It’s an unpleasant smell that permeates the air whenever you take your shoes off – the stench of foot fungus. Even a faint whiff of it can turn heads and wrinkle noses. But why does foot fungus give off such a potent, nauseating odor? The reasons have to do with the biology and chemistry behind this common fungal infection.
Foot fungus, also known by the medical term tinea pedis, refers to a variety of fungal organisms that can infect the top layers of skin on the feet. In many cases, foot fungus initially causes itchy, red, flaky skin between the toes. As it progresses, the infection can spread to the soles of the feet and the sides of the feet, resulting in thickened, yellowing toenails in some individuals. If left untreated, the fungus burrows deep into the layers of the skin on the feet.
The main culprit behind foot fungus is a type of fungus called dermatophytes, which feed on keratin. Keratin is a tough, fibrous structural protein that makes up much of the outermost layer of skin as well as hair and nails. As colonies of dermatophytes grow and spread out across the skin of the feet, they produce keratinase – an enzyme that breaks keratin down into simpler proteins and amino acids which the fungi absorb for nourishment.
This digestive process creates a byproduct – volatile sulfur compounds like hydrogen sulfide, methyl mercaptan, and dimethyl sulfide. In chemistry terms, these sulfur-containing molecules are the source of the offensive “rotten egg” smell that comes from foot fungus infections. The fungi generate more and more of these volatile compounds as they feast on greater amounts of keratin across an expanding infection site.
Excess moisture further enables the production of these smelly sulfur compounds by foot fungi. Damp, sweaty feet provide an ideal warm, humid microclimate for dermatophytes like Trichophyton rubrum to proliferate. Besides moisture, dead skin cell buildup in thick calluses also furnishes an abundance of keratin for the fungi to dine on, fueling the release of even more noxious sulfuric metabolites.
Adding to the problem is that shoes and socks hamper air circulation around the feet. Tight-fitting shoes prevent evaporation of sweat, allowing moisture to be retained against the skin for longer periods. With frequent use, microbes like dermatophyte fungi can begin to colonize inside shoes. Warm, damp conditions inside shoes then promote fungal growth and augmented production of smelly sulfur compounds. Keeping feet dry and aired out is therefore important for control of foot odor.
In rare cases, a serious condition called gangrene can cause feet to smell. Gangrene refers to decay and death of body tissues due to either injury or poor blood supply. Diabetics are at increased risk for gangrene of the feet. Bacteria invade damaged or necrotic flesh, generating foul-smelling gases containing organic acids, methylamines, and hydrogen sulfide – hence the putrid smell. However, gangrene is a far less common cause of foot odor compared to mundane foot fungus.
Apart from maintaining good foot hygiene, using antiifungal creams, sprays and powders can help curb fungal overgrowth and thereby reduce foot odor. When washing the feet, gentle scrubbing followed by thorough drying – especially between the toes – helps diminish moisture and exfoliate layers of dead, fungus-infected skin. Treating shoes and socks with antifungal powders or sprays helps destroy residual fungal spores inside them. In some cases, prescription oral antifungal drugs may be necessary to eliminate an aggressive or recurring foot fungus infection.
The next time your feet smell particularly unpleasant, you can likely blame colonies of tiny fungi that have accumulated between your toes or over your soles. As these opportunistic dermatophytes digest protein from your skin and nails, they generate reeking sulfur metabolites in the process – which are then released to assault your nose. So although having stinky feet may be embarrassing, just know it’s merely a byproduct of microscopic fungi consuming components of your skin! With vigilance about foot hygiene and early treatment, the foul fungi foot odor can at least be contained to minimize any further assaults on your smell receptors.