Studying Sports Drinks

Sports drinks originally were developed and marketed for athletes exercising for long periods of time in heat and humidity. Their benefits are well documented: Consuming sports drinks during exercise helps athletes (and the average consumer) work out longer, and helps decrease heat-related problems.

Studies associate as little as a 2% drop in body water with fatigue and decreased performance. Yet the volume of water most athletes drink when exercising is estimated at less than one-half of their body-fluid losses. During prolonged exercise in the heat, they can become dehydrated at a rate of 1 to 2 liters per hour, about 2 to 4 lbs. of body weight loss per hour.

Even "stop-and-go" competitors, like soccer and basketball players, benefit from sports drinks. According to Research Quarterly for Exercise and Sport (1988, 59: 144-147), drinking carbohydrates before and during ice hockey reduced muscle glycogen use per unit of distance skated, and is especially beneficial when several games are played in a day.

Ingesting carbohydrates along with fluids during strenuous exercise maintains blood glucose concentrations and optimizes energy to the working muscles from blood glucose and glycogen stored in muscles. Early studies on sports drinks concluded that consuming 30 to 60 grams of carbohydrates per hour of exercise improved performance. In most studies, carbohydrate solutions of up to 8% don't hamper gastric emptying. Some athletes may perform better with a higher concentration, and others may handle less better. However, most benefit from a 6% to 8% concentration. Consequently, many sports drinks target 14 to 19 grams of carbohydrates per 8-oz. serving.     

One study determined that the concentration and mix of carbohydrates are more important than the osmolality for maximum fluid absorption. A study in Medicine & Science in Sports & Exercise (1995, 27: 1607-1615), concluded that solutions with several carbohydrate forms, such as sucrose, glucose and fructose, optimize fluid absorption. Another study shows that, at rest, a 6% sucrose or combined sucrose and glucose solution empties from the stomach and moves into the intestines as fast as water. Drinks with 6% carbohydrate content with glucose or maltodextrins as the carbohydrate source emptied more slowly. In this study, published in Medicine & Science in Sports & Exercise (1994, 26: 725-732), the osmolality of the drinks didn't have a major effect on the rate of gastric emptying. Fructose and high-fructose corn syrup take longer to yield glucose, and too-high a level may cause digestive problems.  

Protein may enhance endurance and aid muscle recovery. Branched-chain amino acids have potential in reducing fatigue, and glutamine may help over-trained athletes' immune systems. Although it's clear that protein is essential in recovery drinks, protein in sports drinks is an area for further research and debate. A study in Medicine & Science in Sports & Exercise (2003, 35: S211) tracked eight cyclists completing randomized trials consuming an 8% carbohydrate, 8% carbohydrate plus 2% amino acid, or 10% carbohydrate drink. The amino acids failed to enhance performance of a time trial at the end of cycling. Branched-chain amino acids or a placebo also were given to subjects during and after a one-hour ergometer cycle exercise, with no benefit noted during the exercise, according to the American Journal of Physiology (2001, 281: E365).

During exercise, minerals such as sodium, potassium, chloride, calcium, phosphate and magnesium are lost through body fluids. The main concern is sodium: Most people easily replace the minimal losses of other minerals via the diet. But including sodium in a drink encourages athletes to drink more by keeping the thirst mechanism active for better hydration. Also, sodium is key to preventing muscle and heat cramps during exercise. Reports indicate some "crampers" could lose as much as 11 grams of sodium while exercising in heat and humidity. Although that level can't be replenished in a sports drink alone, it's crucial for athletes to replenish fluids with drinks containing sodium and eat foods with sodium for recovery.      

Hyponatremia, or low blood-sodium levels, is a dangerous condition from drinking too much water, causing an imbalance in fluid-electrolyte levels. In endurance events, it's essential athletes drink fluids containing sodium, especially for those middle- or end-of-the-pack runners, who have more opportunities to drink water and can over-hydrate.  

Vitamins A, C, and E, green tea extract, caffeine, and chromium are added in some beverages to give athletes an edge. Some athletes may want extra insurance they are getting more nutrients. Although many nutrients are unproven as performance enhancers, some people perceive more as better. A study in Medicine & Science in Sports & Exercise (Murry et al, 1995, 27: 1057-062) found that adding nicotinic acid to water or a carbohydrate-electrolyte beverage didn't enhance cyclists' ability to perform high-intensity exercise. Still, some athletes equate B vitamins with burning energy, seeing it as a plus.  

Years of solid research confirm that maintaining optimal hydration during exercise enhances performance and decreases the risk of heat-related illnesses. Although the Feb. 11, 2004 report by Washington, D.C.-based Institue of Medicine emphasizes that the majority of healthy people adequately meet daily hydration needs by "letting thirst be their guide," it's not as simple for athletes. The National Athletic Trainers' Association, Dallas, noted that thirst isn't optimal to determine how much an athlete should drink. Furthermore, athletes should base the replacement of fluids and electrolytes on individual needs.

Susan Kundrat, M.S., R.D.,the owner of Nutrition on the Move, a sports and wellness consulting business in Urbana, IL, is also a part-time outreach dietitian with the University of Illinois Functional Foods for Health Program. Her new sports-nutrition tip book published by Healthy Learning will be out this summer.

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