One of the greatest ironies of exercise is that to achieve health and wellness, many people do unhealthy things.

One prime example: On the few hot days the Washington area has had recently, folks jogged, cycled and exercised in heavy sweat suits or in plastic suits in the vain hope of quick weight loss.

All anyone should be wearing in any weather above 65 degrees is shorts and a T-shirt, experts say. But in the mistaken belief that more sweat yields faster health results, many people try to sweat out the fat by wearing heavy clothing.

That's about the worst thing you can do. What you lose when you sweat hard is fluid. Thus, the instant you drink after exercise, which you should, you "gain" all the weight back. Fat cannot be sweated out and people have the misconception that water loss is fat loss.

"It absolutely is not," said Sheldon Margen, a physician who is professor of Public Health and Nutrition at the University of California at Berkeley.

Another prominent misconception regards the function sweat plays in alleviating body heat. It does not cool the body simply by being produced, as many assume. When the internal temperature rises, fluid is secreted on the outer layer of skin. When the air hits the skin and the accumulated fluid evaporates, the skin cools and lowers the internal body temperature.

The body will tolerate only a limited amount of fluid loss before developing sensations of weakness and faintness, said Margen, and another irony, that of overlooking the feeling of thirst.

"The interesting thing is that people will overlook thirst because they're concentrating so much the signals that come from the brain are lost," said Margen.

A modern analogy is that of a computer programmed to override a basic command by another more sophisticated program. The more primitive part of the brain is telling the body it's thirsty while the mind is overlooking these impulses in the interest of completing a planned workout. And in an activity where an end result is a measure of success or failure, the impulse to override is even greater.

"I feel it happens primarily in competitive sports." said Margen. "It just seems to be a matter of intense concentration about what they're doing."

So when people wear heavy clothes to exercise or ignore the feeling of excessive heat and thirst, they are subverting the very impulse designed to save their physiological systems from damage.

"My mother told me of a husband of one of her coworkers who finished his first marathon, and I don't know how he did it," said Bill Egan, vice president of the Bowie Roadrunners. "He didn't drink any water. He didn't want to stop for it. He wanted to tough it out."

During the summer months, Egan and many of his co-runners design their running courses so they pass water fountains. "On a hot day you just can't take too much water," he said.

What happens when someone chooses to "tough it out" or wear too much clothing is scary. During excessive sweating, water is the primary component. The result, then, is an overabundant concentration of salt in the extra-cellular (outside the cell) fluid. The body tries to compensate and dilute the salt-heavy fluid by draining fluid from within the cell.

The resulting alteration of fluids within and without the cellular structure "probably distorts the function in the cell," said Margen. "And that leads to the possible existence of interference with cellular function within the muscles and the brain."