapid vs. Slow Carbs
Most carbohydrate drinks on the market use glucose and sucrose as their main energy sources, others have used maltodextrin or starches that are rapidly used by the muscle. Some specifically use slow carbohydrates with the claim that this gives “sustained” energy. This is true if you only have one opportunity to ingested carbohydrates, but in most races or training where you can multiple feeding opportunities it is better not to let too much undigested or unabsorbed carbohydrate sit in your stomach or intestine.
Carbohydrates with Different Taste Profiles
Carbohydrates also differ in their taste: glucose, fructose and sucrose are examples of very sweet carbohydrates (sugars). Maltodextrin and starches have very low levels of sweetness and in some cases no detectable sweetness at all.
Practical Recommendations
The practical recommendations that resulted from these studies are simple:
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Use a carbohydrate that can be used rapidly (glucose, sucrose, maltodextrin or soluble starches)
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You can deliver carbohydrates fast from sports hydration drinks such as our C30 Drink or energy gels like the C30 Gel (and even some solid sources if carefully selected).
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Ingest no more than 60 grams per hour if you use a carbohydrate source that contains mostly one type of carbohydrate (recommendation 30 to 60 grams per hour).
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Avoid drinks that contain only fructose or galactose or other carbohydrates that are slowly oxidised.
But, if you want to deliver more carbohydrate and make carbohydrate intake during exercise even more effective, read our article entitled "What Are Multiple Transportable Carbohydrates?" It discusses why our bodies can't use more than 60 grams of carbohydrate per hour and comes up with a solution that results in higher oxidation rates and better performance.
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Related Articles
What Should I Eat on the Morning of My Race?
Drinks, Gels, Bars or "Real Food?" What Should I Use on Race Day?
References
Open Access (free) review articles on this topic:
Jeukendrup A. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Med. 2014 May;44 Suppl 1:S25-33.
Jeukendrup AE. Nutrition for endurance sports: marathon, triathlon, and road cycling. J Sports Sci. 2011;29 Suppl 1:S91-9
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