Is Polyester Bad For Testosterone? A Straight Answer
The straight answer
Testosterone in young men has fallen 25% in seventeen years. Heat suppresses production. Polyester holds heat against the skin. The research does not join those dots for you, so here is what it actually says.
No hand-waving, no hedging. Three separate bodies of evidence, what each one found, and what it means for what you put on every morning.
Mean total testosterone, US men aged 15 to 39
A drop of roughly 25%, and it held after adjusting for BMI, activity level and comorbidities. 4,045 men, NHANES data. Source: Lokeshwar et al, European Urology Focus, 2021.
That is the number most men have seen. What almost nobody explains is the mechanism, and there are two of them.
Mechanism one: heat.
Testosterone and sperm are produced in the same place, and that place needs to run a couple of degrees cooler than the rest of you. It is why the anatomy sits outside the body.
Polyester is dense and non-porous. It moves sweat to the surface and holds warmth underneath, against the skin. Not as a flaw. That is what the fibre does.
In a liner, in a waistband, that is the part pressed hardest against you for the entire session.
Sperm concentration by underwear type, 656 men
Men in loose-fitting boxers had 25% higher sperm concentration, 17% higher total count and 14% lower FSH than men in tighter underwear. 656 men, 1,186 semen samples. Source: Mínguez-Alarcón et al, Human Reproduction, 2018. This compared fit rather than fabric, but the mechanism it demonstrates is heat.
Heat suppresses production. That part is not disputed by anyone. The only question is what you are wrapping it in.
Mechanism two: what comes off the fabric.
A US national health survey found higher urinary phthalate metabolites associated with lower serum testosterone, across men, women and children. Phthalates are used in garment prints, plastisol logos and coatings.
Then there is what the polyester itself is made with. Antimony trioxide is the catalyst used to produce PET and is present in 80 to 85% of virgin polyester. Lab testing found 125 to 470 micrograms per gram in finished fabric, a fraction of which migrated into artificial sweat under standard conditions.
You sweat on it for an hour a day.
Human testicular samples containing microplastics. Every one tested. PET, the plastic in your gym shorts, was among the most common polymers found. Toxicological Sciences, 2024.
What the evidence establishes.
Testosterone is falling
605 to 451 ng/dL between 1999 and 2016 in men aged 15 to 39. Roughly 25%, holding after adjustment for weight and activity. Lokeshwar et al, n=4,045.
Phthalates track with lower testosterone
NHANES data found higher urinary phthalate metabolites associated with lower serum testosterone across every group measured. Meeker and Ferguson, JCEM, 2014.
Polyester migrates into sweat
125 to 470 µg/g antimony in finished polyester, a fraction of which released into artificial sweat under ISO test conditions. Biver et al, 2021.
Sperm counts have halved
A 51.6% decline in Western men between 1973 and 2018, and the rate has more than doubled since 2000. Levine et al, 42,935 men.
So why has nobody told you this?
Every product developer at every major sportswear brand knows polyester traps heat against the skin. It is basic material science. They compete on stretch and colourways because those are the arguments plastic can win.
You are cheaper to keep uninformed.
The layer against your skin is the one to change first.
Start with the liner and the underwear, because those are the parts held against you for the entire session and they are almost always synthetic even when the shell is not.
Nero shorts use a 100% GOTS organic cotton shell and a 100% merino liner at 17.5 micron. Merino absorbs moisture into the fibre and releases it as vapour, and regulates temperature in both directions rather than holding heat in one. The boxers are 100% organic cotton and nothing else.
No polyester. No nylon. No elastane. Not a percentage, not a trace.
We cover the fertility research separately in does wearing polyester affect male fertility, and the fuller hormone breakdown is on our testosterone page.
The short version
Testosterone in young men fell 25% in seventeen years. Sperm counts have more than halved since 1973. Microplastics have been found in every human testicular sample tested, and PET is among the most common polymers in them.
Heat suppresses production, and polyester is a fabric that holds heat against skin. Phthalates track with lower testosterone, and they are in garment coatings. Antimony sits in the polyester itself and migrates into sweat.
Nobody is going to hand