by Chad Kerksick PhD August 05, 2010 3 min read
With a background in athletic training, I’ve seen my fair share of heat injuries and problems and I’m amazed at how many times people (athletes or not) get into trouble with the heat. Our bodies produce heat and lots of it when we exercise and when the temperatures outside are high, it becomes nearly impossible for our bodies to cool themselves.
This is why we sweat. The sweat evaporates on our skin creating a cooling mechanism for our body. Sweat contains water and important electrolytes (sodium, calcium, magnesium) that our body needs for our cells to function properly. You would be amazed at how quickly someone can sweat, but commonly documented sweat rates hover around 1.5 liters every hour, with the highest reported amount to be 3.5 liters per hour. On average, spend two hours outside and you’ve filled up a three liter bottle (or more); a body weight loss of four pounds. When we lose all this water and electrolytes, our blood gets thicker (blood is +90% water) and as a result our heart rate goes up. This compounds the cycle because more work is required of the body and as a result more heat is produced.
The more fluid you lose from sweat, the harder it is for your body to regulate its temperature and this is when trouble can begin. Unfortunately, no easy strategy exists for someone to be able to quickly determine how much fluid they have lost and as a result determine how at risk you may be for dehydration and various forms of heat injury.
So nothing can be done? No, not exactly and a few measures do exist that are worth mentioning to increase awareness and hopefully prevent problems from occurring. First, losing just one percent of your body weight from sweat will alter your perception and performance and the more you lose the more your performance will be decreased. A two to three percent loss of your body mass as water will significant reduce performance and put you at significant risk for dehydration. You can lose this amount of weight in as little as one hour depending on how much you sweat. You can determine a rough estimate of your sweat rate by measuring your naked body weight before and after the exercise bout (or how long you spent working in the yard for that matter). For every pound of body weight lost you should plan on drinking an additional two cups of fluid to replace the lost fluid. Thus if you lose two pounds after an hour of exercise, you should drink four cups of fluid to replace what was lost. Secondly, it’s important to understand that using how thirsty you are as the determining factor when you need to drink is not a good practice.
Our thirst mechanism is delayed and research has shown that by the time you feel thirsty you have already lost enough fluid to decrease your performance. Lastly, when you sweat you need to replace the lost fluid, electrolytes and replace the carbohydrate your muscles have burned for fuel. Sports drinks are formulated in optimal amounts to replace these things. While drinking water is good, it only replaces fluid and for those exercising or working in the heat for much longer than 45 minutes, you will also need to replace lost electrolytes and carbohydrates, so drinking only water would be better than nothing, but not as good as sports drink. Finally, use some common sense. If you start to feel disoriented, nauseated, fatigued, sick, etc. these are signs the heat may be getting to you. Stop what you are doing, get in the shade and a cold beverage (be sure and get it in you and not on you) and rest. The summer is a great time to get outside and enjoy the sun, but too much sun and not the right precautions can result in devastating consequences.