In very simple terms, a high-fat diet cannot satisfy your appetite as well as a high-protein diet when the calories are equal in both. This would lead you to eat more calories on a high-fat diet in order to feel full. One gram of fat holds ~ 9 calories compared to 1gm of protein yielding only ~4 and because of this and respective water content, the volume of food in equal caloric diets is far greater in the high-protein diet, also adding to fullness. Additionally, it takes very little energy (calories) to metabolize fat compared to protein. Dietary fat can be stored very easily as body fat when energy needs are met with other foods. But protein must go through a much longer process to be used or stored as energy, meaning you use over twice the calories when you metabolize dietary protein. For these and other reasons, protein has been proven in studies to be the most satiating of the three macronutrients (protein, carbohydrates and fats), meaning protein makes you feel full sooner. Also high-protein diets are simple because they limit most other foods and people get tired of eating protein all day, leading to an automatic reduction in calories. Not all high-protein diets are healthy, but as long as you keep the total protein less than 40% of your daily calories and include lots of fruits and vegetables (a few whole grains where possible), short-term use can be a safe and effective weight loss solution.