In the European and American underwear market in 2026, sustainability, comfort and functionality have become the "iron triangle" for brand survival. As consumers become increasingly picky about the safety, environmental protection and wearing experience of underwear fabrics, choosing the right core fabrics directly determines whether a brand can stand out in the fierce market competition. Sustainable underwear is no longer an option, but a necessity for brands to establish themselves. Currently, two natural fabrics, bamboo fiber and cotton, are dominating the market. This article provides an in-depth analysis of the core differences between the two, helping underwear brands targeting the EU and US markets to make the wisest fabric choices based on consumer demand and market trends in 2026.