Why your armpits smell like onions (and what it actually means)

If your armpits have a distinct onion-like smell, you are not imagining it. That sharp, sulfurous note is chemically different from the sour or cheesy smell that most people associate with body odour. It comes from a specific molecule, and it is produced through a specific biological pathway that science has mapped in detail over the past two decades.

This article explains exactly what that molecule is, where it comes from, which bacteria produce it, why some people have it and others do not, and what the scientific evidence says about managing it. Every claim is backed by peer-reviewed research.

1. The molecule behind the smell: 3-methyl-3-sulfanylhexan-1-ol

The onion-like component of armpit odour is a molecule called 3-methyl-3-sulfanylhexan-1-ol, commonly abbreviated as 3M3SH. It is a thioalcohol: a class of organic compounds containing a sulfur-hydrogen bond, which is what gives them their potent, sulfurous character.^[1]

Gas chromatography studies have identified 3M3SH as one of the most potent contributors to human axillary malodour, alongside (E)-3-methyl-2-hexenoic acid (3M2H) and 3-hydroxy-3-methylhexanoic acid (HMHA).^[1][2] These three molecules produce different smells. The organic acids, 3M2H and HMHA, produce the sour, rancid notes most people associate with body odour. 3M3SH, the thioalcohol, produces the sharper sulfurous, onion-like note.^[2]

In plain terms What exactly is the onion smell from my armpits?

The onion-like smell from your armpits is a specific chemical compound called 3M3SH. It is different from the "normal" sour body odour smell, which comes from different chemicals entirely. The fact that you can tell the difference by smell means your nose is detecting a real chemical distinction.

2. How your body creates the precursors

3M3SH does not exist in fresh sweat. It is produced on the skin surface when bacteria break down odourless chemical precursors secreted by apocrine sweat glands. The pathway from precursor to volatile odorant involves three steps inside the body before bacteria ever touch it.^[3]

First, inside the apocrine gland cell, an enzyme called glutathione S-transferase conjugates the thioalcohol precursor with glutathione, a small peptide the body uses as a chemical carrier.^[3] Second, the transporter protein ABCC11 moves this glutathione conjugate from inside the cell across the membrane into the gland's secretory lumen.^[3][4] Third, within the secretory vesicle, another enzyme called gamma-glutamyltransferase (GGT1) removes the glutamate residue, leaving behind a smaller conjugate: cysteinylglycine linked to the thioalcohol precursor, written as Cys-Gly-3M3SH.^[3][4]

This Cys-Gly conjugate is the odourless precursor that reaches the skin surface in apocrine sweat. It has no smell. The smell is produced only when bacteria on the skin break it apart.

In plain terms Why does fresh sweat not smell?

Your sweat glands package a smell-producing ingredient inside a chemical wrapper. The wrapped version has no odour. Three separate steps inside the gland prepare and deliver this package to the skin surface. The smell only appears once bacteria on your skin unwrap it.

3. The bacterium responsible: Staphylococcus hominis

The bacterium primarily responsible for converting the Cys-Gly-3M3SH precursor into the volatile thioalcohol 3M3SH is Staphylococcus hominis.^[6] It does this using a PLP-dependent enzyme called ShPatB, classified as a cysteine-thiol lyase (C-T lyase), which cleaves the carbon-sulfur bond in the precursor, releasing the volatile thioalcohol.^[5] This enzyme has evolved novel selectivity for cysteine-conjugated thioalcohol substrates that distinguishes it from generic C-S lyases.^[5]

The specificity of this enzyme matters. S. hominis possesses a PatB variant with a hydrophobic pocket that accommodates the branched thioether side chain of 3M3SH precursors. Staphylococcus epidermidis, the most abundant staphylococcal species on the skin, lacks the PatB gene entirely and cannot produce thioalcohols.^[5] Two closely related species, S. haemolyticus and S. lugdunensis, also carry PatB and can produce thioalcohols, but S. hominis has the highest catalytic efficiency for Cys-3M3SH and is the species most strongly correlated with axillary odour.^[5][6]

The full conversion pathway inside the bacterial cell involves at least two steps. First, the intact Cys-Gly-3M3SH dipeptide conjugate is transported into the S. hominis cell by a dedicated proton-coupled oligopeptide transporter (SH1446).^[7] Once inside, a dipeptidase removes the glycine residue, and then ShPatB cleaves the carbon-sulfur bond in the remaining Cys-3M3SH, releasing volatile 3M3SH.^[5]

In plain terms Which bacterium causes the onion smell?

A small group of closely related bacteria, led by Staphylococcus hominis, have a specialised enzyme that can cut apart the odourless precursor delivered by your sweat glands. The cut releases the onion-smelling molecule. Most other skin bacteria, including S. epidermidis (the most abundant staphylococcal species), lack this enzyme entirely. S. hominis is the primary contributor because it has the highest activity against the precursor and the strongest correlation with body odour.

4. The other pathway: why some people smell cheesy instead of oniony

The onion-like thioalcohol pathway is only half the story. A separate group of bacteria produces an entirely different set of odorants using a different enzyme and different precursors.

