By Roberta H. Anding, M.S., Baylor College of Medicine and Texas Children’s Hospital

Edited by Kate Findley, The Great Courses Daily

With rising outdoor temperatures has come an increase in heat-related deaths among professional athletes and casual exercisers alike. That is why it’s vital to monitor your fluid intake while exercising outdoors, particularly on a hot day. Professor Anding explains how.

Calculating Sweat Rate

During an intense workout, it is important to stay hydrated. Professor Anding recommends drinking gatorade if your workout is more than 30 minutes and you’re in a hot, humid environment. Another thing you can do to avoid dehydration while you exercise is to calculate your sweat rate. 

Your sweat rate can be calculated based on how many pounds you lose during a physical activity. For each pound you lose immediately after exercising, you’ve lost an additional 16 ounces of sweat. 

To rehydrate, you need anywhere between 16 and 24 ounces of extra fluid. You may be wondering why you can’t just replace 16 ounces of sweat with 16 ounces of water. The reason is that some of that water is going to be lost in urine, and what you need to do is rehydrate the body’s muscles. 

For example, suppose a football player loses 10 pounds in a morning practice. How much sweat did he lose? The answer is more than a gallon of sweat, at 10 pounds times 16 ounces per pound, which equals 160 ounces of fluid. 

This example illustrates why the general recommendation of drinking 64 ounces, or eight eight-ounce glasses of water per day, is not a one-size-fits-all solution. If this particular individual only drinks that much water per day, he’s going to become significantly dehydrated.

Signs of Dehydration 

Dehydration is characterized by rapid weight loss (as shown by the example of the football player losing 10 pounds during practice), confusion, dry skin that’s hot to the touch, and possibly an elevated core body temperature. Most of us are not going to measure core body temperature because it requires a rectal thermometer, so knowing that information would involve a medical evaluation.

For many athletes who have died of heat-related illnesses, reports in the media have stated that their core body temperature at the time of death was 106 degrees Fahrenheit. One woman died hiking in the mountains in California, and her core body temperature at the time of death was 110 degrees, which is incompatible with life. 

In a hot climate, dehydration can be dangerous and can result in thermal injury—a heat-related injury—because the core body temperature is not properly regulated. One of the functions of water is to dissipate, or get rid of, heat. When you become dehydrated, you have no more water to dissipate the heat, increasing your susceptibility to heat exhaustion or heat stroke.

“Maybe 20 years ago, we would have coaches who would try and toughen up their athletes by not allowing them to take water breaks,” Professor Anding said. “Well, they’re not toughening up that athlete. They’re actually increasing the likelihood of a heat-related injury.”

Staying Hydrated during Exercise

Sports physicians recommend that athletes—particularly adolescents—get fluid breaks every 10 to 15 minutes to prevent dehydration during exercise. This may not always be practical during a game, but it certainly would be during practice. 

Individuals who are acclimating to a different environment are more prone to dehydration when exercising outside. According to research, it takes anywhere between 10 and 14 days to adjust to a new environment.

This process of acclimation must take place outside, doing the activity. You can’t just go to somebody’s house and sit inside. 

The best way to treat dehydration if it’s not significant is to drink fluids such as water or gatorade. If the dehydration level is significant, you should seek medical attention to receive fluid replacement through an IV saline solution. 

Many professional athletes sweat so much that during halftime, they actually get hooked up to IVs because it’s not possible for the gut to handle all that extra fluid. How much fluid can you handle? 

Professor Anding suggests about a liter to two liters. Thus, if you’ve lost one to two quarts in sweat, it’s not tenable to replace all of that with oral rehydration alone.

This article was edited by Kate Findley, Writer for The Great Courses Daily.

Professor Roberta H. Anding is a registered dietitian and Director of Sports Nutrition and a clinical dietitian at Baylor College of Medicine and Texas Children’s Hospital. She also teaches and lectures in the Baylor College of Medicine’s Department of Pediatrics, Section of Adolescent Medicine and Sports Medicine, and in the Department of Kinesiology at Rice University.