Leggings that smell like fish usually come down to one of three things: bacteria thriving in synthetic fabric, normal sweat chemistry reacting with that fabric, or less commonly, a vaginal health condition transferring odor to clothing. The good news is that most causes are straightforward to address once you know what’s behind it.
Synthetic Fabric Traps Odor-Causing Bacteria
Most leggings are made from polyester, nylon, or spandex blends, and these synthetic fibers are significantly worse at harboring smell than natural fabrics like cotton or wool. A study comparing polyester and cotton clothing after exercise found that the two materials encouraged completely different bacterial populations. Polyester specifically promoted the growth of Micrococci, a type of bacteria found almost exclusively on synthetic shirts and barely present on cotton ones. These bacteria are aggressive odor producers.
The reason is structural. Synthetic fibers are smooth and non-absorbent, so sweat sits on the surface rather than being wicked into the fiber itself. This creates a warm, moist film that bacteria love. Cotton absorbs moisture into its fibers, which paradoxically makes it less hospitable to the surface-dwelling bacteria that generate the strongest smells. Your leggings essentially become a petri dish during a workout or a long day of wear, and the odor compounds those bacteria produce can include amines, the same chemical family responsible for the smell of spoiling fish.
How Sweat Becomes a Fishy Smell
Sweat itself is nearly odorless when it leaves your body. The smell develops when skin bacteria break down the proteins and fatty acids in apocrine sweat (the type produced in your groin, underarms, and other high-friction areas). Bacteria like Staphylococcus hominis convert these odorless secretions into volatile chemicals, including sulfur compounds that can smell musty, sour, or distinctly fishy depending on the specific byproducts.
Diet plays a role too. Foods rich in choline (eggs, liver, certain legumes) and marine fish are broken down in your gut into trimethylamine, or TMA, the exact compound responsible for the smell of rotting fish. Normally, your liver converts TMA into an odorless form before it can accumulate. But if you eat large amounts of these foods, or if your liver’s conversion process is less efficient than average, small amounts of TMA can end up in your sweat. When that sweat gets trapped against synthetic leggings for hours, the fishy smell concentrates.
When It Could Be a Health Condition
If the fishy smell is strongest in the crotch area of your leggings and you notice it even on days you haven’t exercised, a vaginal infection may be the source. Bacterial vaginosis (BV) is the most common culprit. BV causes an overgrowth of certain vaginal bacteria that produce trimethylamine, the same molecule behind fishy-smelling fish. Research has confirmed that trimethylamine is present in the vaginal discharge of women with BV but absent in women without it. The smell often intensifies after sex or during your period because changes in pH release more of the compound.
Trichomoniasis, a common sexually transmitted infection, can also produce a fishy-smelling discharge. The CDC notes that symptoms include a thin, clear, white, yellowish, or greenish discharge with a fishy smell, along with itching, burning, or redness. Trich can’t be diagnosed by symptoms alone and requires a lab test, so if the odor is persistent and comes with any irritation or unusual discharge, getting tested is the fastest path to an answer.
Trimethylaminuria
In rare cases, a metabolic condition called trimethylaminuria (sometimes called “fish odor syndrome”) causes the body to excrete trimethylamine through sweat, breath, and urine. People with this condition lack the liver enzyme needed to fully convert TMA into its odorless form. The result is a persistent fishy smell that shows up in clothing, especially in areas where sweat accumulates. This is uncommon enough that it’s worth considering only if the smell affects all your clothing, not just leggings, and persists regardless of washing or fabric type.
Getting the Smell Out of Leggings
Standard laundry detergent often fails to remove fishy odors from synthetic fabric because the bacteria and their byproducts cling to polyester fibers even through a wash cycle. A few strategies work better:
- White vinegar soak: Soaking leggings in a basin with one part white vinegar to four parts cold water for 30 minutes before washing helps neutralize amine compounds, which are the alkaline molecules causing the fishy smell.
- Baking soda in the wash: Adding half a cup to your wash cycle helps neutralize odors that detergent misses.
- Cold water, not hot: Hot water can set odors into synthetic fibers. Wash leggings in cold water on a gentle cycle.
- Skip fabric softener: Softener coats synthetic fibers with a waxy layer that traps bacteria and odor underneath, making the problem worse over time.
- Air dry fully: Tossing damp leggings into a drawer or gym bag creates the exact environment bacteria need to multiply. Hang them to dry completely.
If the smell returns within a single wear despite thorough washing, the fabric may be permanently colonized. Some leggings simply reach an end of life for odor, especially cheaper polyester blends worn frequently for exercise.
Preventing the Smell in the First Place
Switching to leggings with a higher nylon content or those marketed as antimicrobial can help, though no synthetic fabric performs as well as natural fibers for odor resistance. Wearing moisture-wicking underwear (cotton or merino wool) underneath creates a buffer layer that absorbs sweat before it reaches the leggings. Changing out of leggings promptly after exercise, rather than running errands in them for another few hours, makes a noticeable difference because the bacterial population on the fabric roughly doubles every 20 to 30 minutes in warm, moist conditions.
If you suspect diet is contributing, reducing intake of fish, eggs, and choline-rich supplements for a week or two and seeing if the smell changes can be a simple test. For anyone noticing the smell specifically from vaginal discharge rather than general sweat, addressing the underlying cause will solve the clothing odor far more effectively than any laundry trick.