Corynebacterium species in the axilla possess a zinc-dependent N-alpha-acyl-glutamine aminoacylase that cleaves a different class of odourless apocrine precursors: N-acylglutamine conjugates.^[8][9] The products of this reaction are volatile fatty acids, principally (E)-3-methyl-2-hexenoic acid (3M2H) and 3-hydroxy-3-methylhexanoic acid (HMHA). These are the molecules responsible for the sharp, rancid, goat-like smell that most people think of as "body odour."^[8][10]

This means two distinct biochemical pathways operate simultaneously in the armpit. The thioalcohol pathway (S. hominis producing 3M3SH) creates the onion-like component. The volatile fatty acid pathway (Corynebacterium producing 3M2H and HMHA) creates the sour, cheesy component. Your personal body odour is a blend of both, and which one dominates depends on which bacteria are more abundant on your skin.^[11][12]

5. Your microbiome balance shapes your odour

16S rRNA gene sequencing studies have shown that the relative abundance of Corynebacterium and Staphylococcus species in the axilla strongly influences odour type and intensity.^[11] People with higher proportions of Corynebacterium tend to produce more volatile fatty acids, resulting in a stronger, more acidic body odour. People with higher proportions of specific Staphylococcus species, particularly S. hominis, tend to produce more thioalcohols, resulting in a more sulfurous, onion-like odour.^[11][12]

This microbial balance is individually specific and stable over short time periods: intra-individual variation over 30 hours was found to be smaller than inter-individual variation.^[11] The same person tends to maintain a consistent ratio of these bacterial genera, which is why body odour profile tends to be consistent from day to day.^[11] Factors that can shift the balance include changes in skin pH, antibiotic use, hormonal changes, and the products you apply to your skin.^[11][13]

There is also a sex-based difference. Males tend to harbour higher proportions of specific Corynebacterium and Staphylococcus haemolyticus operational taxonomic units, and exhibit stronger fatty and acid-spicy odour descriptors. This is consistent with higher androgen levels that influence apocrine gland activity.^[11]

In plain terms Why does my body odour have its own consistent character?

Your armpit hosts two main groups of bacteria, and each group produces a different type of smell. The balance between these two groups varies from person to person and stays fairly stable over time. That is why your body odour has its own consistent character. If your smell is more "oniony" than "cheesy," it means the thioalcohol-producing bacteria are more dominant on your skin.

6. The gene that decides whether you produce body odour at all

Whether you produce the precursors for thioalcohol body odour in the first place depends on a single gene: ABCC11. This gene encodes the transporter protein that moves glutathione-conjugated odour precursors from inside the apocrine gland cell into the secretory lumen. Without a functional ABCC11 transporter, the precursors never reach the skin surface, and bacteria have nothing to convert.^[3][14]

A single nucleotide polymorphism in ABCC11 (rs17822931, c.538 C>T) determines whether the transporter functions. People who carry at least one copy of the functional allele produce apocrine odour precursors normally. People who carry two copies of the loss-of-function allele produce a non-functional ABCC11 protein (a single amino acid substitution, Gly180Arg) that is targeted for proteasomal degradation and cannot transport the precursors. These individuals produce little to no body odour from their axillae.^[14][15]

The loss-of-function ABCC11 allele is one of the most geographically differentiated genetic variants in humans. It reaches frequencies of 80 to 95 per cent in East Asian populations (Chinese, Japanese, Korean), meaning the majority of people in these populations produce little axillary body odour. In African and European populations, the frequency is below 5 per cent.^[16]

The same gene also determines earwax type. People with the loss-of-function allele have dry, flaky earwax rather than the wet, sticky type.^[15] Both earwax and apocrine sweat pass through secretory pathways that depend on ABCC11. If your earwax is dry, your body produces fewer of the precursors that bacteria convert into body odour.

In plain terms Can your genes determine whether you have body odour?

A single gene controls whether your sweat glands deliver odour-producing raw materials to your skin. In most of East Asia, the majority of people carry a version of this gene that means their sweat glands deliver very little of these raw materials, so they produce minimal body odour. The same gene determines whether your earwax is dry or sticky.

7. Why some people notice the smell and others cannot

Even when 3M3SH is present, the ability to detect it varies from person to person. The difference is genetic.

The perception of specific odorants is mediated by olfactory receptor proteins in the nasal epithelium, and the genes encoding these receptors are highly polymorphic. The best-studied example in body odour science is androstenone, a steroid compound present in male sweat. Genetic variation in the olfactory receptor OR7D4 determines whether a person perceives androstenone as offensive, pleasant, or undetectable.^[17]

A similar mechanism likely operates for thioalcohols such as 3M3SH. Genetic variation in olfactory receptor genes may explain why some people are acutely sensitive to the onion-like component of body odour while others barely notice it. Research into the specific receptor-gene associations for thioalcohol perception has not yet reached the same level of certainty as the androstenone findings.

In plain terms Why can some people not smell body odour that others find strong?

