Fluid Absorption Rates
Average fluid absorption rates range from 0.8 to 1.2 liters per hour (27.4 to 40.6 oz.). Unfortunately, while the sweat-rate range and the fluid-absorption ranges are close, some athletes sweat at higher rates per hour than their fluid-absorption rate. In short races, the rate discrepancy isn't much of a problem; however, for longer races the rate difference can lead to dehydration and decreased performance.
Nausea and vomiting can occur when athletes attempt to consume more fluids and fuel than their bodies can handle. Even if you manage to hold down the extra fluids, carrying beyond what your body needs is just unnecessary weight.
More: 15 Hydration Facts for Athletes
Champion Fluid Absorption Rates
I was unable to find any hard numbers for the maximum fluid-absorption rate documented in a laboratory. Most scientific literature suggests there is a range of absorption rates that varies from person to person. On the high end, I have worked with three people that can consume in excess of 41 oz. per hour.
On the low end, some people develop intestinal fullness, nausea and vomiting when ingesting fluid rates as low as 0.6 to 0.8 liters per hour.
Additionally, scientists have found there is a specific water carrier in the body (the water-channel protein, aquaporin) that influences fluid absorption. There is speculation that people without aquaporin--or very low levels of it--may have a reduced capacity to absorb fluid.
More: The New Rules of Hydration
Case Study
During my coaching career I've had the opportunity to work with three champion sweaters and a couple of athletes I would consider low-volume sweaters.
The best data I have for a champion sweater is a male triathlete that weighs around 185 pounds. He recently completed a 90-minute run at a very aerobic pace of 10 minutes per mile at a temperature range of 84 to 92 degrees Fahrenheit with 70 percent humidity. He consumed fluid containing electrolytes, water and some electrolyte tablets during the run. His fluid consumption rate was 2.84 liters (96 oz.) per hour. He still lost weight.
This particular athlete has tested and retested himself to determine his sweat rate and to determine how much fluid he can comfortably absorb. When he runs on a hot day, even when he is acclimated to the heat, he routinely sweats at a rate of 3.6 liters (121.7 oz.) per hour. He routinely consumes 2.84 liters (96 oz.) per hour. Even consuming fluids at a relatively high rate, he loses 26 oz. of fluid or about 1.7 pounds per hour. Up to about 3.7 pounds (roughly 2 percent of his body weight), the weight loss has minimal affect on his performance.
Your Numbers
If you are looking to crack the code on your sweat rate and related hydration rate, you need to start collecting data during training and racing sessions. Know that your sweat rate is not a single number, but changes depending on several factors not limited to your fitness level, ambient temperature, humidity, clothing, exercise pace, stress level and rest level.
More: How to Prepare for Racing in Hot Weather
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