Even tiny amounts of gluten in foods are troublesome for people with celiac disease, and restaurants may be the hardest places to avoid the protein, finds a new study. More than half of gluten-free pizza and pasta dishes in restaurants tested positive for the presence of gluten; about one-third of supposedly gluten-free foods had detectable gluten. Gluten-free pasta samples were positive in 51 percent of tests; gluten-free pizza contained gluten for 53 percent.