Wednesday, February 3, 2016

Sweet Sweat, Part 2: "Never Let Them See You Sweat"

We humans are built to sweat.  And we do, up to a gallon per hour under extreme circumstances. To accomplish this prodigious feat, each of us has 2-4 million sweat glands.  These are spread all over our bodies, but with greater concentrations in regions with more hair, such as armpits, groin, scalp and yes, probably eyebrows (see Part 1).  Among mammals only primates and horses cool their bodies by evaporating large amounts of sweat from these glands, and they are the only two mammals who perspire in their armpits.

Being efficient sweaters means that we humans are very good at regulating the core temperature of our bodies. This ability has given us at least two evolutionary advantages over our less sweaty brethren.  First, it allowed early humans to engage in endurance activities like running down and killing other animals for food. Second, the heat generated by our big brains burning a lot of calories can be managed through the evaporative cooling produced by sweating. In other words, Big Brain <==> Big Sweat. (Note to you heavy sweaters out there -- no, it doesn't follow that you are smarter that someone who doesn't sweat as much as you do.....)

Mayo Clinic's Depiction of Sweat Glands
There are two kinds of sweat glands: eccrine and apocrine.  Eccrine glands open directly onto the skin and are found over most of the body, with higher concentrations in the hands, feet, forearms, forehead, chest and back. Apocrine glands open into hair follicles and so are found in areas where follicles are abundant, such as armpits, groin, scalp, and yes, even eyebrows (see Part 1).  There is considerable variation from person to person in the total number of sweat glands, but the region-to-region relative differences are the same. Thus, one person might have few glands in say, the eyebrows, whereas another has more in that region.  But both people have more glands in their arm pits than in their eyebrows.  You might say it's more normal to have fewer glands in the eyebrows (again, see Part 1)....

The sweat from the two kinds of glands differs in composition and in potential stinkiness.  Eccrine glands produce perspiration that is mostly water, with some salt and trace elements. In most people this sweat is odorless, both as it emerges and after it has dried. Apocrine glands, on the other hand, are a boon to the $3 billion a year deodorant industry because they produce a milky, chemically complex sweat that is 20% proteins and fats, and skin bacteria love it.  The bi-products of their digestive process are odoriferous -- and we have been conditioned to regard the smell as offensive.  Nearly 90% of adult Americans are proactive about body odor by taking frequent showers and baths and using commercial deodorants and antiperspirants.  An interesting paradox is that many soaps increase the natural pH level of the skin, making it more friendly to bacterial growth. Thus, frequent showering may actually encourage higher concentrations of bacteria and therefore make chemical deodorants even more necessary.  One way around this dilemma besides going back to the "once-a-week-bath-whether-you-need-it-or-not" routine is to use pH-balanced bath soap.

Perspiration is triggered by three primary sources:  environmental heat, exercise, and stress.  Oh, and "power surges" brought on by hormonal changes in women -- definitely a topic worth a whole other blog and so it won't be considered here.  Stress has a strong social component, as indicated by a recent survey of American & Canadian adults in which 62% of the participants reported stressful work situations brought on sweating, and nearly half said this occurred when interacting with their boss or with colleagues.  In the same survey,  2/3 said they perceived someone who was sweating as being nervous and uncertain.  Thus the adage "Don't ever let them see you sweat" to avoid the attribution that you are unsure of yourself and not in control. Probably the most famous historical case of this was the first Kennedy/Nixon debate in which Nixon was visibly perspiring, leading to a widespread belief that he lost the debate.  Stress sweat comes on suddenly and almost always activates both kinds of glands, which means it often produces the most odor -- stress stinks, literally.

Heat and exercise are perhaps the two most "natural" sources of sweating, and they illustrate our somewhat ambivalent attitudes toward perspiration.  On the one hand we spend $3 billion a year on antiperspirants and on the other we deliberately seek out certain situations where the objective is to sweat buckets.  For instance, in terms of hot environments, consider saunas, steam baths, and "sweat lodges" as examples where sweating is desirable, healthy, and even sacred.  Vigorous physical exercise, with its inevitable liquid result, is eagerly sought after in natural environments as well as in fitness centers with hi-tech equipment.  In the U.S, alone, people spend about $ 22 billion per year on health clubs, where if you aren't seen sweating, you're regarded as just not working hard enough, an evaluation applied equally to men and women in that context.

