Does sweat smell? Surprisingly, not on its own, because fresh sweat is almost odourless. The smell we call body odour comes from bacteria living on our skin, which feed on certain compounds in our sweat and turn them into the pungent molecules we notice. And whether that happens much at all depends partly on your genes.
Two kinds of sweat
Your body has two main types of sweat gland, and they do quite different jobs. Eccrine glands cover almost all of your skin and are especially plentiful on your palms, soles and forehead. They release a watery sweat, mostly water and salt, that cools you down as it evaporates. It’s part of the body’s temperature control system, which I explain in what is homeostasis?. This kind of sweat has very little smell.
Apocrine glands are found mainly in the armpits and groin, and they become active at puberty, which is why body odour is mostly an adolescent and adult issue. They release a thicker sweat containing fats and proteins, including the odourless chemical ingredients that bacteria can turn into body odour. Warm, moist, sheltered places like the armpits are also ideal homes for those bacteria, which is why that’s where the smell tends to be strongest.
The bacterium behind body odour
Your armpits are home to a whole community of bacteria, but only a few of them are responsible for the smell. In 2020, researchers at the University of York, working with scientists from Unilever, pinpointed what they called the “BO enzyme”. It’s found in only a handful of bacteria, especially one called Staphylococcus hominis.
As the University of York explains, the bacterium takes an odourless compound released by the apocrine glands and converts it into a type of chemical called a thioalcohol, one of the most pungent parts of body odour. To prove it, the team moved the enzyme into a related bacterium that normally causes no smell, and it began producing the odour too, as their paper in Scientific Reports describes.
Remarkably, the enzyme appears to have evolved in these bacteria tens of millions of years ago, long before modern humans existed. That suggests body odour has been around for a very long time, and it may once have played a role in how our primate ancestors communicated.
The gene that lets some people skip deodorant
Here’s the really surprising part: some people produce almost no underarm odour at all, and whether you do comes down largely to a single gene called ABCC11. People with one version of it lack the compound in their armpits that bacteria feed on to make body odour. Oddly, they also have dry rather than sticky earwax, according to Scientific American. Genetic epidemiologist Ian Day of the University of Bristol described the gene as basically the single determinant of whether you produce underarm odour.
The odour-free version is rare in people of European descent, but common across East Asia. In a 2013 study of 6,495 British mothers, just 117, about 2%, carried it. And in a twist, the University of Bristol reported that 78% of those women, who don’t produce underarm odour, still used deodorant on all or most days.
What actually helps with body odour
Since the smell comes from bacteria breaking down sweat, what helps most is dealing with both:
- Wash daily. Use soap and water, paying attention to your armpits and feet, and dry them well.
- Use a deodorant or antiperspirant. A deodorant targets odour-causing bacteria and masks smells, while an antiperspirant reduces how much you sweat. Many products do both.
- Change clothes regularly. Swap into clean clothes, socks and underwear often, because bacteria and sweat build up in fabric.
- Let sweat dry. Wear loose, breathable clothing and shoes in hot weather.
Scented shower gels and perfumes can cover up a smell for a while, but they don’t tackle the bacteria behind it.
When to see a doctor about sweat or odour
Body odour is normal, and everyone with the common version of the ABCC11 gene has some. But see a doctor if your body odour changes suddenly or noticeably without an obvious reason. The same goes if you sweat so much that it interferes with daily life even when you’re not hot or exercising. That’s a condition called hyperhidrosis, and it can be treated. A sudden change in odour can occasionally be linked to other health conditions or medicines, so it’s worth getting checked rather than simply reaching for a stronger deodorant.
Sweat isn’t the villain
So the next time someone complains about smelly sweat, you can point out that the sweat is innocent. It’s a cooling system doing its job, and the smell comes from a few particular bacteria, armed with an enzyme they’ve carried for millions of years, turning odourless compounds into something much less pleasant. And depending on your genes, you might not need that deodorant at all.
Everyone’s body is a little different, so if a change in sweating or smell is worrying you, your GP is the best person to check it. You can also call Healthdirect on 1800 022 222 for advice.
If you’re curious about how the body keeps itself cool and balanced, see what is homeostasis?, and for another surprising body response to temperature, see what are goosebumps?
Sources
- University of York: Life in the pits, scientists identify the key enzyme behind BO (2020)
- Scientific Reports: The molecular basis of thioalcohol production in human body odour (2020)
- Scientific American: People without gene for underarm odour still wear deodorant (2013)
- University of Bristol: Deodorants, do we really need them? (2013)