Robin asks: Why do armpits smell bad compared to other parts of your body?

Those millions of bacteria have to live somewhere.

Sweat and Odor

There are two universally-accepted types of sweat glands.

Eccrine Glands

The most abundant of the sweat glands, eccrine are found on most of the body and secrete a sweat that is: “A sterile, dilute electrolyte solution that contains primarily sodium chloride (NaCl), potassium and bicarbonate . . . .”

Most of us are familiar with the reason our eccrine glands produce sweat: “Continuous secretion . . . provides a mechanism for thermoregulation via evaporative heat loss [and] maintenance of electrolyte balance . . . .”

Apocrine Glands

This type of gland is found in hairy places on the body, such as the armpits and between the legs. Near the skin’s surface, inside the hair follicle, apocrine glands secrete: “A milky fluid that most commonly [occurs] when you’re under emotional stress. This fluid is odorless . . . .”

Despite its innate lack of smell: “[Apocrine sweat] is rich in precursors of odoriferous substances (cholesterol, triglycerides, fatty acids, cholesterol esters, squalene). It also contains androgens, carbohydrates, ammonia and ferric iron.”

In addition, this gland produces pheromones, the “chemical signals that instigate behavioral responses (e.g. sexual attraction).”

Armpit Funk

At any given moment, there are 100,000,000,000,000 (one hundred trillion) bacteria living on your body. So, of course, a few are going to wander down to the smorgasbord that is your armpit, itself “home to one million bacteria per square centimeter.”

Dining on the aforementioned precursors, notable axillary stinkers include Corynebacterium spp., Staphylococcus spp., Micrococcus spp. and Propionbacterium spp., and, depending on their food of choice, any of a range of offensive odors can be produced: “The sulfur-containing molecules are the worst, giving armpits their characteristic nauseating, onion-like smell. . . [while others produce] a cumin spice-like odor . . . . Two possible pheromones, androstenol, which is musky, and androstenone . . . [may also contribute. Finally,] isovaleric acid has a cheesy, sweaty foot smell, as does propionic acid.”

A Wealth of Information

Researchers have determined that some mammals are born with distinct bouquets, organized into odortypes: “An individual’s odortype is determined in part by genes in [the] . . . major histocompatibility complex (MHC), which plays a role in the immune system . . . .”

Acting as “olfactory nametags,” an individual’s unique fragrance helps others identify him, and apparently, it cannot be changed, no matter how much garlic and cumin he eats: “In behavioral tests, ‘sensor’ mice were trained to use their sense of smell to choose between pairs of test mice that differed in MHC genes, diet or both. . . . The results . . . indicate that genetically determined odortypes persisted regardless of what the mice ate, even though dietary changes did strongly influence the odor profiles of individual mice. Both the sensor mice and chemical analysis could still detect the underlying odortypes.”

The study’s authors concluded that: “If this can be shown to be the case for humans, it opens the possibility that devices can be developed to detect individual odorprints in humans.”

Later research explored the extent to which disease in a body gives off an aroma by taking advantage of the olfactory virtuosity of our canine friends: “A Belgian Malinois shepherd was trained . . . to scent and recognize urine of people having PCa [prostate cancer] . . . After a learning phase and a training period of 24 mo, the dog’s ability to discriminate PCa and control urine was tested in a double-blind procedure. . . . . The dog . . . correctly designated the cancer samples in 30 of 33 cases. Of the three cases wrongly classified as cancer, one patient was rebiopsied and a PCa was diagnosed.”

It Could Be Worse

Some people suffer from a condition where they’re body odor smells like rotting fish. Called trimethylaminuria, people with this disorder cannot break down trimethylamine: “Which is found in eggs, liver, legumes and some grains. [Commonly] it is broken down by bacteria [and] . . . is normally oxidized in the liver to odourless TMAO [trimethylamine], which is excreted from the body.”

The compound smells like fish because it is commonly found in them. It is “believed… it… increases osmotic concentration and thus depress the freezing point of body fluids.”

In people who suffer from trimethylaminuria, the smell is emitted in their “sweat, urine and breath,” a condition often leading to social isolation and derision, such as the much-reviled Caliban of Shakespeare’s The Tempest: “What have we here? a man or a fish? dead or alive? A fish: he smells like a fish; a very ancient and fish-like smell . . . . A strange fish!”

Trimethylaminuria is a rare inherited condition that only occurs in people who inherited two copies of the defective gene, one each from their mother and father. It is estimated that 1 in 10,000 suffers from this syndrome.

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Bonus Fact:

A third type of sweat gland, called apoeccrine, has been suggested by some scientists, although a 2007 study that examined axillary (armpit) skin failed to find evidence of it “either by histology or by immunofluorescence.”

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