Sweat itself is close to odorless. What smells is bacteria feeding on it, and different fabrics grow different bacteria in very different amounts. A study out of Ghent University put this to the test directly: the same people, the same workout, one shirt in polyester and one in cotton. The polyester lost, badly.
Why does sweat smell different depending on what you're wearing?
Fresh sweat has almost no smell on its own. The compounds your skin secretes, mostly fatty acids, are too large to be volatile, meaning they don't evaporate and reach your nose. Bacteria living on your skin and in your clothing break those compounds down into smaller molecules that do evaporate, and that's what you're actually smelling.
Which bacteria show up, and how many of them, depends heavily on the fabric they're growing on. That's the part most people never think about when they blame the smell entirely on how much they sweat.
What did the research actually find?
Researchers at Ghent University, led by Chris Callewaert, collected t-shirts from 26 healthy volunteers after an intensive spinning class, then incubated the shirts for 28 hours before a trained odor panel assessed them. The results, published in the journal Applied and Environmental Microbiology, were not close. The polyester shirts were rated significantly more unpleasant, more intense, and more sour and musty than the cotton shirts worn by the same people during the same workout.
When the researchers looked at what bacteria were actually growing on the fabric, one species stood out: Micrococcus, found in almost every polyester shirt and rarely on the cotton ones. That single bacterial group appears to be doing most of the damage.
Why does polyester grow different bacteria than cotton?
It comes down to the surface of the fibre itself. Polyester is hydrophobic, meaning it repels water, but it holds onto skin oils and sebum well. That combination gives odor-causing bacteria both shelter and a steady food source. Cotton absorbs water more readily, which changes the environment enough that the same bacteria don't establish themselves in the same numbers.
Later research digging into the mechanism found something else worth knowing: cotton's higher water content means some bacteria can stay active even after the fabric has dried. So natural fibres aren't magic either. They're just working with a different, generally less favourable environment for the specific bacteria that cause the worst odors.
Polyester's surface holds onto sweat oils and shelters odor-causing bacteria, mainly a species called Micrococcus, far more than cotton does. That's a documented, measured difference, not a marketing claim.
Does washing more often just fix it?
Only partly, and often not for long. Bacteria that have colonised synthetic fibres can be difficult to fully remove at low wash temperatures, which is exactly the temperature most performance fabric is designed to be washed at. Anyone who's had a polyester shirt come out of the wash and still smell faintly sour has run into this firsthand.
Some brands respond by adding antimicrobial treatments, often silver-based, directly into the fabric. These work in the short term, but they don't distinguish between odor-causing bacteria and everything else living on your skin, which is a blunter solution than it sounds.
Does this apply to every synthetic fabric equally?
No, and this is where the research gets more interesting than a simple polyester-versus-cotton headline. The same broader research into textile microbiology has found that nylon is a particularly good environment for the bacteria behind foot and acne-related odour, while wool, despite being a natural fibre, supported a wide range of bacterial growth in early testing. One fibre, viscose, stood out in preliminary work as a surface where almost nothing grew at all.
The pattern that holds up isn't "natural always wins." It's that fibre chemistry, how hydrophobic it is, how it handles moisture, what it's coated with, determines which bacteria move in. Cotton and hemp happen to land on the better end of that spectrum for the bacteria that cause the worst gym-bag smells, which is a real, measurable property rather than a marketing story.
Do natural fibres actually solve this?
They shift the problem rather than eliminate it entirely. Any fabric worn during intense exercise will eventually need a wash. But hemp and organic cotton don't offer the same hospitable surface to Micrococcus that polyester does, so the pieces tend to hold up for longer between washes without that sour, embedded smell setting in.
It's also why hemp specifically has a long-standing reputation among people who train often. It isn't a treatment sprayed onto the fibre. It's a property of the fibre itself, which is the part that doesn't wash out over time.
Our cotton-hemp and Tencel™ jerseys are built around exactly this. No synthetic fibres, no antimicrobial coatings, just fabric that doesn't give odor-causing bacteria much to work with. See the current collection, or read our full FAQ on fit and care for wash instructions that keep it that way.