In a country obsessed by weight loss, I should have been overjoyed: I shed three pounds in 35 minutes last Saturday -- without puking! -- courtesy of a four-mile run in the July swelter. (I dropped from 172.5 pounds to 169.5, if you must know.) Of course, as a responsible fitness columnist I knew I had to drink that weight back, with water and, perhaps, a sports drink. But it got me wondering if sweating away about 2 percent of my body weight in less than 40 minutes was cause for alarm.

"That's a pretty high sweat rate, but some people will lose 3 percent [of their body weight] or more in a strenuous workout," said W. Larry Kenney, a professor of physiology and kinesiology at Pennsylvania State University.

Perspiration rates range widely, Kenney said -- very heavy sweaters will lose up to 10 pounds in a strenuous, one-hour workout. He recommends people weigh themselves before and after workouts to plan post-exercise rehydration. A fairly easy equation: Drink 24 ounces for every pound lost (one pound equals 16 ounces; the extra half-pound accounts for what you will eliminate via urination).

So, to stave off dehydration, I needed to drink 72 ounces after returning from my jog. (I drank nothing while out running, which is fine for exercise sessions under one hour, Kenney says.)

The good news: I had a couple of hours to rehydrate. As long as you drink back what you lost within two or three hours of exercising, "you should be okay," Kenney said, provided you were adequately hydrated before heading out. The American College of Sports Medicine recommends drinking about 17 ounces of fluid roughly two hours before exercise to allow your body to process and eliminate excess fluid.

"We get especially concerned with 'repeat exercisers' in high heat," Kenney said, "like people who go work out and then come home and do some gardening without drinking anything."

And, if we may get even a little more gross here, if you are a salty sweater -- i.e., if your sweat frequently leaves whitish marks on your clothes or baseball cap (like mine does) or stings your eyes -- Kenney recommends drinking sodium-containing sports drinks (along with plain water) to ensure you get enough salt in your diet to replace lost sodium.

We sweat to regulate our core temperature, which rises due to heat created by the contraction of our muscles and heat absorbed from the environment. We do not perspire more in high humidity, but we notice the moisture more because it evaporates far more slowly than in drier environments.

Hydrating raises blood volume and thus "you don't have to work as hard" to cool your core or get oxygen to your muscles, Kenney said. "This allows you to maintain your sweating rate and supports your cardiovascular system. So you can keep exercising" without putting dangerous strain on your heart, he said.

You may have heard recently about hyponatremia, a dangerous condition caused by drinking too much water and thus diluting blood sodium. Hyponatremia almost exclusively affects endurance athletes who exercise continually for hours while rehydrating with copious amounts of water. If you are not exercising for several hours and forcing water regularly, you don't need to give this condition another thought.

Shower off, swab on some deodorant and meet us in the Moving Crew chat room to fitness, today at 11 a.m. at www.washingtonpost.com.

-- John Briley

To avoid dehydration, don't wait long after exercise to replace liquid lost to sweating.