As you might imagine, foot odor is usually caused by sweaty feet. But it's a little more complicated than that.
Foot odor is different from other types of body odor. Most body odor emanates from the genitals and armpits, which contain apocrine sweat glands. The rest of the body, including the feet, is covered with eccrine sweat glands. What's the difference? All sweat is basically odorless, but the sweat from apocrine glands contains protein and amino acids that feed bacteria living on the skin. The bacteria generate body odor.
Sweat from eccrine glands, on the other hand, is watery and thin. It tends to evaporate pretty quickly and doesn't usually cause a smell. But your feet have about half a million of the body's 2 to 5 million sweat glands, so there's a higher concentration of sweat. Some researchers believe that this evolved back when we walked around barefoot all of the time, and the constant contact with the ground meant that our feet got hotter and needed more sweat glands to stay cool. The sweat just evaporated off bare feet.
Today, when your feet sweat, you're usually wearing socks and shoes, and the sweat gets trapped inside. Bacteria are to blame, but unlike with other types of body odor, the bacteria on your feet don't feed on sweat. One culprit is Brevibacterium linens. This bacterium lives on your feet because it likes to eat the dry, dead skin. It also thrives in warm, moist environments -- like sweaty socks. When the Brevibacteria eat, they break down an amino acid into a gas. That gas, methanethiol, has been described as smelling like rotten eggs or cabbage. If your foot odor is more vinegary, it might come from Staphylococcus epidermis. This bacteria eats another amino acid found on the skin and produces isovaleric acid. When either -- or both -- of these bacteria go crazy, the result is stinky feet.
Now you know why you have foot odor. Next, we'll look at ways to get rid of it.