With spring upon us, it is getting warmer in most places and the rising temperatures mean when people start heading outside, the stress associated with exercise can be amplified by added thermal stress. While exercising indoors seems like a solution, there is no guarantee of a cool work out as most exercise equipment is stationary, which limits air flow and the evaporative cooling we all need to stay comfortable when it is warm.
When our core body temperature increases by 1-2 degrees Fahrenheit (about 0.5 to 1 degrees Centigrade) most of us start to sweat. The purpose of this sweat is to wet the skin and upon evaporation, the skin cools. When you start sweating skin blood flow also goes up dramatically. If the skin and the blood flowing through are both cool, they keep core temperature down while exercising and burning calories.
This design is thought to have evolved so that early humans could hunt in warm dry environments. Humans can’t out run most game over short distances but because we sweat and have high skin blood flow we stay cool and can out last many faster animals over time. So what does dehydration do and what are some of the things we need to do to keep this cooling system working?
First, if we are dehydrated by as little as 2% of our body weight we don’t start to sweat until our body temp is higher, we sweat less, and our temperature increases more than it should. Dehydration also causes the heart to pump less blood. This can make skin blood flow lower so it is harder to keep things cool and there can be less blood flow going to the muscle doing the exercise. None of these are good things.
In addition to affecting our endurance, dehydration can affect things like the ability to shoot a basketball or other skills in sports. Dehydration also makes thinking and judgment worse. Less is known about exactly how much dehydration it takes to reduce ability in strength and power events, but with enough dehydration performance will surely start to decline.
Under most circumstances, there are a few simple solutions to combating dehydration. One important thing to do is to start your workout, especially on warm days, with a full tank. In other words, sip some water most of the day or have a couple of glasses of water a few hours before training or competition starts. And remember: there is no added benefit to denying yourself water while you train. Cold fluids can also help keep body temperature down and might also provide a psychological boost.
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For most athletes and weekend warriors, the majority of work outs last less than an hour and hydrating prior and during the session will likely be able to fulfill your body's need. When training or competition goes longer than one hour, it's important to be aware of the need for electrolytes and glucose. There are also specific training strategies that can be used to improve your ability to exercise and compete in the heat that will get a deeper dive as I cover this topic further. Finally, here are some excellent resources from the American College of Sports Medicine on exercise and fluid replacement and exertional heat illness during exercise.
Training in hot weather or environments can improve your ability to compete in the heat and there simple strategies that are available to deal with extra stress on especially warm days. However, it all starts with having a full tank and water available while you are training or competing. Hydration is one place you don’t want to play catch up.
Michael Joyner, is an expert in human performance at the Mayo Clinic, these views are his own. You can follow him on twitter @DrMJoyner