You've heard of heirloom tomatoes. Now meet heirloom flours, ground from grains first domesticated thousands of years ago. Also called "heritage" or "ancient," these grains are now in supermarket breads, health-food store pasta and cookies. They are billed as exceptionally nutritious and, in some cases, safe for people who can't tolerate gluten.

Some dietitians say they aren't necessarily better than modern whole grains, and new research casts doubt on the gluten safety of some products. Any grain that...