A low-carb or no-carb diet requires food restriction, which enhances cravings for those foods and creates a list of "bad" foods to be avoided. Even after the diet ends, the sense that carbohydrates are "bad" will remain. If the person then eats carbs, he feels guilty. If he doesn't, he misses out on essential nutrition.
This kind of black-and-white thinking and the possibility of denying your body what it needs can pave the way for the development of deeper restriction (anorexia nervosa) or binge-related disorders (bulimia nervosa or binge-eating disorder).
