Material choice is not a contest between “natural” and “synthetic.” Different fibers solve different problems, and most training bras use blends to balance softness, stretch and shape retention.
Cotton
Soft and familiar, often blended with elastane for stretch and recovery.
Cotton guide →Synthetic blends
Nylon or polyester blends often dry quickly and are common in seamless and sports designs.
Synthetic blend guide →Modal & rayon-family fabrics
Often selected for a soft hand feel; performance depends on the full blend and construction.
Modal guide →Read the whole fiber blend
A label such as “cotton bra” can still include elastane. That small stretch component can materially change fit recovery. Likewise, a nylon/elastane seamless bra can feel very different from a polyester sports bra even though both are synthetic.
Material does not determine comfort by itself
Soft fiber content can still be combined with firm elastic, rough labels or prominent seams. Evaluate material together with construction and then check real-world fit.
Care changes performance
Heat, agitation and repeated washing can affect elastic recovery, padding and surface feel. Follow the garment care label rather than relying on a universal washing rule.