WHAT IS SWEAT AND WHY IS IT IMPORTANT?
Thank you to LMNT for this awesome article!
Most people understand that sweat keeps you cool, but there’s more to sweating than thermoregulation. There’s fascinating physiology underlying sweat and sweat glands—a science I recently brushed up on by reading a comprehensive review in the journal Temperature. In the piece, sweat expert Dr. Lindsay Baker covered questions including:
Does age influence sweat rate?
How does sweating impact electrolyte balance?
What effect does sweating have on skin health?
Is sweating really as detoxifying as people believe?
And much more
We’ll dive into the answers to all those questions in this article. But first, let’s start with some basics.
What Is Sweat?
Sweat—a combination of water, minerals, and non-micronutrient components—are secreted by your sweat glands in response to rising core body temperature. Your sweat subsequently evaporates, dissipating heat along with it to keep you cool.
Sweat is mostly water and salt, but it also contains small amounts of potassium, magnesium, calcium, iron, zinc, copper, arsenic, lead, lactate, urea, ethanol, ammonia, glucose, amino acids, enzymes, pesticides, and toxins like BPA. I’ll dig more into the detoxification question shortly.
Sweat Glands 101
There are 3 types of sweat glands: eccrine, apoeccrine, and apocrine.
Eccrine glands are your dominant species of sweat glands, numbering 2–4 million across your body. They’re most densely concentrated on your hands and feet (think: sweaty palms), but it’s primarily the glands on your face, trunk, arms, and legs that keep you from overheating.
Apoeccrine glands are clustered around your armpits and have insignificant cooling power.
While eccrine and apoeccrine sweat is mostly water and salt, apocrine glands act as scent portals, secreting pheromones (body odor) for social or sexual functions. Is body odor really a turn-on? My wife doesn’t think so, but other mammals may disagree.
Sweat glands activate when your body temperature rises and skin-based plus internal thermoreceptors alert a region in your brain called the hypothalamus. First, a bunch of sweat glands (mostly eccrine glands) are “recruited” to deal with the job. Then, each gland gradually increases sweat secretion. The result—assuming your sweat can evaporate in its current environment—is a cooler human.
What Influences Sweating?
Many factors influence sweat rate, including:
Environment. Hotter, sunnier, more humid, less breezy climates increase sweating.
Clothing. Covering the skin (especially with heavy protective gear) can prevent your sweat from evaporating, so you stay hotter and then sweat even more to compensate.
Exercise intensity. Vigorous activity builds metabolic heat and stimulates sweating.
Heat acclimation. Training in the heat for 5–14 days consecutively can boost sweating up to 20% in subsequent sessions. It’s a neat adaptation that helps us cool off on a hot day.
Aerobic training. Endurance training increases the activity of acetylcholine, a neurochemical that stimulates sweat response.
Hydration status. Dehydration means lower blood volume and greater osmolality (concentration of blood electrolytes). Either of these raises your sweating threshold so you sweat less.
Body mass. Bigger people generate more metabolic heat (and therefore perspire more) for a given workload.
Biological sex. Greater (typically) male body mass can contribute to higher sweat rates.
Circadian rhythm. We sweat more easily in the early morning than in the afternoon.
What about age? Older folks generally sweat less, putting them at higher risk for heat exhaustion and heatstroke. That said, seniors who experience impaired sweating should also factor in aerobic fitness and heat acclimation as potential causes.
Why Do We Sweat?
Until now, I’ve mainly focused on sweating to stay cool. But we also sweat to maintain skin health and prevent electrolyte imbalances. Let’s dive deeper into all three of these functions.
#1: Thermoregulation
Less than 30% of the energy expended when you exercise goes toward motion. A big chunk of the rest becomes heat generated from contracting muscles. This metabolic heat—plus any heat from your external environment—warms you up.
But your body doesn’t like it too warm, so it strives to keep your core temperature within a narrow band. Even mild temperature elevations of just a few degrees Fahrenheit can impair performance, cognition, and the function of major organ systems. (For more on this, check out this article on heat warnings and heat illness.)
When you sweat, you transfer heat from your body to water on your skin. That water soon evaporates, taking the heat with it so you can keep playing tennis, enjoying the beach, jazzercising, or dancing with your dog.
#2: Skin health
Sweat delivers water, amino acids, lactate, sodium, and potassium that reduce moisture loss to your skin’s surface. Consequently, a good sauna session may improve dermatitis, dry skin, or other dermatologic conditions—though more research is needed. (Read this article on skin hydration for more on this topic.)
Another interesting factoid: Researchers have discovered antimicrobial compounds (dermcidin, cathelicidin, and lactoferrin) in sweat that may protect against skin infection. When you play full-contact sports like jiu-jitsu, you take all the skin defense you can get.
#3: Mineral balance
Sodium is such an essential nutrient that sweat glands have some neat adaptations to keep levels up. For instance, if your diet remains low in salt for a period of 1–3 days, your sweat glands will learn to reabsorb more sodium than is typical. Your kidneys will also begin conserving sodium even faster when you’re sodium deficient, and that comes with hormonal side effects that can raise blood pressure. (You heard me right: a low-sodium diet can actually increase blood pressure – but I digress.)
If you exercise in the heat, your body acclimates by reducing your sweat sodium concentration 30–50%. Your sweat glands seem to know: If you’re bushwhacking in the jungle, you must retain sodium! Regardless, folks can still lose 3.5–7 grams of sodium when exercising in warm climates, so you need to replace what’s lost with a salt shaker or salty electrolyte drink.
Remember, sodium and chloride (together, salt) are the primary minerals excreted through sweat. You lose trace amounts of other minerals, but not enough to cause an imbalance. A heart-healthy starting point is 4–6 grams of sodium per day; if you sweat a lot, you may need much more to stay salty and avoid symptoms resulting from low sodium.
Is Sweating Detoxifying?
Some studies have found traces of heavy metals, pesticides, BPA, and other toxins in sweat. These findings (plus clever marketing from sauna companies) have convinced many people that sweating is the ultimate way to clear harmful chemicals from your body.
Sweating may help with detoxification, but I need more evidence. In many BPA studies, for instance, researchers used sweat collection methods susceptible to surface contamination. The favored method (scraping sweat from skin) also picks up sebum, a non-sweat secretion more likely to contain fat-soluble toxins. In one of these papers, 16 of 20 participants had BPA in their “sweat,” while only 2 of 20 had BPA in their serum. Strange.
I have similar skepticism about claims pointing to the excretion of heavy metals (like arsenic and lead) through sweat. From the review I cited earlier: “Studies suggesting a larger role of sweat glands in clearing waste products or toxicants from the body may be an artifact of methodological issues rather than evidence for selective transport.” I think that’s a fair point.
Is Sweat Good for You?
My doubts about detoxification aren’t meant to diminish the positive effects of sweat or your sauna routine. Air conditioning aside, humans couldn’t survive in warm climates without sweat. As a bonus, sweating helps with skin health and mineral balance too.
Don’t forget: The more you sweat, the more you need to hydrate with fluids and electrolytes. And for the sake of your friends, family, romantic partner, and the person behind you in the checkout line, don’t forget to shower either.