Doctors advise cotton at the gusset, and synthetic keeps you dry on a long day. Both are true. Here is which fabric belongs in which situation, and why a bra cannot be cotton at all.
Most advice about underwear fabric stops at three words: cotton is best. It gets repeated so often that a drawer full of seamless pieces starts to feel like a mistake, and a woman who has just bought a smooth synthetic set quietly wonders whether she has done something unwise. She has not. The real answer depends on which garment you mean and which part of you it touches, and once that is clear the whole question gets easier.
What follows is what underwear is actually made of, what the percentages on the label mean, why there is no such thing as a cotton bra, where cotton genuinely earns its place, and where synthetic quietly outperforms it.
What is underwear actually made of?
Five fibres do almost all the work in modern intimates, and each is chosen for a property the others do not have.
- Nylon gives a smooth, fine surface that can be knitted without side seams and fused at the edges. It is soft against skin, resists abrasion, and is what makes a piece disappear under clothes.
- Spandex is the only fibre here with real recovery. It stretches and returns to its original length, which is what lets a band, a waistband or a shaping panel hold anything at all.
- Polyester holds a moulded shape. It absorbs almost no water, dries fast, and keeps a curve heat-set into it, which is why moulded bra cups are made of it rather than nylon.
- Cotton is soft, absorbent and breathable, with almost no recovery, so it stretches out and stays stretched.
- Modal and viscose are soft cellulose fibres, cooler against the skin than cotton and quicker to lose their shape.
Three of those go by more than one name, which is the source of most label confusion. Nylon and polyamide are the same fibre. Spandex and elastane are the same fibre. Lycra is a brand name for spandex. A label reading 76 percent polyamide and 24 percent elastane and a label reading 76 percent nylon and 24 percent spandex describe an identical fabric.
What are ILARIA pieces made of?
Every piece we make is a nylon or polyamide knit with spandex in it, apart from one moulded cup that is polyester. The proportions are what differ, and the proportions are what you can read behavior from.
| Piece | Blend | What the blend does |
|---|---|---|
| WideEase Sculpt Bra | Cup 100% polyester, wings 52% nylon and 48% spandex | A moulded cup that keeps its curve, on wings with the strongest recovery in our range |
| SmoothForm Wireless Comfort Bra | 59% nylon, 41% spandex | Stretches far and pulls back hard, which is how one alpha size covers several bra sizes |
| Everyday V-Neck Bra | 60% polyamide, 40% spandex | A fine high-stretch knit that moves with you rather than holding a fixed shape |
| SculptEase Bra | 85% nylon, 15% spandex | More fibre, less stretch, the softest surface and the lightest hold |
| SmoothLayer Seamless Bodysuit | 85% nylon, 15% spandex | Medium shaping you can keep on all day rather than for an evening |
| SoftSculpt Shaping Bodysuit | 85% nylon, 15% Lycra | The same balance, with a textured knit that lets air through |
| ZipSculpt Shaping Bodysuit | 78% nylon, 22% spandex | A denser knit cut small, which is what produces firm compression over a wide area |
| CurveHold High-Waisted Thong | 78% nylon, 22% spandex | The same knit concentrated into a wide waistband so the hold sits at the tummy |
| StreetForm Short Sleeve Bodysuit | 76% polyamide, 24% elastane | Knitted in two layers, which gives it the opacity of a T-shirt rather than of underwear |
| SoftLine No-Show Briefs | 77% nylon, 23% spandex | Almost the same blend as our firmest piece, cut loose, so it stretches without compressing |
| Everyday Seamless Briefs | 60% polyamide, 40% spandex | A second-skin knit fine enough to vanish under a fitted skirt |
The briefs and the thong have a cotton gusset. Everything else is synthetic from edge to edge, and the rest of this article explains why that is the right construction for each of them.
What does the percentage on the label actually tell you?
Spandex content tells you two things at once: how far the fabric will stretch, and how hard it pulls back. It does not tell you how tight the garment will feel, because that is decided by how the piece is cut.
Our SoftLine briefs are 77 percent nylon and 23 percent spandex. Our ZipSculpt bodysuit is 78 percent nylon and 22 percent spandex. The blends are within one percent of each other and the two pieces do opposite jobs, because SoftLine is cut generously enough to stretch over you without pressure, while ZipSculpt is cut small enough that the same recovery becomes firm compression. Fabric sets the potential and the pattern decides what you feel.
