An onion-like armpit smell comes from bacteria breaking down sweat from apocrine glands, and can be affected by diet, stress, and hormones.
Cause
Armpits have apocrine glands that release a milky sweat with proteins and lipids. Bacteria break it down into odor compounds, including thioalcohols (onion-like) and acids.
Why onion
Foods like onion, garlic and curry, as well as stress sweat, can strengthen the smell. Hormones and puberty change body odor.
What helps
- Wash daily with antibacterial soap.
- Use antiperspirant (aluminum) and deodorant.
- Shave or trim.
- Wear breathable fabrics.
- Wash clothes in hot water.
Medical causes
Diabetes, kidney or liver disease, thyroid conditions or infection can change odor.
See a doctor if
The smell is sudden or strong despite hygiene.
Why armpits smell
Fresh sweat has little odor. Armpits have two types of sweat glands: eccrine glands, which release watery sweat to cool the body, and apocrine glands, which release a thicker fluid with proteins and fats, mainly after puberty. Skin bacteria, including Corynebacterium species and Staphylococcus hominis, break down apocrine sweat into volatile compounds such as thioalcohols and short-chain fatty acids, which create body odor.
Why onion
Some of the odor compounds are sulfur based, notably 3-methyl-3-sulfanylhexan-1-ol, which smells like onion. Sulfur compounds are strong even at very low levels. Diet can also change body odor: onions, garlic, cruciferous vegetables, curry, red meat and alcohol release sulfur compounds that are excreted in sweat. Stress sweat comes from apocrine glands and often smells stronger.
What helps
- Wash daily with antibacterial soap, scrubbing the armpits, and dry thoroughly.
- Use antiperspirant (with aluminum chloride) to reduce sweat, and deodorant to mask or fight bacteria. Apply antiperspirant at night to dry skin.
- Shave or trim underarm hair, which holds moisture and bacteria.
- Wear breathable fabrics like cotton and wool, and wash clothes after each wear. Synthetic fabrics hold odor more.
- Wash shirts in hot water and treat stains with enzyme cleaners.
- Cut back on strong-smelling foods and alcohol to see if odor changes.
- Try a switch of deodorant type, like benzoyl peroxide wash to reduce bacteria.
Medical causes
- Hyperhidrosis, excessive sweating, affecting about 5% of people.
- Bromhidrosis, medically strong body odor.
- Diabetes, kidney disease or liver disease, which can change body smell.
- Thyroid problems, hormonal changes and menopause.
- Trimethylaminuria (“fish odor syndrome”), a rare genetic condition.
- Infections like erythrasma, caused by Corynebacterium, which gives a coral-pink fluorescence under UV light.
- Medications and supplements.
See a doctor if
The smell is new, sudden or persistent despite good hygiene, comes with heavy sweating, night sweats, weight loss, fatigue or other changes, or if a rash appears. Prescription antiperspirants, botulinum toxin injections, or microwave treatment (miraDry) can help severe cases.
Sweat chemistry and bacteria
Sweat itself is nearly odorless. Smell develops when bacteria on the skin break down proteins and lipids from apocrine glands in the armpits into volatile compounds such as thioalcohols, which can smell like onions or sulfur. The apocrine glands become active at puberty, which is why children rarely have body odor. Sulfur rich foods like onions, garlic and cruciferous vegetables, as well as spices such as curry, change the smell of sweat. Stress sweat contains more proteins and often smells stronger. Washing with antibacterial soap, using antiperspirants containing aluminum salts, wearing breathable fabrics and shaving armpit hair reduce odor. Rare causes include trimethylaminuria (fish odor syndrome), diabetes, liver or kidney disease, and thyroid disorders. A sudden change in body odor with other symptoms like weight loss, thirst or fever should be assessed by a doctor.
Based on data from the Mayo Clinic and the American Academy of Dermatology
