If you have a problem with overperspiration, you may find that your clothing--especially your undergarments--becomes uncomfortable. Damp undergarments can chafe and irritate your skin. If you’re working or playing outside, damp undergarments can cause you to become chilled.
Cotton fibers are the least desirable for people who perspire a lot. They retain moisture, making cotton undergarments more likely to chafe and become uncomfortable. Cotton underwear also will not insulate when it becomes damp. This isn't always a bad thing--in the heat of the summer, it can be cooling. But if you're engaging in outdoor activities and might be caught out in inclement weather, choosing undergarments that insulate even when damp can protect you from hypothermia.
Wool is no longer the thick, itchy fabric you might remember from your childhood. Merino wool undergarments are thin and soft, and they insulate even when damp, making them a better choice than cotton for people who sweat a lot.
Of natural fiber choices, silk is the best for people who perspire profusely. Silk is light and does not chafe. It draws moisture away from your skin--a process called "wicking" by outdoor enthusiasts--keeping you feeling dry and comfortable.
While some synthetic materials, like nylon, are uncomfortable for people who sweat, microfiber undergarments developed by outdoor clothing companies are comfortable and breathable. These have superior wicking properties and don't become damp even when you sweat. They are also good insulators, making them the best choice if you are shopping for long underwear. Often synthetic underwear is a little more expensive, but it is also more durable and will last longer than most natural fibers.
An additional consideration is odor control. Many synthetic fibers, while excellent at wicking away moisture, retain odors from your sweat. For daily wear, look for synthetic garments that offer odor control, or choose silk. For wear during activities like hiking or backpacking, make good insulation your top priority.
Standard underwear, bras and long underwear are made with each kind of fiber. Experiment to determine which you find most comfortable.