Read the two numbers together this way. A high spandex share, 40 percent and above, means a fabric that stretches a long way and returns with force, which is what lets one alpha size fit a range of bodies rather than a single measurement. That is why our SmoothForm bra at 41 percent and our Everyday pieces at 40 percent cover more bodies per size than a rigid fabric could. A lower share, around 15 percent, leaves more fibre in the knit and gives a softer, more opaque, more breathable fabric with gentler hold, which is where SculptEase and our medium bodysuits sit.
More spandex is not a better fabric, it is a different one. The trade is comfort and breathability against stretch and grip.
Why is there no such thing as a cotton bra?
A bra holds weight, and the weight sits on the band. For the band to hold, the fabric has to stretch when you fasten it and pull back to its original length afterwards, every day, for a year or more. Cotton does not do that. It stretches and stays stretched, which is why a cotton band goes loose within a month and the straps end up carrying what the band should have carried.
Spandex is the fibre that supplies recovery, so every functioning bra contains it. A bra described as cotton is a cotton face over a synthetic structure.
The WideEase wings show the principle at its extreme. At 48 percent spandex, nearly half the fabric exists to pull back, which is what a deep band under a fuller bust needs to do all day without giving up by spring. It is also why the band feels firm on the loosest hook when it is new, since a band with that much recovery is meant to be worked, not to sit slack.
None of the health guidance on underwear fabric concerns bras. It all looks at the vulvar area, where warmth and moisture behave differently. For the garment on your upper body, synthetic is not a compromise, it is simply how the thing works.
Why is one bra cup polyester when everything else is nylon?
Because a moulded cup has to keep a curve, and polyester is the fibre that keeps one.
Both nylon and polyester soften under heat and hold the shape they cool into, but polyester holds it more stubbornly and takes on almost no moisture, so the cup does not soften and lose its line on a humid day. That is why the WideEase cup is 100 percent polyester while its wings are nylon and spandex. The cup is doing a structural job and the wings are doing an elastic one, and no single fibre does both well.
The honest trade is warmth. A moulded cup is a solid layer rather than an open knit, so it holds more heat than an unlined bra does. If a hot day is the priority, a wireless knit bra is the cooler choice, and if a smooth unbroken line under a fitted shirt is the priority, the moulded cup is what delivers it.
Where does cotton genuinely matter?
In one specific place: the gusset, the panel of fabric that sits against the vulva.
The American College of Obstetricians and Gynecologists advises wearing cotton underwear, or at minimum underwear with a cotton panel, and avoiding tight-fitting trousers and underwear [1]. Cleveland Clinic gives the same guidance and explains the reason: cotton is breathable and wicks away the sweat and moisture that yeast and bacteria thrive on. It is also less likely to irritate sensitive skin than a synthetic sitting right against it [2]. The moisture argument is the whole of it, which is also why a drier synthetic can be the better choice everywhere the gusset is not.
That advice is the reason every pair of briefs and the thong in our range has a cotton gusset while the fabric around it stays synthetic. The panel sits where doctors say cotton belongs, and the knit around it does the work cotton cannot.
One honest caveat, which comes from Cleveland Clinic rather than from us: a cotton panel helps, and it does not breathe the way a fully cotton pair does [2]. If you are prone to irritation, a cotton gusset is a sensible middle and not a complete answer.
When does synthetic work better than cotton?
Whenever staying dry matters more than the name of the fibre.
Cotton absorbs moisture and holds it against the skin. Once it is wet it stays wet, gets heavier, and the friction of wet cotton against skin is what causes chafing during repetitive movement. Synthetic knits move moisture to the surface and dry quickly, which reduces both the wetness and the friction.
That trade shows up in ordinary life more than in the gym. A long day on your feet in August, a flight, a commute in a warm car, a wedding in a fitted dress. In each of those, the cotton pair that started out more breathable finishes the day damp, while the synthetic one is dry.
It is worth being accurate about the limits. In low and moderate activity the difference is small, and the clear advantage appears in heat and in hard effort. If your day is cool and quiet, the fibre matters less than either camp suggests.
The other reason synthetic exists
Nothing else disappears under clothes. A bonded or laser-cut edge lies completely flat because the fibre can be fused, and cotton cannot be fused the same way, so cotton pieces are hemmed and a hem stands away from the body. Under a ribbed knit dress or pale trousers, that hem is the line you can see.
Our SoftLine briefs are the clearest example. The edges are fused rather than sewn and the waist has no separate elastic band at all, which is only possible in a nylon knit. The same pair in cotton would need a hem and a waistband, and both would print through a fitted skirt.
The same applies to shaping. An even, smooth layer that holds its shape across a whole garment is only possible with spandex through the entire knit, which is why every bodysuit we make is synthetic from shoulder to hem.
