To function daily, your body gleans energy from three food-derived macronutrients: carbohydrates, fats and proteins. How you divvy up those macronutrients in your diet is a matter of personal preference. But what if you could train your brain to prefer one macronutrient over the other? In a study appearing in Endocrinology, Ryan and her colleagues, including Molecular, Cellular and Integrative Physiology Ph.D. student and lead study author Karlton Larson, identified the hormone fibroblast growth factor-21 (FGF21) as a control for regulating dietary protein intake in male mice. They found that male mice injected with the hormone increased their intake of dietary protein over carbohydrates and fats.