In this issue we will explore fluid-replacement drinks, also known as carbohydrate electrolyte drinks. They should not be confused with ergogenic-enhancement drinks, phosphate-buffering drinks, and carbohydrate-loading drinks, which will be discussed in future articles.
Fluid-replacement drinks are designed for three primary functions: hydration/rehydration; electrolyte/mineral replacement; and blood glucose/energy enhancements.
As we discussed last month, the athlete who is low on fluid has a distinct competitive disadvantage. Furthermore, the exercising athlete is only able to absorb one ounce of fluid every three or four minutes, while fluid losses can exceed one ounce per minute. Therefore, we must encourage our patient athletes to consume extra amounts of fluid before they become thirsty. Studies have shown that athletes will consume greater amounts of fluid if it tastes good. Fluid-replacement drinks contain various types of sugars and minerals that increase palatability.1,2,3 A fluid-replacement drink with good balance will increase the absorption of carbohydrates, electrolytes, and water.4 In my literature review, I came across exercise-induced hypotonic hyponatremias in athletes who are exercising vigorously and consuming only plain water or non-electrolyte solutions.5 Consumption of a good fluid-replacement drink can avoid this unfortunate circumstance.
There are many reasons for the inclusion of minerals and electrolytes in fluid-replacement drinks:
Sucrose:
It is this author's opinion that sucrose or simple sugar is not the optimal source of carbohydrates for a sports drink. As most of you know, it causes a sharp rise in the blood plasma levels of insulin, which in turn leads to a rapid decrease in blood glucose levels. When intestinal fluid uptake of a 3.6 percent glucose polymer, 1.8 percent fructose, 1.6 percent sucrose solution (7 percent total) was compared to equal amounts of plain water, net fluid absorption from the small intestine was significantly reduced in those subjects who consumed the 7 percent carbohydrate solution.12 In another study, a 7 percent solution consisting of 5 percent glucose polymer and 2 percent fructose emptied the stomach and was absorbed in the small intestine at rates very close to that of plain water.13 This is another example of the powerful affect even small amounts of sucrose can have on human physiology.
The obvious question is, what if sucrose is the only available source of carbohydrates for the athlete? Is sugar and water better than plain water? The answer depends on what the doctor needs to accomplish. When compared to an artificially-sweetened noncarbohydrate drink, the cycling athletes who consumed a dilute sucrose solution had increased endurance during their trial and a faster sprint performance at the end of the test.14 Thus, a dilute sucrose drink is better than plain water for energy enhancement. However, the athlete who is dehydrated or dehydrating should choose plain water over a water-sucrose combination.
Glucose
The stimulating effect of glucose on water absorption in the small intestine is a key mechanism in the advent of fluid-replacement drinks.15 Glucose also increases passive and active sodium transport in the small intestines.16 This is accomplished because free glucose can be used to generate energy in the gastrointestinal mucosal cells directly.17
Fructose
Fructose has many interesting effects in applications for fluid-replacement drinks. Fructose solutions leave the stomach faster than equal molar glucose solutions,18 but are absorbed more slowly than glucose from the small intestine.19 Fructose stimulates potassium absorption in the small intestine.20 Fructose also increases water absorption in the small intestine, but not as much as glucose.21 It is non-insulogenic and spares glycogen.22 Finally, fructose is sweeter than glucose, glucose polymers or sucrose, and as previously-referenced studies have noted, athletes will consume greater amounts of beverages that are pleasing to the palate.
Glucose Polymer
Glucose polymer or maltodextrin are medium chains of the monosaccharide glucose. They are larger than mono- or disaccharides, but smaller than a starch. It, therefore, has some of the best properties of simple sugars and starches. Glucose polymers clear the stomach faster than glucose.23 They also have a lower osmotic pressure than either fructose or glucose. This results in a decreased amount of intestinal secretion needed for its uptake. Glucose polymer ingestion maintains a greater plasma volume during exercise than simple carbohydrate drinks or plain water.24 This means that beverages with the right amount of glucose polymer are absorbed by the body better than plain water. The thinking of many people (this author included) that nothing was better for hydrating the athlete than plain water is now becoming obsolete.
Next month in Hydration, Part III, we will continue this series with a discussion of the optimal ranges of carbohydrates and electrolytes a good fluid replacement drink should contain.
G. Andersen, D.C.
Brea, California
References
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