What matters more than the fabric
Fit. Both ACOG and Cleveland Clinic put looseness alongside fibre in their advice, and a tight cotton pair traps as much heat as a loose synthetic one, with added pressure. If a pair leaves a mark on your skin, that is the size talking, whatever it is made of.
Why synthetic underwear is washed cold and never tumble dried
Spandex is the fibre that limits the life of everything here, and heat is what destroys it. A hot wash, a tumble dryer or a radiator breaks down the elastic filaments, and once they have gone the garment still looks intact while holding nothing. A band that has lost its recovery rides up your back, and a shaping piece that has lost its recovery simply feels like a slippery layer.
Ultraviolet light does the same work more slowly, so a piece dried in direct sun ages faster than the same piece dried in the shade, and pale shades fade at the same time.
That is the whole reason our care instructions read the way they do: hand wash cold up to 30 degrees, no bleach, no tumble dryer, no iron, dry flat in the shade, and never wring. Twisting a high-spandex fabric stretches the elastic filaments past their recovery point in exactly the places that matter. Washed that way, a piece with 20 to 40 percent spandex holds its shape for a year or more of regular wear, and washed hot it will not last a season.
Which fabric for which situation
| The situation | What to reach for | Why |
|---|---|---|
| Bras, all of them | Nylon and spandex | Only spandex recovers, and the band carries the weight |
| A smooth line under a fitted shirt | A moulded polyester cup | Polyester keeps the curve it was shaped into, where a knit follows the body |
| An ordinary day at a desk | Either, with a cotton gusset | Low heat and low movement, so the fibre matters least here |
| Heat, travel, long hours upright | Seamless synthetic | Wet cotton stays wet and rubs, synthetic moves moisture out |
| Under fitted or pale clothing | Bonded-edge synthetic | Fused edges lie flat where a cotton hem prints through |
| All-day shaping | A 15 percent spandex knit | More fibre, gentler hold, more air through the fabric |
| An evening in something fitted | A 22 percent spandex knit cut close | The denser knit is what makes firm compression possible |
| Sensitive skin or a tendency to irritation | Full cotton, loose | Doctors advise cotton and a fit that is never tight |
| Overnight | Cotton, or nothing | Doctors suggest going without at night for ongoing vulvar sensitivity |
How to build a drawer that covers all of it
Most women need two kinds rather than one. Seamless synthetic pairs with a cotton gusset for fitted clothes, heat and long days, and a few full cotton pairs for sensitive days, quiet days and nights. Bras are synthetic regardless, because there is no working alternative.
You can see our seamless panties and underwire bras grouped separately, and our size guide covers the fit half of the question. If it helps to have another woman’s read on what to keep and what to retire, our community is an open door and never a hard sell.
Frequently asked questions
Is synthetic underwear bad for you?
For a bra, no. All of the guidance on underwear fabric concerns the vulvar area, not the garment on your upper body. For briefs worn every day, doctors advise cotton or a cotton panel at the gusset, because synthetic holds more moisture against the skin there, which is why every pair of ours has a cotton gusset.
What is the difference between nylon and polyamide?
There is none. Nylon is a type of polyamide, and the two words appear on labels interchangeably depending on where the garment was made. Elastane, spandex and Lycra are the same situation: one fibre, three names, with Lycra being a brand.
Why does a bra contain so much spandex?
Because the band does the supporting, and support means stretching to fasten and pulling back afterwards. Our WideEase wings are 48 percent spandex, which is high even for a bra, and that is what keeps a deep band holding under a fuller bust rather than riding up by the afternoon.
Is a cotton gusset enough?
It helps, and it is not the same as full cotton. Cleveland Clinic states directly that a small cotton panel does not fully protect you from the synthetic fabric and will not breathe like 100 percent cotton. For most women on most days it is a reasonable middle, and for anyone prone to irritation full cotton is the safer choice.
What should I wear in hot weather?
Something that stays dry, which usually means a seamless synthetic pair with a cotton gusset rather than full cotton. Cotton absorbs sweat and holds it against the skin, and wet cotton is what causes chafing on a hot day. In a bra, an unlined knit runs cooler than a moulded cup.
My skin reacts easily. What should I choose?
Full cotton, and a size that never leaves a mark. Doctors advise cotton underwear and clothing that is not tight-fitting, and for anyone with ongoing vulvar sensitivity the same guidance adds going without underwear overnight.
How long does a synthetic piece last?
A year or more of regular wear if it is washed cold and dried flat in the shade. Heat is what ends it, so a tumble dryer will cost you more months than daily wear ever will.
Sources
-
Vulvovaginal Health. The American College of Obstetricians and Gynecologists.
-
Newlin E. 5 Underwear Tips To Avoid Problems Down There. Cleveland Clinic.
