Protein has profound effects on your metabolism. Though they both contain four calories per gram, protein forces your body to use more calories in digestion than carbohydrates do. This thermic effect accounts for one of the ways that a high-protein diet speeds up metabolism. In addition, protein builds metabolically active muscle and prevents body fat storage.
Protein has a thermic effect, meaning that 25 to 30 out of every 100 calories from protein get burned in the process of digesting it, according to "The Abs Diet" by David Zinczenko. Thermic refers to the heat generated from this increased energy output. Carbohydrates and fats have a much lower thermic effect.
Decreased Insulin Levels
Eating protein with meals prevents insulin surges, which lead to high blood sugar and fat storage, according to "The Fat Burning Bible" by Mackie Shilstone. Carbohydrates, particularly high-glycemic sugars, spike insulin, shutting down fat burning. Keeping insulin levels low by including protein at your meals and snacks keeps fat burning all day long.
The amino acids from proteins build the structure of your muscles by turning on protein synthesis, which means building muscle. Lean muscle tissue increases metabolic rate because it needs calories to exist even when your body is at rest, according to "Xtreme Lean" authors Jonathan Lawson and Steve Holman. They recommend taking in 35 or more grams of protein immediately after resistance training for this reason.
When the body runs out of its preferred fuel, carbohydrates, it must turn to stored calories in the form of body fat and muscle. Without a ready supply of amino acids from dietary protein, the body turns to muscle for a source of energy. Muscle loss inevitably leads to decreased metabolic rate, according to Zinczenko. Eating protein frequently spares lean muscle so that the body burns fat for energy.
According to "The Cortisol Connection" by Shawn Talbott, the stress hormone cortisol not only breaks down muscle, it stops fat burning cold. In fact, cortisol primes the body to store fat in case of emergency, particularly around the midsection. Eating protein prevents this by blunting cortisol release from the adrenal glands.
Overeating protein can lead to weight gain. However, it is far less likely to pack on the pounds than carbohydrates and fats. Your body can also use protein for energy, say Lawson and Holman. By the same pathway that the body turns protein into glucose for fat storage, it can also use it to fuel your workouts. Through gluconeogenesis the liver turns amino acids into glucose for energy purposes.