Just as some people cannot taste the bitterness in Brussels sprouts because of their genetics, some people are more (or less) sensitive to the onion-like smell of thioalcohols because of differences in their smell receptor genes. If someone tells you they cannot smell it, they may genuinely be unable to detect it.

8. What works against onion-like body odour

Understanding the biochemistry changes the approach to managing it. Standard antiperspirants work by blocking sweat glands with aluminium salts, reducing the total volume of sweat that reaches the skin surface. This can help, but it does not specifically target the thioalcohol pathway. A more targeted approach addresses the bacterial enzymes responsible for producing the odorant.^[18]

Zinc salts contribute through odorant capture: zinc ions form stable coordination complexes with thioalcohol molecules, binding the sulfur atom and preventing the compound from volatilising.^[19][20] This complexation mechanism reduces the onion-like component of body odour at the release stage. Zinc-based approaches do not directly inhibit the C-T lyase enzyme responsible for producing thioalcohols. Targeted enzyme inhibition requires chemistry specifically designed to block the PatB active site, which is a different intervention from zinc complexation.^[18]

The choice of zinc compound matters. Some zinc formulations provide both antimicrobial and complexation effects. Others provide odorant complexation without significant antimicrobial activity, preserving the skin microbiome while reducing odour.^[20] The distinction is relevant for people who experience skin sensitivity to more aggressive antimicrobial agents.

An alternative strategy targets the microbiome composition itself. Research has shown that lowering the skin's pH in the axilla reduces Corynebacterium abundance and shifts the bacterial community towards Staphylococcus species.^[13] Since Corynebacterium are the primary volatile fatty acid producers, this shift can reduce the acid and cheesy component of odour. For the thioalcohol component specifically, strategies that reduce S. hominis colonisation or inhibit its C-T lyase activity are the most direct biochemical approaches.

Why single-ingredient approaches have limits

Zinc-based deodorants can reduce the onion-like component of body odour, but they address the thioalcohol pathway at only one or two control points. For people with higher apocrine output, which includes those going through perimenopause, hormonal shifts, or sustained physical stress, the rate of precursor secretion can exceed what a single mechanism can neutralise. The S. hominis population within biofilm-protected reservoirs in hair follicles continues producing thioalcohols even while zinc on the skin surface is complexing the volatiles that have already formed.

This is why some people find that zinc-based deodorants help but do not fully resolve the onion-like component of their body odour. The product is working. It is addressing the steps it was designed to address. It is just not covering enough of the pathway to keep up with the volume of precursors being delivered to the skin.

What the thioalcohol pathway requires

The biochemistry described in this article points to five control requirements for thioalcohol management:

• C-T lyase inhibition to slow the enzymatic production of 3M3SH at the source.^[18]
• Thioalcohol sequestration to capture volatile molecules that are produced before they reach the air.^[19]
• Selective antimicrobial action targeting the odour-producing S. hominis population without disrupting the beneficial commensal bacteria that maintain colonisation resistance.
• Biofilm disruption to reach the protected bacterial populations that surface washing and topical zinc cannot access.
• pH management to maintain the acidic environment that influences the overall balance of odour-producing species on the skin.^[13]

Most deodorants address one or two of these requirements. A zinc oxide formulation provides enzyme inhibition and some thiolate complexation. An antimicrobial formulation may reduce bacterial load but does not sequester the volatiles already produced. A pH-adjusting product shifts the microbial balance but does not directly inhibit thioalcohol production. Each approach contributes. For people with higher precursor volume, the question is whether enough of the pathway is being addressed simultaneously.

In plain terms Why doesn't my deodorant stop the onion smell completely?

The onion smell is produced through a multi-step process: your sweat glands deliver precursors, bacteria break them down, and the volatile molecule reaches the air. Most deodorants target only one or two steps in this chain. If your body is producing a high volume of precursors (due to hormonal changes, stress, or genetics), blocking one step may not be enough to keep up. A formulation that addresses multiple steps simultaneously, stopping the enzyme, capturing the molecule, managing the bacteria, and reaching bacteria inside biofilms, has a better chance of keeping pace with the production rate.

When to see a doctor

A sudden change in body odour that does not correlate with any change in diet, medication, or hygiene may indicate an underlying medical condition. Metabolic disorders such as trimethylaminuria can produce distinctive body odours sometimes confused with severe bromhidrosis. Hormonal changes associated with thyroid conditions, diabetes, or liver and kidney disease can also alter body odour. These conditions may affect odour through the bacterial conversion pathways described in this article, through direct excretion of volatile compounds, or through mechanisms that science has not yet fully characterised.

If your body odour has changed suddenly, persists despite good hygiene, or is accompanied by other symptoms, this warrants medical evaluation. A doctor addresses the underlying condition. The Volatile Control System addresses whatever component of the odour is manageable at the skin surface, across multiple pathways simultaneously. If the VCS cannot fully resolve the odour, no other topical product will. What remains belongs to medicine. SD Labs provides scientific education to help you understand your body. We do not diagnose or treat medical conditions.

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