That onion-like smell coming from your underarms, feet, or groin is almost certainly not caused by onions themselves. It comes from sulfur-containing molecules called thioalcohols, which are among the most pungent components of human body odor. The surprising part is that your sweat is odorless when it leaves the gland. Bacteria living on your skin break down specific compounds in sweat and convert them into those sharp, oniony volatiles. How strongly you produce this smell depends on a tangle of factors, from the species of bacteria colonizing your skin to your genetics, your diet, what you wore to the gym, and even your emotional state.
How Bacteria Create the Onion Smell
Fresh sweat does not smell. The apocrine glands concentrated in your armpits, groin, and around your nipples release a milky fluid containing odorless precursor molecules, including amino-acid conjugates and lipids. These precursors are essentially food for the bacteria that live on your skin. As bacteria metabolize them, they release volatile byproducts, and the sulfur-containing ones are the molecules your nose registers as “onion” or “body odor.”
Two groups of bacteria do most of the heavy lifting. Corynebacterium species, which are lipophilic (fat-loving) bacteria common on human skin, break down odor precursors to release short-chain fatty acids and other volatile compounds that contribute to the general sour or sweaty note of body odor.1PubMed Central. Human Skin Odor Throughout Life: A Literature Review But the really sharp, onion-like edge comes from a different player: certain species of Staphylococcus. Researchers identified a specific enzyme in odor-forming staphylococci, a cysteine-thiol lyase (C-T lyase), that cleaves the odorless thioalcohol precursors in sweat and releases the pungent sulfur molecules responsible for the worst of the smell. When scientists transferred this single enzyme into non-odor-producing staphylococci, those bacteria started producing the same sulfurous stink, confirming the enzyme is both necessary and sufficient for thioalcohol formation.2PubMed Central. The molecular basis of thioalcohol production in human body odour
The evolutionary backstory is worth a quick note. That C-T lyase enzyme did not evolve recently. Genetic analysis suggests it moved into the ancestor of these odor-forming staphylococci through horizontal gene transfer roughly 60 million years ago and has since been fine-tuned to act specifically on human sweat precursors.2PubMed Central. The molecular basis of thioalcohol production in human body odour In other words, specific bacteria have been perfecting the art of making us smell for longer than our species has existed.
Why Stress Sweat Smells Worse
You have probably noticed that the sweat you produce during a stressful presentation smells worse than sweat from a long run. This is not your imagination. Your body has two main types of sweat glands. Eccrine glands cover most of your body and produce the watery, mostly odorless sweat that cools you during exercise. Apocrine glands, concentrated in your armpits and groin, respond more strongly to emotional and hormonal triggers. They secrete a thicker fluid loaded with the lipid and protein precursors that bacteria feast on.
Research into the emotional dimension of body odor has shown that human body odor changes measurably with shifts in emotional state, and that people exposed to those odors can be affected by them in detectable ways.3PubMed Central. Emotional expression in human odour The practical upshot: stress, anxiety, and fear ramp up apocrine output, delivering more raw material for bacteria to convert into that onion-like smell. Exercise sweat is mostly water and salt. Stress sweat is a bacterial buffet.
Foods That Push the Smell Toward Onion
If you ate a plate of garlic pasta last night and smell like an allium this morning, your diet is a contributing factor. Garlic, onions, leeks, shallots, and other members of the allium family are rich in organosulfur compounds. When you digest garlic, for example, these compounds get reduced and converted into allyl methyl sulfide (AMS), a sulfur metabolite that your body cannot break down quickly. AMS is excreted through your breath and through your skin, which is why “garlic breath” can persist for hours and why you may notice a general onion-like smell from your pores the next day.4PubMed Central. Allyl Dimethyl Sulfonium: A Novel Urinary Biomarker of Allium Consumption
Other sulfur-rich foods can produce a similar effect. Cruciferous vegetables like broccoli, cabbage, and Brussels sprouts contain sulfur compounds that are metabolized into volatile byproducts. Spices like cumin and fenugreek are also commonly reported to intensify body odor. Alcohol can temporarily increase sweating and alter skin chemistry. And high-protein diets, particularly those heavy in red meat, tend to shift body odor in a less pleasant direction because protein metabolism generates more nitrogenous and sulfurous waste products that get excreted through sweat.
