Weight loss is extremely difficult because reduced caloric intake increases hunger sensations and slows metabolic rate. A low-calorie, mixed diet triggers protein mobilization for fuel, which results in muscle-wasting.
A review of literature by Suzanne Devkota and Donald Layman from the University of Chicago concluded that substituting protein for fat and carbohydrate in the diet reduces insulin levels and suppresses hunger and food cravings. Protein, particularly sources high in the amino acid leucine, triggers protein synthesis and helps maintain muscle mass during periods of caloric restriction. People trying to lose weight should consume protein, particularly during breakfast and lunch. This will help curb appetite and maintain muscle mass.
(Current Opinion in Clinical Nutrition and Metabolic Care, 13: 403-407, 2010)