This brings us full circle in this exploration of perspiration, back to the question whether there is really a sex difference in sweating.  We can easily dismiss the romantic notion that ladies "merely glow," but do women and men perspire at the same level under the same conditions?  Fortunately, there is some pretty solid scientific evidence that addresses that question in a recent experimental study by several Japanese researchers published in the journal Experimental Physiology (Ichinose-Kuwahara et al., 2010).  A less technical but more readable presentation of this research is in a New York Times article by Gretchen Reynolds.

The researchers compared the sweating rates of healthy men and women who were either very fit (amateur endurance athletes) or who exercised very little.  The average age of the participants was about 21, which unfortunately doesn't allow confidently generalizing the results to geezers, but is still informative.  The sweat output of the participants was measured while they rode a stationary bike under increasingly higher pedaling intensity.  The temperature of the room was held at 86d, pretty toasty.  The researchers measured both the number of sweat glands that were activated and the amount per gland at several sites on each participant's body.

For both men and women, those who were very fit sweated more during exercise than those who were less fit, a finding that has also been found in other research studies.  The functional importance of this is that the body's core temperature can be kept below a critical level longer if sweat glands start pumping earlier during strenuous exercise.  Regular work-outs condition the body to do this.

Now for the question at hand. The sweat rate for men was higher than for women in both the fit and unfit groups, although the difference was small for less fit groups.  Men achieved this soggy superiority by sweating more per gland, not by having more glands activated. This same result has been found in other studies that didn't involve exercise and instead looked at sweating under different temperature and humidity conditions -- i.e., "passive sweating."  The bottom line appears to be that men's sweat glands pump out more perspiration, period.  The best evidence to date is that this is associated with testosterone levels, not a structural difference between men and women.  For instance, prior to puberty when hormones begin to run amok, both boys and girls sweat at the same rate and from the same number of glands.

One last point. Sweating cools the body and lowers core temperature.  This means that women in the exercise study tended to be hotter than men in both the fit and less fit groups.  A possible inference from this result is that if you sweat less, you may perceive the same environment as warmer than someone who sweats more because your body is actually hotter.  Of course it is also possible that people who don't ordinarily sweat much may reverse the direction of attribution --  "If I'm sweating, the environment and me must be really hot."  I suspect this attributional method of assessing comfort level is quite common. Either way, this could account for many marital disagreements, present company included.

For me the conclusion of all this is that sweating in humans has important implications in terms of economic impact, physiological functioning, evolutionary processes, and social relations.  And it is a prime example of the common feature of our species to elevate something that is basic and essential to something that is a complex phenomenon with additional qualities that are to a fairly large extent the product of our big brains.  Whew!  I think it's time for a shower......
Sources & Resources

Perspiration, Sweat Glands, Deodorants, Sweat Lodges, Hot Flashes  - Wikepedia
Sweating and body odor - Mayo Clinic
The science behind sweating - The California Aggie
Sweating: - MedlinePlus Medical Encyclopedia 
Statistics and Facts on Health and Fitness Clubs - Statistica
Sex differences in the effects of physical training on sweat gland responses during a graded exercise - Ichinose-Kuwahara et. al.-  Experimental Physiology (2010)
Do Women Sweat Differently Than Men? - The New York Times
Sweating Survey - International Hyperhidrosis Society
Stress Sweat -
Body Odors & pH Balances |


Anne said...

Amazing article about an until-now little know topic!

Coleen Hanna said...

Interesting! I couldn't help but think about Donald Trump elevating Marco Rubio's supposed heavy sweating during a debate to a major ISSUE! I hope you don't decide to do a blog entry on the size of a man's hands in relation to the size of other body parts....