Foot odor, medically known as bromodosis, results from a biological reaction on the skin’s surface. The skin on the feet contains an exceptionally high concentration of eccrine sweat glands, totaling around 250,000 per foot. While the sweat produced by these glands is initially odorless, it creates a warm, moist environment perfect for bacteria to thrive. Resident skin bacteria break down the sweat’s components, releasing volatile organic compounds that the nose perceives as a foul smell.
Hyperhidrosis and Excessive Sweating
The most frequent medical cause of persistent, severe foot odor is hyperhidrosis, a condition involving excessive sweating. The feet, along with the palms, are densely covered with eccrine glands that primarily secrete water and salts for temperature regulation. When these glands are overactive, they produce a continuous supply of moisture, effectively “watering” the bacteria on the skin.
This excessive moisture leads to the overgrowth of microbes, particularly species of Staphylococcus. These bacteria metabolize the amino acid L-leucine, which is present in sweat, breaking it down into a compound called isovaleric acid. Isovaleric acid is a short-chain fatty acid responsible for the characteristic pungent, cheesy smell associated with severe bromodosis.
Hyperhidrosis of the feet is classified as primary when it has no identifiable underlying cause, often linked to genetic factors or nervous system overactivity. Secondary hyperhidrosis is less common and results from another medical condition or medication. In either case, the volume of sweat provides a constant nutrient source and habitat for odor-producing bacteria, increasing the production of foul-smelling organic acids.
Infectious Causes: Fungal and Specific Bacterial Conditions
Beyond general bacterial overgrowth fueled by sweat, specific infections can cause or intensify foot odor. Tinea pedis, commonly known as Athlete’s Foot, is a superficial fungal infection that thrives in damp, occlusive conditions. The fungus causes the skin between the toes to become macerated, soft, and cracked. This breakdown of the skin barrier provides easy entry and a rich, moist environment for secondary bacterial colonization, which is often the direct source of the odor. Gram-negative bacteria, such as Proteus species, can infect the damaged tissue, leading to a distinct, often more intensely foul smell than the fungal infection alone.
Pitted keratolysis (PK) is a bacterial infection that is a direct cause of strong foot odor. PK is typically caused by Corynebacterium species, which thrive in excessive moisture and produce enzymes that digest the keratin protein of the stratum corneum, the outermost layer of the skin. This digestion process creates numerous small, crater-like pits on the soles and heels of the feet. The bacteria also release sulfur compounds, such as thiols and sulfides, which are responsible for the condition’s characteristic pungent, sometimes sulfuric or rotten-egg-like smell.
Systemic Health Issues Reflected in Foot Odor
Foot odor can sometimes be a secondary symptom of a systemic health issue. Diabetes is a common example, where long-term high blood sugar can lead to nerve damage (neuropathy) and poor circulation in the feet. The loss of sensation means minor cuts or blisters can go unnoticed, quickly developing into deep, infected ulcers. These diabetic foot infections, often involving multiple types of bacteria, can produce severe, putrid odors that signify advanced tissue damage.
Severe malfunction of the body’s primary filtration organs can also result in altered body odor that includes the feet. When the kidneys or liver are failing, they cannot properly filter metabolic waste products from the blood. These accumulated waste compounds, such as urea and sulfur-containing molecules, can then be excreted through the sweat. Kidney failure may lead to a smell resembling ammonia or bleach, while liver failure can cause the body to excrete sulfur compounds, sometimes contributing to a noticeable, unpleasant odor.