The Cooper Institute

Founded in 1970 by the "Father of Aerobics"
Kenneth H. Cooper MD, MPH


Hydrating for physical activity in the heat: don't wait for a sprinkler to save you!

Posted in
Live well

Monday, Jun 14, 2010

I was out running a few Saturday’s ago and was plagued by extreme thirst. It was in the morning but as it turns out, unknown to me, it was the hottest day we had had this summer, hitting triple digits in the afternoon. So it was already pretty hot when I started out. How hot I didn’t realize until I later checked the temperature and humidity; 88°F, 70% humidity. This put the heat index at close to 100! Humidity, especially when combined with that high of a temperature, reduces the effectiveness of sweating to cool the body by reducing the evaporation of sweat from the skin. This causes us to sweat more in a further attempt to cool the body which is being stressed by both environmental heat and the heat being produced by our working muscles. No wonder I was thirsty! I was so thirsty that all I could think about was water. Every sprinkler I passed I swore was calling my name and eventually I gave in, not once, but twice. My first attempt didn’t yield very positive results. I got a few drops of water in my mouth but mostly just got drenched. My second attempt several minutes later was much better. The stream was more of a straight stream than a mist. The only downside was that I had to crouch low to the ground to get to it. But I didn’t care, I needed water and I was getting it! And no, I didn’t care who saw me or what they thought.

This incident served not only as a good laugh for me and the many others I shared it with but also as a wake up call that summer is indeed upon us and that I needed to pay better attention to my hydration status. So I thought this was a perfect opportunity to share some fluid guidelines with you so that you don’t wind up in the same predicament as me.

As was mentioned, evaporation of sweat is used to cool the body. If the fluid that is lost from sweating is not replaced, dehydration can occur. Even a slight level of dehydration (> 2% loss of body weight) can increase the strain on the cardiovascular system and significantly affect exercise performance1. Many other factors can be affected by dehydration as well such as decision making skills, hand-eye coordination, and the ability to cool and deal with heat stress1,2,3,4. Preventing significant loss requires consuming the proper amounts of fluid before, during, and after exercise. Here are some guidelines to follow4:   • Start hydrating 4 hours before activity by drinking 2-3 cups (16-24 ounces) of fluid. • If signs of dehydration are present despite this (i.e. not needing to urinate), drink another 1-2 cups (8-16 ounces) 2 hours before activity. • Drink 6-12 ounces every 15-20 minutes of activity. • After activity, drink 3 cups (24 ounces) for each pound lost. 

And important to note is that thirst can be an unreliable mechanism for detecting fluid loss. So the fact that I was thirsty during my run probably meant that I was significantly dehydrated, and had been so before I had even started out that day. There are several ways to determine your level of fluid loss so that you can be certain to replace the proper amount.   • Weigh yourself before and after you exercise. This will tell you how much weight you have lost in the form of water or sweat. • Check the color and quantity of your urine*. If your urine is dark in color and only in a small volume, you need to drink more fluids. When it is pale yellow and of substantial volume, you have returned to normal water balance. Remember that dehydration is accumulative so this may take several days.    *Urine is not a good indicator of hydration status if diuretics are present in the body.

Remember that many fluids can be used to replace sweat losses including water, seltzer, juice, sports drinks, milk, soft drinks, even foods with liquid in them (i.e. soups, fruit). Check back next week for a discussion on sports drinks.

What do you do to stay hydrated? Do you have a sprinkler story of your own? Or maybe just the opposite, you drank too much water in a short period of time and suffered from water poisoning? Has your performance ever been affected by dehydration? Do tell. 1 Armstrong, L.E., D.L Costill, and W.J. Fink. Influence of diuretic-induced dehydration on competitive running performance. Med. Sci. Sports Exerc.17 (4): 456-461, 1985.

2 Casa, D. J., P. M. Clarkson, and W. O. Roberts. American College of Sports Medicine roundtable on hydration and physical activity: consensus statements. Curr. Sports Med. Rep. 4:115–127, 2005.

3 Cheuvront, S. N., R. Carter III, and M. N. Sawka. Fluid balance and endurance exercise performance. Curr. Sports Med. Rep. 2:202–208, 2003.

4 American College of Sports Medicine; Sawka MN, et al. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 39:377-390, 2007.