Tuesday, May 8, 2018

Why do We Sweat?

If we were any other species of mammal, including our closest relatives, the great apes, we would not perspire, and the deodorant industry would not exist. Humans perspire a lot; horses a bit. Dogs (and wolves, coyotes, fox, lions) pant. Prey species such as deer are fast in a sprint, but not equipped for long distance running without overheating. It’s us, the hairless ape, that is unique.

The major function of perspiration is to cool. When water on skin evaporates there is a transfer of huge amounts of heat away from the skin. Cooling of the skin’s surface cools blood circulating underneath, which cools the body as a whole. Panting, while not as effective, serves the same purpose. Kangaroos, which neither sweat or pant, lick their forelimbs, achieving evaporative cooling that way.

No evaporation, no cooling. Hence the truth behind “It’s hot, but it’s a dry heat.” In humid weather, we still attempt to cool by sweating, but the moisture soaks our clothing and drips off without the benefit of evaporation. (Standing in front of a fan helps.) In passing, worth a mention that a traditional sauna practice reverses the heat exchange process. Sauna is very dry heat, so a person can be comfortable in temperatures of 140-180 degrees Fahrenheit, versus 110 to 115 degrees tops for a steam bath. “Löyly,” the practice of throwing water on superheated stones during a sauna, converts the water to steam, which then condenses on cooler surfaces, such as skin. The condensation process transfers heat to the skin. The intense wave of heat experienced about 30 seconds after water hits hot stones is the opposite of evaporative cooling.

What is sweat? First, it is initially sterile, and hence not initially smelly. However, our skin is inhabited by billions of moisture-loving bacteria. The smells we associate with sweated up clothing are from the happily replicating bacteria that consumed our skin gland secretions for food and produced their own smelly waste products. Men and women have different mixtures of skin bacteria, and thus different smelling sweat. Interestingly, some studies show that homosexual women and men are more sexually attracted to the smell of same-sex sweat, versus heterosexual women and men who are more turned on by opposite sex sweat, but whether this is genetic or driven by one’s sexual orientation is not known. For some people, skin harbors Propionibacteria which product propionic acid, a compound that smells a lot like the chemically related acetic acid in vinegar. Time to wash those clothes and take a shower!       

Back to sweat. Sweat is 99.9 percent water and one-tenth of one percent minerals and organic compounds. Sodium makes up the majority of the minerals, then potassium and small amounts of calcium, zinc, copper and iron. Sports performance researchers have looked into heat adaptation. Results suggest that sweat at the end of a long, hot day has much the same composition as in the morning. However, over days in a hot environment, mineral content decreases by as much as a third. Thinking is that the body has adapted to conserve minerals while still managing evaporative cooling.

Other causes of sweating are emotional sweating, which can include sweaty hands, not seen during thermal-triggered sweating. A third cause is a reaction to eating very spicy foods.

Sports drinks (Gatorade, Powerade) are a multi-billion dollar industry based on the theory that modest amounts of minerals (primarily sodium, but some potassium and magnesium), plus calories will have a performance benefit over water during a prolonged period of exercise. There is a kernel of truth there. Given water to drink, actively exercising people will drink less than the water being lost to perspiration. And that’s generally okay, as athletic performance begins to suffer only after two percent weight loss. A salty-tasting, slightly sweet beverage will cause people to drink more compared to plain water. More is not necessarily better, just more. The sodium provides no performance benefit. The carbohydrates do provide usable energy, but that only really matters for hours of strenuous exercise.

People in the U.S. consume too much sodium. Our kidneys dump the excess in urine, but the effects of high sodium consumption include hypertension and higher risk of stroke and coronary heart disease deaths. National surveys estimate that average adult consumption is 3,400 milligrams per day, whereas recommendations are to consume less than 2,300 milligrams, and  1,500 milligrams per day is defined as an adequate amount. Only for people doing prolonged, vigorous exercise, say a hundred mile-bicycle ride, might there be a benefit for calorie-containing beverages or snacks during the event. That’s for energy. After the event, normal foods and beverages will replenish whatever minerals were lost.  

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