Beyond the metabolic/hormonal effects of low-carbohydrate or high-protein diets, these diets may also be effective because of changes in the reward and palatability value of the diet (and food variety, a related factor). Getting rid of pastries, donuts, soda, fries, and pizza goes a long way, and taking one or more items off the table (e.g. bread) tends to reduce total food intake even if everything else remains the same. The potato-only diet is a good example of this (19, 20). Over on Mark Sisson's forum, a number of people have been repeating Chris Voigt's experiment of eating nothing but potatoes, with similar weight loss results (21). To be clear, I'm not recommending this approach, but it certainly illustrates the power of a monotonous diet. Interestingly, besides being extremely filling per calorie, potatoes are also one of the most insulin-stimulating foods, according to the paper cited above.