The Dangers of Dehydration

The upshot of all this dripping sweat is a dropping pace. One study found that for each one percent of body weight that you lose because of dehydration, your running performance falls by three percent. Bear in mind that one percent of body weight isn’t much— if you weigh 150 pounds, that’s only 1.5 pounds, or 24 ounces. It’s not unusual to lose three or four pounds of water in an hour of running on a hot day. This loss causes your performance to sag by more than five percent, which can be as much as a minute slower
per mile. Are you starting to see why it’s so tough to
feel comfortable while running in the heat?
You also need to consider cumulative dehydration.
Cumulative dehydration is what happens when it’s hot for several days in a row, and you don’t replace all the fluid you lost after each run. Suppose that during a hot week, an 150-pound runner doesn’t rehydrate himself completely each day. By the end of the week, he has a cumulative loss of two pounds of water—that’s more than one percent of his weight. As a result, his performance is off by more than three percent by week’s end, but he may not make the connection to dehydration.
A lot of runners, myself included most summers,
go through the hottest parts of the year chronically
dehydrated. As a result, our running suffers.