Polyester and non-organic cotton are the dominant fibres used in sportswear today. They just happen to be the cheapest any brand can use but also the most damaging to our environment.
However, there are better, slightly more expensive fibres they could be using; such as hemp and nettle. These natural crops require less water and harmful pesticides to grow compared with conventional cotton, which also requires a hot climate. Where as nettle can be grown in the UK. As you know polyester is a synthetic fibre derived from fossil fuel; petroleum, made from ethylene and terephthalate.
When environmental costs are taken into consideration nettle and hemp become the cheapest fibers to use and non-organic cotton and polyester the most expensive.
Golf Refugees are currently working on using nettle fibres for a new range of sportswear called 'neat'. Which stands for 'nettle enhanced athlete textile'.