The dietary effect is temporary. If you stop eating allium-heavy foods for a few days, the sulfur metabolites clear your system and that onion edge fades. This is the easiest cause to rule out: think about what you have been eating for the past 24 to 48 hours.
Your Genes Set the Baseline
Some people naturally produce much less body odor than others, and the biggest known genetic factor is a single gene called ABCC11. This gene encodes a transporter protein expressed in apocrine sweat glands. The “wild-type” (more common globally) version of ABCC11 actively pumps odor precursors out of the gland and onto the skin surface, where bacteria convert them into volatile odorants. A single-nucleotide change at position 538 (from G to A) produces a variant protein that is essentially nonfunctional and is barely expressed in apocrine tissue.5PubMed Central. Clinical and Molecular Evidence of ABCC11 Protein Expression in Axillary Apocrine Glands of Patients with Axillary Osmidrosis
People who carry two copies of the A variant (one from each parent) have a nearly complete loss of the characteristic underarm odor compounds. Their sweat lacks the amino-acid conjugates that bacteria would otherwise convert into thioalcohols, and the steroidal odorant precursors are also significantly reduced.6PubMed. A functional ABCC11 allele is essential in the biochemical formation of human axillary odor This variant is especially common in East Asian populations. The same polymorphism also determines earwax type: the dry, flaky earwax common in East Asian populations tracks with the low-odor ABCC11 variant, while the wet, sticky earwax common in European and African populations tracks with the odor-producing version.7PubMed. Functional characterization of variants in human ABCC11, an axillary osmidrosis risk factor
What this means practically is that if you have always had minimal body odor and dry earwax, your ABCC11 genetics are probably doing you a favor. If you have always been a strong-smelling person despite good hygiene, you are likely producing a high volume of precursors that bacteria eagerly convert. Genetics sets the ceiling; everything else determines how close you get to it.
Why Polyester Smells Worse Than Cotton
If your gym shirt smells terrible after one workout but your cotton undershirt survives a full day at the office, the fabric is a real factor. A study that compared polyester and cotton T-shirts worn during fitness sessions found that after exercise, polyester shirts smelled significantly more intense and less pleasant than cotton shirts, as rated by a trained odor panel.8PubMed Central. Microbial odor profile of polyester and cotton clothes after a fitness session
The reason comes down to which bacteria thrive on which surfaces. Micrococci, a group of bacteria associated with stale and sour odors, were found almost exclusively on synthetic fabrics. Cotton, by contrast, harbored a different microbial community that produced less offensive volatiles. The fiber composition of your clothing literally selects for different bacterial populations, and the ones that love polyester happen to be the ones that generate the worst smells.8PubMed Central. Microbial odor profile of polyester and cotton clothes after a fitness session This is worth keeping in mind if you notice that your onion-like smell seems to cling to workout gear long after washing. Polyester fibers can harbor odor-producing bacteria in ways that cotton does not.
Practical Ways to Reduce the Smell
Since the onion smell is fundamentally a product of bacteria acting on sweat, the most effective strategies target one or both sides of that equation.
- Wash strategically: Soap and water remove the bacterial film and the precursor-laden sweat. Antibacterial soaps or washes containing benzoyl peroxide can reduce the bacterial population in high-odor areas like armpits and groin. Focus on these zones rather than lathering your entire body with harsh products.
- Use antiperspirant, not just deodorant: Deodorant masks the smell. Antiperspirant contains aluminum salts that temporarily block eccrine and apocrine ducts, reducing the volume of sweat that reaches your skin surface. Less sweat means less food for bacteria. Applying at night, when sweat rates are low, gives the aluminum salts time to form the duct-blocking gel before morning activity.
- Choose natural fibers when possible: Cotton, linen, and merino wool all support less odor-producing microbial communities than polyester and nylon. If you need synthetic activewear, look for fabrics treated with antimicrobial finishes, and wash them promptly after use.
- Watch your diet: If onion or garlic-heavy meals consistently precede your worst body-odor days, the connection is real. You do not have to eliminate these foods entirely, but spacing allium-heavy meals away from days where you expect to sweat heavily can help.
