The promise: You'll burn more calories by digesting protein, and, because protein takes a while to leave your stomach, you also feel fuller longer.
The problem: By consuming 30 to 50 percent of your calories in bacon, eggs or even tofu, you won't get the carbohydrates you need. Yes, carbs are often maligned for their almost addictive pull and simple white carbs have been linked to weight gain, diabetes, inflammation and heart disease. But this food group remains the most efficient fuel source for the body, explains Sasson (who is, by the way, a fan of lean protein). Carbs are crucial for tissues and organs like the brain and the heart. If you don't supply your body with adequate amounts of carbohydrates, it's forced to use energy from muscle as well as fat.
The risks: Breaking down too much muscle and fat for fuel can lead to the production of ketones, which can cause nausea, bad breath, headaches, muscle soreness and, over time, kidney stones, gout and even kidney failure. Other potential unpleasant side effects include constipation (from a fiber deficit), intense fatigue and weak bones (animal protein can increase blood acidity, leading your body to use its calcium to neutralize it). So even though high-protein, low-carb diets are preferred by some fitness buffs, the American Heart Association specifically advises against them.