There are different ways to classify carbs. Chemically, they can be mono, di-, or poly-saccharides. Monosaccharides consist of a single sugar molecule. There are 3 we get in our diet - glucose, fructose, and galactose. Disaccharides are combinations of these single sugars and include sucrose (1 glucose + 1 fructose), lactose (1 glucose + 1 galactose), and maltose (1 glucose + 1 glucose). Collectively, these are also referred to as 'simple sugars'. Starches are polysaccharides that consist of several glucose molecules. They are therefore considered complex carbs.
It's interesting to point out that the carbohydrate source in nearly all sports and energy drinks commercially available is simple sugar. Check out the label on any of the popular brands and you'll likely see one or more of the following simple sugars: glucose, fructose, high fructose corn syrup, and dextrose. With the exception of fructose (more on this in subsequent articles), simple sugars are digested and absorbed quickly. Many formulations now use maltodextrin, which is technically a complex carbohydrate or "glucose polymer" consisting of anywhere from about 3 to 20 glucose units loosely bound together to form one molecule. Maltodextrin allows for provision of more carbohydrate with less osmolality (more on this later), but it behaves much like glucose in terms of its rate of absorption into the blood. In the last few years there has been another form of carbohydrate introduced into sports drinks that is truly a complex carb - it's called waxy maize. Waxy maize is a starch derived from corn with a high content of a particle starch called amylopectin. It's essentially a very long string of glucose molecules bound together in a branching pattern. Sort of like glycogen for plants. In some cases these are huge molecules with molecular weights greater than 500,000 g/mol. Compare that to glucose at 180 g/mol.
Digestion Rate and Glycemic Index
Glycemic index (GI) is a method to rate carbohydrate containing food based on their acute glycemic impact. A standard amount of carbohydrate is fed to a group of people, and serial blood draws are obtained over a 2 hour period. The blood glucose area under the curve is calculated and compared to a standard (usually white bread or glucose). A food with a high GI increases blood glucose fast whereas a low GI results in a muted blood glucose response. The main determinant of the GI is the rate of digestion and movement through the gastrointestinal tract. You might think that simple carbs are digested fast and complex ones slow. However, it's not that straight forward. Several complex carb (eg, potatoes) actually have a very high GI and simple carbs (eg, fructose) have a low GI. For the purposes of sports drinks, most contain fast acting carbs with a high GI. It's hard to predict the GI of many of the new waxy maize products because of the different preparation methods that have a significant impact on how the starch is digested and absorbed. Some are marketed as fast-acting and probably do have a fast absorption (high GI) whereas other clearly have a low GI.
Fast Versus Slow Carbs Before Exercise
When it comes to selecting a sports drink for pre-exercise use, a key practical question is whether you should look for a fast or slow acting carb source. As I have said, the majority of sports drinks contain fast acting carbs. The rationale is to provide a quick hitting source of glucose for fast energy. But what does the research show? Studies comparing low to high GI carbs before exercise have found that low GI carbs consistently impact metabolism during exercise including attenuated glucose and insulin responses, increased circulating fatty acids and glycerol, increased fat oxidation, decreased reliance on carbohydrate fuel sources, muscle and liver glycogen sparing, and increased hepatic glucose production late in exercise as athletes approach fatigue. Several studies have also noted that low GI carbs result in significant increases in exercise performance (1-5).
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Man and Machine
In examining the physiology of slow release carbs for sport performance, the automotive metaphor is hard to resist. Think about gas for your car. You wouldn't want to empty the entire gas tank into the engine all at once. Bad news for performance. But that's what happens if you ingest a fast acting carbohydrate before or during exercise. There are only about 1-2 teaspoons of sugar in your blood and often times sports drinks can contain 5-10 times this amount of carbohydrate. So if that carbohydrate gets absorbed quickly into the blood it's like dumping your entire gas tank into the combustion chamber - its overkill. You might say but what's wrong with having extra glucose in the blood, especially if it will end up being used by the muscle eventually? The principle concern with rapidly elevating glucose is that it stimulates insulin secretion. Insulin does many things in the body, but one of its most potent effects (well within the physiologic range of insulin values) is that it potently inhibits fat breakdown and fat oxidation. Fat represents the largest reservoir of energy for an athlete since they can only store a limited amount of glycogen. Since show release carbs do not increase insulin to the same extent, they allows for a larger percentage of fuel to come from fat while still maintaining glucose levels and likely sparing muscle glycogen. Circling back to the automotive analogy, taking a slow release carb is like a hybrid car that can run on electricity (fat), but has gas (glucose) as a fuel source as well.
Most sports and energy drinks consist of simple sugars designed to provide rapid increases in glucose. Research however points to lower glycemic impact carbs as having a metabolic and performance edge. It may be worthwhile to rethink the paradigm that a spike in blood glucose and insulin is necessary or even desired before exercise. The same logic may apply to consuming carbs during and recovery after exercise, but more on that in future articles.
- Thomas et al. Carbohydrate feeding before exercise: effect of glycemic index. Int J Sports Med. 1991 Apr;12(2):180-6.
- DeMarco et al. Pre-exercise carbohydrate meals: application of glycemic index. Med Sci Sports Exerc. 1999 Jan;31(1):164-70.
- Kirwan et al. Effects of moderate and high glycemic index meals on metabolism and exercise performance. Metabolism. 2001 Jul;50(7):849-55.
- Wu et al. A low glycemic index meal before exercise improves endurance running capacity in men. Int J Sport Nutr Exerc Metab. 2006 Oct;16(5):510-27.
- Chen et al. Effect of preexercise meals with different glycemic indices and loads on metabolic responses and endurance running. Int J Sport Nutr Exerc Metab. 2008 Jun;18(3):281-300.