According to the American College of Sports Medicine, or ACSM, the average individual can sweat at a rate exceeding 30 g per minute during bouts of activity. According to a 1991 study by D.L. Costill, fluid loss, through sweat and/or exhalation, especially during endurance events such as marathons or long-distance cycling, can result in a loss of body water content by 13 to 14 percent. You must adequately restore the water and minerals lost during hot-weather exercise or you will experience serious declines in performance and exercise tolerance.
Why Water Matters
During exercise or activity the body needs fluids to regulate body temperature. Sweat acts as the body's coolant during exertion to dissipate the heat produced as a byproduct of energy production. The body is approximately 60 percent water, soproper hydration before and during activity is critical. Unfortunately, most people wait until they are thirsty to begin drinking, which is often too late to adequately address dehydration.
How the Body Absorbs Fluids
The body primarily absorbs fluids in the gastrointestinal tract. Upon drinking, fluids enter the stomach and then are emptied into the intestinal tract for absorption by the blood. Marie Dunford and J. Andrew Doyle, authors of "Nutrition for Sport and Exercise," report that for the average adult-sized male at rest, the maximal gastric emptying rate is between 1.0 L and 1.5 L per hour. The rate at which the stomach empties is determined by a number of factors, including the fluid volume, temperature and acidity of the liquid and the external environment of the individual. Additionally, Tim Noakes, MD, states in the "Lore of Running" that fluids containing carbohydrates and/or electrolytes, including glucose, sucrose or maltodextrin, empty at a much slower rate from the stomach both at rest and during exercise
How Exercise and Heat Affect Absorption
A 1991 study by D.L. Costill showed that low- and moderate-intensity exercise, or exercise performed at less than 70 to 80 percent of your VO2 max, does not cause the stomach to empty any slower or faster than what is experienced at a resting state. However, intense exercise, at a level above 80 percent of VO2 max, causes gastric emptying to significantly slow down.
According to Noakes, studies show that severe environmental conditions -- ambient air temperatures greater than 95 degrees Fahrenheit or relative humidity that is greater than 20percent -- severely impair gastric emptying from the stomach.
How to Maximize Absorption
One important way that you can maximize the body's ability to uptake liquid is to drink constantly. According to Noakes and the ACSM, the stomach can achieve higher rates of gastric emptying because it remains in a distended state when fluid is constantly ingested. Research indicates that ingesting fluids on regular intervals of at least 10 minutes keeps the stomach distended without distressing the body system, allowing for maximal absorption of water and other carbohydrate-based beverages, maintaining and preserving athletic performance, especially in hot and humid conditions.
References
- "Lore of Running"; Tim Noakes; 2001
- "Proceedings of the Nutrition Society"; Water and Mineral Balance During Exercise; D.L. Costill; 1991
- "Amercian College of Sports Medicine"; Position Stand, Exercise and Fluid Replacement; Victor Convertino,et al.; January 1996
- "Nutrition for Sport and Exercise"; Marie Dunford, et al.; 2008
