Sport drinks

It is already mentioned that ingesting carbohydrate before, during, and following exercise can benefit performance by ensuring adequate fuel for energy production during exercise and for replenishing glycogen stores following exercise. While selecting a wise diet can provide for most of the athlete’s nutritional needs, nutritional supplements can also be of great value. In addition, adequate fluid intake is necessary for preexercise hydration, hydration during exercise, and rehydration following exercise. Sport drinks are uniquely designed to meet both the energy and fluid needs of the athlete. Performance benefits from these drinks have been clearly documented, not only in endurance activities, but in burst activities as well.

Composition of sport drinks

Sport drinks differ from one another in a number of ways besides in taste. Of major concern, however, is the rate at which energy and water are delivered. Energy delivery is primarily determined by the concentration of the carbohydrates in the drink, and fluid replacement is influenced by the sodium concentration of the drink.

Energy delivery – the carbohydrate concentration

A major concern is how rapidly the drink leaves the stomach, or the rate of gastric emptying. In general, carbohydrate solutions empty more slowly from the stomach than either water or a weak sodium chloride(salt) solution. Research suggests that a solution’s caloric content, a reflection of its concentration, might be a major determinant of how quickly it empties from the stomach and is absorbed in the intestine. Since carbohydrate solutions, increasing the glucose concentration of a sport drink significantly reduces the gastric emptying rate. For example, 400 ml(14oz) of a weak glucose solution(139 mmol/L) is almost completely emptied from the stomach in 20 min, but emptying a similar volume of a strong glucose solution(834 mmol/L) can require nearly 2h. However, when even a small amount of a strong glucose drink leaves the stomach, it can contain more sugar than a larger amount of a weaker solution simply because of its higher concentration. But, if an athlete is trying to prevent dehydration, this would deliver less water and thus be counterproductive.

Most sport drinks on the market contain about 6 to 8g of carbohydrate per 100ml( 3.5oz) of fluid(6% to 8%). The carbohydrate source is generally glucose, glucose polymers, or a combination of glucose and glucose polymers, although fructose or sucrose has also been used. Research studies have confirmed enhanced endurance performance with use of solutions in this range of concentration and with these sources of carbohydrates when compared to water. Carbohydrate solutions above approximately 6% slow gastric emptying and limit the immediate availability of fluid. However, they can provide a greater amount of carbohydrate in a given period of time to meet the increased energy needs.

Rehydration with sport drinks – the sodium concentration

Just adding fluid to the body during exercise lessens the risk of serious dehydration. But research indicates that adding glucose and sodium to sport drinks, aside from supplying an energy source, stimulates both water and sodium absorption. Sodium increases both thirst and palatability of the drink. Recall that when sodium is retained, this causes more water to be retained. For rehydration purposes, both during and following exercise, the sodium concentration should range between 20 mmol/L and 60 mmol/L. There is an important loss of sodium from the body with sweating. With high rates of sweating and large volumes of water intake, this can lead to critical reductions in the sodium concentration of the blood and possibly lead to hyponatremia.

What works best?

Athletes will not drink solutions that taste bad. Unfortunately, we all have different taste preferences. To further confound the issue, what tastes good before and after a long, hot bout of exercise will not necessarily taste good during the event. Studies of taste preferences of runners and cyclists during 60 min of exercise showed that most chose a drink with a light flavor and no strong aftertaste. But, will athletes drink more if given a sport drink as compared to water? In one study, runners ran on a treadmill for 90 min and then recovered while seated for an additional 90 min. Both exercise and recovery conditions were controlled in an environmental chamber at a temperature of 32°C(86°F), 50% humidity. Three trials were conducted, two with two different sport drinks(6% and 8% carbohydrate) and one with water. Subjects were encouraged to drink throughout each trial. The volume consumed during exercise was similar for all three drinks; but during recovery, the runners drunk about 55% more of each of the two sport drinks than water.