Most of us probably wouldn’t contemplate going for a run in jeans or tackling a HIIT class in an everyday bra, yet the underwear beneath our gym kit tends to receive considerably less thought.
But with sweat, friction and tight-fitting gym clothes all potentially affecting intimate skin, perhaps it deserves more consideration. From moisture-wicking seamless briefs to plastic-free thongs, there are now plenty of pairs designed - or marketed - specifically for exercise, but do we actually need designated ‘workout underwear’, and could your trusty cotton knickers be doing more harm than good during a sweaty session?
Experts explain what happens when we sweat down there, which fabrics perform best and the underwear mistakes to avoid at the gym.
Can the wrong underwear cause thrush or a UTI?
There is an important distinction between causing an infection and creating conditions that may contribute to irritation.
"Tight, sweaty clothing combined with repetitive movement can cause rubbing and irritation of the delicate vulval skin, particularly in women who are already prone to eczema, dermatitis or vulval conditions,” says explains Dr Nikki Ramskill, women’s health GP and founder of The Female Health Doctor clinic.
"Thrush tends to thrive in warm, moist environments, so staying in damp, non-breathable clothing for prolonged periods may contribute to conditions that favour yeast overgrowth in women who are susceptible.”
But she stresses that sweaty underwear doesn’t directly cause urinary tract infections.
"UTIs are caused by bacteria entering the urinary tract, so sweaty underwear itself isn’t a direct cause of a UTI,” says Ramskill. "However, changing out of damp clothing and maintaining sensible vulval hygiene is worthwhile, particularly if you’re someone who experiences recurrent infections.”
What happens down there when we exercise?
Sweating around the groin is entirely normal, but the combination of moisture, heat, tight clothing and movement can create uncomfortable conditions.
"The issue is that this area is already warm and enclosed, so when you add sweat, friction and tight clothing, you can create a damp environment that may irritate the skin,” says Pilates instructor Nikkii Behrens, founder of Nikkii Health and Rare Street Pilates.
"Fabric makes a difference because some materials hold onto moisture, while others help move it away from the skin and dry more quickly. Breathability and fit are just as important as the fabric itself.”
That means the knickers that work perfectly well for an ordinary day may not necessarily perform as well during a sweaty spin class.
Is cotton always best?
Cotton underwear has long been recommended for its breathability, but the gym is one situation where it may not always be the obvious winner.
"For everyday wear, breathable cotton is an excellent option, particularly for women with sensitive vulval skin,” explains Ramskill.
But for exercise, she says, "cotton can absorb a lot of sweat and stay wet, so a well-designed moisture-wicking sports fabric may actually be more comfortable during a sweaty workout.
"The key words I’d look for are breathable, moisture-wicking, soft and non-irritating.”
Pour Moi‘s lingerie buyer Emma Woodrow explains that cotton is hydrophilic, meaning it absorbs moisture rather than moving it away from the skin.
"This ultimately leads to that unpleasant ‘damp’ feeling many gym-goers will relate to,” she says.
"As such, my main recommended style for working out is synthetic seamless knickers with a cotton gusset.”
However, synthetic performance fabrics bring their own environmental considerations. Fabrics produced on scale are often treated with chemicals to help make them softer, brighter, water-resistant and odour-repellent. Among them are PFAS - dubbed "forever chemicals” - which are used for their water-, grease- and heat-resistant properties.
These chemicals don’t necessarily just sit inertly in fabric. Research published in Environment International suggests some PFAS can permeate the skin barrier and enter the bloodstream.
And so heavily treated underwear, worn close to some of the body’s most sensitive skin, may give consumers another reason to check the label.
What about plastic-free underwear?
Unfortunately, many moisture-wicking performance fabrics are synthetic, so readers trying to reduce their use of plastic-based clothing may prefer natural or plant-derived fibres.
For lower-impact workouts where you’re not sweating heavily, there may be no reason to abandon the comfortable underwear you already own.
Behrens says that "you absolutely don’t need to buy designated workout underwear simply because it’s marketed that way. If your everyday underwear is comfortable, breathable, fits properly and works well during exercise, that’s perfectly fine”.
For yoga, Pilates, walking and other lower-impact exercise, look for breathable natural-fibre underwear - particularly if reducing synthetic fabrics is a priority.
If you sweat heavily, however, bear in mind that natural fibres such as cotton can remain damp for longer, so changing promptly afterwards becomes particularly important.
Is seamless really better?
If you’re constantly attempting to discreetly retrieve your underwear halfway through a squat, the problem may be the cut rather than the material.
"For the most part, I’d always pick seamless styles as my preference for the gym,” says Woodrow.
"Beyond performance, seamless styles also work best for avoiding visible lines or bunching, since women often work out in leggings or tighter lycra-material trousers. After all, it’s tricky to get your best workout in if you’re constantly pulling at your knickers!”
But there’s no need to throw away anything with a seam. "If you’re someone who prefers lower-impact workouts, such as slower yoga, your regular, most comfortable underwear will do the job,” says Woodrow.
"Seamless just earns its place when you’re moving dynamically or sweating a lot!”
Ramskill says seams can be the culprit of irritability, too.
"Sometimes it isn’t the fabric causing the problem at all, but a seam repeatedly rubbing the same part of the vulva or groin.”
The best test may simply be whether you’re thinking about your underwear while exercising.
"Your underwear shouldn’t be something you’re constantly aware of while exercising,” Ramskill says.
"If you’re repeatedly adjusting it, it’s digging into the groin, riding up, rubbing, or you’re consistently coming away from exercise with redness, itching, soreness or chafing, it’s probably not working for your body.”
The most important bit might be what you do afterwards
Whatever underwear you choose, the experts emphasise the importance of not spending the rest of the day in sweaty clothes.
As for how long you stay in your workout gear until you should change, "ideally, as soon as reasonably possible,” says Behrens.
"You don’t need to panic if you can’t change the second your workout finishes, but I wouldn’t recommend spending hours in damp leggings and underwear.
"Showering and putting on clean, dry clothing after training helps reduce prolonged moisture, friction and irritation. If you can’t shower immediately, even changing into dry underwear and clothes is worthwhile.”
So perhaps skipping a long coffee straight after a workout is the way forward to prevent irritation. – dpa
