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It's a mystery that haunts laundry baskets, gym bags and changing rooms the world over. Why do some gym clothes smell so much worse than others after exercise, asks Gabriel Weston.

When we exercise we all make different choices about what we wear - whether it's a decade-old baggy T-shirt, last season's football strip, or high-tech performance sports gear that's been specially designed for the task at hand.

But if we're being honest with ourselves we've probably all noticed that there are certain items in our gym bags that always smell worse than others. And now and then most of us have probably wondered why.

For Trust Me, I'm A Doctor we tracked down the latest research and ran our own experiment to see if science could provide the answer to this pongiest of problems.

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Gabriel Weston is one of the presenters of Trust Me, I'm A Doctor, which returns for a new series at 20:00 BST on Thursday 1 September on BBC Two - or watch later on BBC iPlayer

Studies have actually been done comparing the smelliness of different fabrics. Two such studies, at the University of Alberta and the University of Ghent, used highly trained odour analysts to sniff various fabrics after they'd been worn, and they both came to the conclusion that polyester gets much smellier than natural fibres like cotton or wool.

But interestingly, this difference can't be blamed solely on our sweat, because sweat itself doesn't smell. Instead, odour is produced when the bacteria that live naturally on our skin feed on a particular kind of oily sweat that comes from places like our armpits and groins. So what does account for the more intense aroma that clings to synthetic fabrics?

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Inspired by existing fabric research, we ran an experiment to find out whether wearing cotton or polyester clothing could affect our skin bacteria and therefore the subsequent smell. A group of volunteers took part in two high-intensity spin classes wearing T-shirts of 100% cotton and 100% polyester. In the run up to both classes they bravely eschewed deodorant - much to the delight of their families, friends and work colleagues. At each class we swabbed their armpits before and after the exercise and gathered up their T-shirts for analysis.

When Prof Andrew McBain and Dr Gavin Humphreys from the University of Manchester analysed our samples, they found up to 300 different types of bacteria were inhabiting our volunteers' armpits. The most common strains included Staphylococci which are associated with normal body odour, and Corynebacteria which produce more unpleasant smells. Interestingly, though perhaps not surprisingly, research suggests that Staphylococci tend to be more dominant in female armpits, whereas men tend to have more of the stinky Corynebacteria.

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But when it came to looking at whether different fabrics affected the armpit bacteria, our experts found no significant differences. They also found that while there were plenty of stinky Corynebacteria on the skin, these were not being transferred over to either of the T-shirts. This all suggested that it's not actually our skin bacteria that causes the odour on our clothes, and that there must be something going on in the synthetic fabric itself to explain why it ends up being so smelly.

Dr Rachel McQueen, at the University of Alberta in Canada, has studied polyester, cotton and merino textiles and proposes that one of the reasons for their contrasting smells is the different make-up and behaviour of natural and synthetic fibres. An example of this is the way they deal with moisture.

Natural fibres like cotton absorb moisture, including the smelly compounds produced by bacteria, which get trapped inside the fibres where they can't reach our noses. Synthetic fibres on the other hand, do not absorb moisture. Instead they attract oils. This means that they hang on to the "oily soils" from our sweat which sit on the surface of the fibres, waiting to be guzzled by whatever odour-producing bacteria happen to come along.

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Researchers at the University of Ghent also made a fascinating discovery when they tested cotton and polyester fibres that had been worn for exercise. What Prof Nico Boon, Dr Chris Callewaert, and their colleagues, found was that a particularly smelly bacterium called Micrococcus grew in abundance on synthetic fibres, but didn't enjoy living on cotton or on skin.

So next time you notice some particularly pungent polyester clothing in your gym bag, you can let yourself off the hook - instead the culprits appear to be the synthetic fibres themselves which provide the kind of environment in which stinky bacteria can thrive.

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