- Keep skin dry: Bacteria thrive in warm, moist environments. Thoroughly drying armpit and groin areas after showering, and using absorbent powders in folds of skin, reduces bacterial growth rates.
Emerging research has also looked at botanical compounds that may shift the underarm microbiome away from odor-causing species. One study tested a bioconverted extract from lotus seed on underarm bacteria and found that after a week of application, the populations of Corynebacterium and Anaerococcus (both strongly associated with odor production) dropped significantly, along with the genes linked to volatile fatty acid production.9PubMed Central. Effect of a bioconverted product of Lotus corniculatus seed on the axillary microbiome and body odor Products like this are still in early stages, but the principle of selectively targeting odor-producing bacteria rather than wiping out the entire skin microbiome is gaining traction in cosmetic science.
When It Might Be a Medical Issue
There is a clinical name for persistently strong body odor that goes beyond what hygiene can manage: bromhidrosis. People with this condition tend to have increased numbers and larger volumes of apocrine gland tissue in the armpits compared to people without it.10PubMed Central. Advances in the treatment of axillary bromhidrosis More glands producing more precursors means more substrate for bacteria and a stronger smell, even with excellent hygiene habits.
The prevalence varies enormously across populations. Survey data suggest roughly 6% of Chinese individuals report bromhidrosis, compared to about 90% of people of European descent and even higher rates among people of African descent.10PubMed Central. Advances in the treatment of axillary bromhidrosis Those numbers track closely with the distribution of the ABCC11 gene variant discussed earlier. In populations where the odor-reducing variant is rare, strong underarm odor is essentially the biological default, not a disorder. But in any population, there is a spectrum, and the people at the high end may benefit from medical intervention.
Treatment options for bromhidrosis range from prescription-strength antiperspirants and topical antibiotics to more interventional approaches. Botulinum toxin injections can reduce sweating in the armpits for several months at a time. Laser treatments and surgical techniques that selectively destroy or remove apocrine gland tissue have also been developed. If you have tried every practical measure and still feel that your body odor is unusually strong or has changed suddenly, a dermatologist can help determine whether something beyond normal variation is going on.
A sudden change in body odor is worth paying particular attention to. Conditions like uncontrolled diabetes can produce a sweet or fruity smell, kidney problems can cause a urea-like or ammonia note, and liver disease can produce a musty odor. Hormonal shifts during puberty, pregnancy, and menopause also noticeably alter body odor. If the onion-like smell appeared recently or intensified sharply without an obvious cause like diet change or new medication, mention it to your doctor rather than assuming it is just sweat.
The Microbiome Is Personal
One of the more interesting findings in body odor research is how individually variable the underarm microbiome is. Two people with identical hygiene routines and similar diets can smell quite different because they harbor different ratios of Corynebacterium, Staphylococcus, Micrococcus, and other skin-resident bacteria. The particular cocktail of volatile molecules your bacteria produce creates what researchers sometimes call an “odortype,” a chemical fingerprint that is relatively stable over time for a given individual.
An intriguing question has been whether this odortype is linked to immune-system genes, specifically the HLA (human leukocyte antigen) complex that also plays a role in mate-choice studies in animals. The idea was appealing: your immune genetics might influence the chemical composition of your sweat, and others might unconsciously detect those differences. However, research examining whether HLA-identical siblings share similar odor profiles found that siblings who were genetically identical at HLA did not have notably similar patterns of released odorants compared to siblings who shared no HLA type at all.11PubMed Central. The specific biochemistry of human axilla odour formation viewed in an evolutionary context So while the romantic notion that you can “smell” someone’s immune compatibility persists in popular culture, the biochemical evidence for it remains thin.
What does reliably shape your personal odor profile is the bacterial ecosystem on your skin, which is influenced by your genetics (especially ABCC11), your environment, your hygiene products, and even the people you live with, since skin bacteria transfer through physical contact. Switching deodorants, antibiotics, or even moving to a new climate can shift the microbial balance enough to change how you smell. If you have ever noticed that your odor changed after traveling or after a course of antibiotics, you were observing your microbiome in flux.