Hydrate or die?

Happy first day of summer, all. I normally poo poo some of the fear-based marketing associated with hydration for outdoor enthusiasts, but biking in 120° Fahrenheit (50° C) weather is no joke.

Handy dandy water spigot on the drinking fountain  #ucsc #rer #rideeveryroad

Of the five heat-related fatalities reported since the weekend in Arizona, one was a mountain biker and another was a hiker, all of whom ran out of water miles from the trailhead.

How do you know if you’ve overdone it? Sports physicians recommend weighing yourself before and after your activity to determine water loss. If you lose 2% to 3% of your body weight during the workout, drink more water. If you lose between 4% to 6% of your body weight in water, you need to slow down and reduce the intensity of your workout. 7% and above, and you should call the doctor. Since carrying a scale into the front country might be inconvenient, you should perhaps stay close to civilization until you know how much your body can handle on an extremely hot day.

It’s possible to swing the pendulum too far the other way and drink too much water, possibly resulting in potentially life threatening water intoxication / hyponatremia. This is apparently a problem more for long-distance endurance activities that last more than three or four hours. To avoid hyponatremia, don’t force yourself to drink more than is comfortable, and drink a sports drink during prolonged exercise.

Healthy humans can acclimate themselves to high heat, but it takes time. If you’re already fit, this acclimation takes less time, but you should still ease yourself into it. I wrote about the physiological process of heat acclimation here.

Temperatures in the USA west should be more moderate the rest of the week.


  1. Excellent topic. Hydration should actually start 1-2 days before exertion in hot weather, and water should be augmented with electrolytes (I use Hammer Endurolytes Extreme). This is a sensitive topic for me because I have a heart condition know as atrial fibrillation, originally brought on (we’re pretty sure) by over-exertion on a hot day. Recently I was cardioverted (electrical shock to reset the heart’s normal rhythm) for the fourth time, all episodes brought on by over-exertion in the heat, each time with me believing I had proper water and energy management and pacing. Older people, of course, need to be most careful. Do NOT drink energy drinks or much caffeine on hot days – ER folks were telling me they often see a-fib in teenagers drinking too much Red Bull, Rock Star, etc.

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