FORTUNE — What started as a way to accommodate a rare food disease has blossomed into the latest food industry craze. Gluten is a protein that helps make wheat, barley, and other grains chewy. It also makes 0.05% of the population very sick. To address that, food producers started substituting corn, rice, and potatoes for wheat in bread and pasta. Last year $6.3 billion of such products were sold, up 33% since 2009, according to Spin research. Why the growth? The diet-conscious are clamoring for, say, wheatless pizza because they believe it contains fewer calories — but surprisingly often it doesn’t. Even so, some big restaurant chains are now joining in. Subway is testing a gluten-free sandwich roll and brownie at about 800 of its locations and says demand is strong. McDonald’s (MCD) offers a gluten-free bun in Spain, Sweden, Finland, and Norway but not in the U.S. Going gluten-free in America isn’t risk-free. Those with the disease might accidently be served a regular roll, triggering a lawsuit. Worries aside, this trend has legs: The number of U.S. restaurants with gluten-free menus rose 61% in 2011. —Caitlin Keating