Gluten is a protein, which humans have been consuming for some 10,000 years, mainly as a component of wheat. For about one percent of the population, gluten causes celiac disease, which involves stomach problems. But now, some one third of Americans are trying to avoid gluten — so many that gluten-free food has become a $10 billion industry that’s still growing. The phenomenon is so much part of the culture that animated series South Park devoted a whole episode to it. Critics call it a fad based more on fear than science. We separate fact from fiction.