On any diet, restricting calories causes your metabolic rate to slow down and affects hormone levels, which make you hungrier, making weight regain more likely. Research shows that carb loading can temporarily raise metabolism and increase levels of leptin, a hormone that blunts hunger, which, together, could help promote weight loss. Research has also shown that carbohydrate-rich foods boost athletic performance and recovery, and that carbohydrates burned for energy spare protein, which then can be used for muscle growth rather than for fuel. Thus, the thinking goes, higher-carb days once in a while can help even non-athletes prevent a metabolic slowdown, enhance the effectiveness of their workouts, trim fat and build muscle. And lower-carb days the rest of the time can encourage the body to burn fat for fuel.