The tendency of modern man is to want to learn by learning rules. But today’s rules often turn out to be tomorrow’s myths. For example, when I started running over half a century ago, one of the rules most athletic coaches accepted (believe it or not!) was that if you’re practicing football or running on a hot day, you shouldn’t drink cold water. For some, the rule was no water at all. After too many young athletes got dehydrated and died, we eventually recognized that rule for what it was—a myth.

Unfortunately, due to our continuing desire for hard and fast rules, that myth was replaced by a new rule, epitomized by what I heard an official shouting to us over the PA system before the start of the New York Marathon one year: “Drink, drink, drink!” The new rule was that a runner should hydrate as much as possible in warm weather. But that rule, too, turns out to be a myth.

Heat Myth #1: The more you drink, the better.

While you do need to keep hydrated, it’s possible to get too hydrated. A few years ago, a young runner died at the Boston Marathon, and the coroner’s report said the cause was a cascade of medical events precipitated by excessive fluid intake. Another time, one of America’s best ultrarunners, Don Choi, had to be helicoptered out of the Western States 100-mile because he’d drunk too much water and been stricken by hyponatremia. You need water, but not so much that you get bloated.

Heat Myth #2: If it’s hot, you need salt.

Well, not exactly! You do need electrolytes, which include potassium and magnesium as well as sodium (salt) for muscles to work, and if you sweat out too much of your mineral supply you can find your muscles failing. But the electrolytes need to be dilute enough to be assimilated by your body while you’re running. It’s best not to take salt tablets at all, but instead to take electrolyte capsules formulated specifically for running. And that’s only if you’re going out for a fairly long time or distance. If you’re going out for a three-miler, the minerals already in your blood from the food you ate last night should be more than enough.

Heat Myth #3: People just aren‘t born to run in heat the way a lot of other animals are.

Wrong! We now know that humans have about the best capability of any animal on Earth to run in heat. A human has a uniquely efficient cooling system—bare skin, which is both a radiator and a conduit for evaporative cooling via sweat and for convective cooling via the “breeze” effect of skin moving against air.

What it all adds up to is that you don’t need to be afraid to run on a hot day if you use good judgment. Keep adequately (but not excessively) hydrated and supplied with balanced electrolytes, make sure you’re sweating enough and have enough skin exposed to have evaporative cooling, and know that you can handle this better than a woolly mammoth or wolf.

Ed Ayres publishes the blog http://enduranceandsustainability.blogspot.com