Picking a fabric isn't really "natural vs. synthetic." Different fabrics solve different problems, and most training bras actually blend a few fibers together to get the right mix of softness, stretch, and shape.
Cotton
Soft and familiar — usually blended with a little stretch fiber.
Cotton guide →Synthetic blends
Nylon or polyester blends that dry fast — common in seamless and sports styles.
Synthetic blend guide →Modal & rayon-family fabrics
Chosen for how soft they feel — the exact blend still matters a lot.
Modal guide →Check the whole fabric blend, not just the headline fiber
A "cotton bra" can still be mostly held together by a small amount of elastane — and that tiny stretch component makes a big difference in how well it holds its shape. In the same way, a nylon-elastane seamless bra can feel completely different from a polyester sports bra, even though both are technically synthetic.
Fabric isn't the whole comfort story
A soft fabric can still be paired with stiff elastic, a scratchy tag, or seams in the wrong place. Look at fabric and construction together, then check the actual fit.
How you wash it matters too
Heat, hard drying, and lots of washing can wear down elastic, flatten padding, and change how a fabric feels. Follow the care label instead of just doing whatever's easiest.