Why do feet smell like cheese?

Those who want to relax and put their feet up in the evening often have to cope with an attack on the nose. The scent cocktail that steams towards you when you take off your shoes is more of a habit. It may differ individually in nuances and intensity, but four bad notes are always represented: sweat, cheese, vinegar and rotten cabbage. The aromas are produced by bacteria, which are particularly abundant on the soles of the feet, where they have a rich nutrient buffet made up of dead skin cells and sweat. They feel particularly comfortable in the warm, humid climate of closed shoes.

Skin cells and sweat in and of themselves are practically odorless. So-called apocrine sweat glands, which also release fragrances directly under the armpits, do not exist on the feet. Foot odor occurs when bacteria, especially spherical staphylococci, convert sweat ingredients and skin flakes into acetic acid, rancid-smelling butyric acid and valeric acid. The latter has a sweaty, cheesy aroma, can be perceived even in tiny doses and is an unmistakable guiding scent for cheesecake.

There is a reason why smelly feet are so called: The aroma of cheeses such as Limburger, Harzer cheese or Blue Stilton is determined by a similar acid mix. The foul-smelling cabbage note from foot odor is due to a sulfur-containing substance called methanethiol, which is also a product of bacterial breakdown processes and is also found in coffee, pastries and oysters. Apparently the dose and mixture determine the stink factor here.

Some people just don't notice the stuff

"Odors are very complex, and how we evaluate them also depends on the context," says Hanns Hatt from the Ruhr University in Bochum. The fact that people turn up their noses when they smell their feet is not a natural result, but rather follows from upbringing, culture and experience. "Foot odor is usually associated with poor hygiene." If you show test subjects a piece of cheese to smell their feet, many would even find the scent quite pleasant.

Before the brain evaluates odor information, however, the corresponding odor molecules must first identify the appropriate receptor in the nose. Hatt's team has identified the receptor for foot odor. To this end, the researchers examined people who cannot smell cheese feet and in whom a receptor showed genetic mutations at crucial points. The knowledge should help to find some kind of anti-odor that blocks this receptor.

Airy footwear, socks made of natural fibers, foot powder and the guiding scents of lemon and rose citral or geraniol are also effective antidotes, which are supposed to disable an enzyme involved in the development of odor. And psychological tricks may help with odor management: for example, the visualization of a delicious cheese or the thought that foot bacteria are also good. After all, the tiny ones supply the skin with nourishing oils and with substances that protect against infections. Therefore, care is also required if your own feet suddenly smell differently. Then the microbial flora could be out of balance and, for example, a fungal infection could be emerging.