The Biochemistry of BO

In 1880, the French novelist J.K. Huysmans, still writing in his “Naturalist” style, published a volume of brief social observations called Croquis parisiens (Parisian Sketches). He devoted one chapter to the odors of the female armpit:

no aroma has more nuances, its range traverses the whole keyboard of the sense of smell . . .

He noted the “pungent scent of goat” that rises from the sleeves of a group of sweaty working class women in Paris. Then he described the “stronger and coarser” odor wafting from women spreading hay in the countryside in the middle of a hot day:

It was excessive, terrible; it stung your nostrils like a flask of ammonia, or rather it gripped you, irritating the mucus membranes with a musky, gamey smell, like wild duck cooked in olives and the sharp odeur of shallots.

Huysmans’s prose is vivid and dramatic yet also full of detail. Anyone who can detect a shallot note in the underarm odor of a French farm woman is either a brilliant olfactory observer or a bullshitter of the first order.

Since the pioneering work of the late Walter B. Shelley in the 1950s we have known that fresh axillary sweat is non-odorous. It is the action of naturally occurring skin bacteria, especially those in the genera Corynebacteria and Staphylococcus, that creates the smelly molecules of BO. Among the stinkers are the steroid-like compounds androstenone and androstenol, discovered in the 1970s, and 3-methyl-2-hexenoic acid (3M2H), isolated in 1991. The former smell musky or urine-like (although many people cannot detect them at all), while the latter has an acrid, pungent bite that pretty much defines ripe underarm odor.

Commercial underarm products reduce BO by limiting sweat production and by killing bacteria directly. Relatively little research has gone into the deep biochemistry of BO formation. By the 1990s it was clear that 3M2H comes from a non-odorous molecule in fresh sweat that is chemically converted by the Corynebacteria. But it wasn’t until 2003 that the entire chain of events was uncovered. The scientists who did it were at Givaudan’s Swiss research center in Dübendorf. Led by Andreas Natsch, they identified an enzyme in Corynebacteria—a bacterial aminoacylase—that cleaves off a glutamine residue from an odorless precursor molecule, thereby creating stinky 3M2H as well as another BO molecule: the cheesy, rancid-smelling, 3-hydroxy-3-methylhexanoic acid (HMHA). The Givaudan team even managed to clone the gene that produces the enzyme. That’s some hot biochemistry.

Since then, the Givaudan team and others have identified another key BO component. Known as 3-methyl-3-sulfanylhexan-1-ol (MSH), this sulfur-containing molecule smells like tropical fruit and onions (scrummy!). MSH is released from an odorless precursor by an enzyme in Staphylococcus bacteria.

A study recently in the headlines looked at sex differences in concentrations of the odorless precursors to fruity-oniony MSH and cheesy-rancid HMHA. Researchers at Firmenich and the University of Geneva (what is it with Switzerland?) spent three winters collecting droplets of fresh sweat from volunteers in the sauna. It turns out that women have far higher amounts of the MSH precursor than do men, which means women (or rather the bacteria that love them) can liberate significantly more of the sulfur volatiles that smell like tropical fruit and onions. Male BO, in contrast, tends to smell cheesy and rancid.

Let’s allow that onions smell like shallots and score this round for J.K. Huysmans. No bullshitter he: in hindsight he was a discerning judge of the feminine armpit. 

In Parisian Sketches Huysmans also declared that the armpits of brunettes, redheads, and blondes have distinctive odors. The jury is still out on that one. But are you willing to bet against him?