This article is an excerpt from Sports Nutrition for Endurance Athletes by Monique Ryan MS, RD, LDN, CSSD. In her comprehensive guide to sports nutrition, Ryan uses her 30 years of experience coaching professional and age-group athletes to simplify this complex subject into proven, real world guidelines. Sports Nutrition for Endurance Athletes shows runners, cyclists, and triathletes how to address specific nutritional needs for short- and long-course racing and busts dozens of myths and misconceptions along the way.
Much research has gone into the development of sports drinks in an effort to come up with the ideal fluid for athletes to consume during their workouts. The basic theory—￼￼￼￼￼￼￼￼￼￼￼￼￼ emptying from your stomach quickly, entering your small intestine, and then getting absorbed into the bloodstream. The carbohydrates from the drink are then available to maintain blood glucose levels, supply energy to exercising muscles, and replenish muscle fuel stores. The fluid from your sports drink also maintains blood volume and offsets sweat losses.
In an effort to develop scientifically sound formulations, sports drink researchers have looked at each of the steps in digestion and absorption to see where the flow may actually slow down or speed up. If your sports drink gets bogged down in any of these steps, it takes the fluid and carbohydrates that much longer to reach your bloodstream and muscles. Consequently, the sports drink is not as performance-enhancing as it could be. Researchers have concluded that two main factors determine how quickly a drink leaves your stomach: the concentration of carbohydrates in the drink and the volume of fluid in your stomach.
Finding the ideal formulation for a sports drink involves striking a delicate balance. Al- though researchers agree that carbohydrates are essential ingredients, they have had to experiment to determine how many carbohydrates to include in the mix. Research has determined that a concentration of about 6 to 8 percent carbohydrates empties from the stomach very efficiently. Thus, most sports drinks fall into this range. This drink concentration allows fluid to reach your bloodstream pretty much as quickly as water but still deliver performance-boosting carbohydrates. Drinks of this type also seem to be well tolerated by most athletes, and tolerance to these products can improve with practice. In contrast, drinks of 10 to 12 percent carbohydrates, such as soft drinks and fruit juices, empty more slowly from the stomach. Although they do provide carbohydrates and energy, these drinks are not as hydrating as sports drinks and should be diluted if they are consumed during exercise.
The volume of fluids in your stomach affects the degree of stomach distention and consequently affects gastric emptying. Increased stomach distention will result in liquids emptying from your stomach more quickly, with about 50 percent of stomach contents being emptied every 10 minutes. Too, the more dehydrated an athlete becomes, the slower the rate of gastric emptying. Once an athlete reaches a very slow rate of gastric emptying, attempting to drink more fluid only exacerbates the condition and can even result in ￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼￼nutritional upheaval, such as vomiting, particularly during very demanding races like the Ironman. Of course, mental stress can have a significant impact upon the gastrointestinal system. Being keyed up for an important training session or race can also slow down the rate of gastric emptying.
To maximize emptying, start exercise with a comfortably full stomach and drink at regular intervals of about 10 to 15 minutes, if possible. How much fluid an athlete can empty from his or her stomach is highly variable. However, emptying rates of 24 to 40 ounces, or 3 to 5 cups (720–1,200 ml), per hour are commonly seen. The most important strategy is to consume fluid whenever the opportunity presents itself. Aim to consume at least one 20-ounce (600 ml) squeeze bottle for every hour of a workout. Refill your bottle as needed whenever possible, depending on the duration and nature of the training session, and the sweat rate that you have calculated in various conditions.
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See what to eat and when with Sports Nutrition for Endurance Athletes. Ryan demystifies optimum daily nutrition and shows simple steps to make the best decisions about what you eat and drink.