There's something else I've learned. It's one thing to buy gluten-free products. It's another to avoid gluten entirely, especially at restaurants where cross-contamination can be unavoidable. (That's a significant issue for restaurants and food service, as gluten-free menus start popping up.) In other words, a truly gluten-free diet is as rigorous as being vegan, vegetarian or any other diet. It's tough to follow and, like most diets, fleeting, especially when you're inside a bakery, dining out with friends or traveling.

Which gets back to the question of gluten free's sustainability as a growth niche. The answer? See the history of diets, especially those that might make you gain weight, and one that is usually mentioned as a possible cause of a laundry list of maladies, not a certain cause. Not only is the diet vulnerable but, so too, it would appear, is Boulder.

-- Written by Herb Greenberg

Herb Greenberg, editor of Herb Greenberg's Reality Check, is a contributor to CNBC. He does not own shares, short or trade shares in an individual corporate security.

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