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Protein, like food in general, is definitely thermogenic. The overall thermic effect of food - all three macronutrients (protein, carbs and fats) blended together as in the typical North
American diet - fries about 10% of your daily calories. That means you have to eat about 10% more food than you would predict based only on your basal metabolic rate (the calories you burn
at rest), exercise and other activities just to maintain your weight.
Most dietitians estimate that 20-25% of protein's calories are lost to its own metabolism and the rise in body temperature it causes, known as the "thermic effect" or "specific dynamic action" of protein. Carbohydrates and fat do the same thing, but to a lesser extent. For fat, only about 3% of its calories are lost to heat; carbs lose 10-15%. Compared to fat and carbs, protein is a thermogenic macronutrient.
Not all of the calories lost in protein consumption are burned. Some calories are lost to biochemical reactions needed to deal with proteins nitrogen component. These reactions (which make up what is known as the urea cycle) use a lot of energy to turn nitrogen (as ammonia carried by the amino acid glutamine) into urea, a compound that can be safely excreted by the kidneys. For more than one reason, protein contributes fewer net calories per gram than you might think.
So if you replace some of your carbs and fat with protein, will the extra caloric burn help you lose weight? Apparently yes. A study of obese volunteers found that taking a protein supplement helped them lose a significant amount of weight (although it may not have been due entirely to protein's thermic effect, as will be discussed). Indeed, the weight-loss effect of protein may explain why some bodybuilders swear that extra protein makes them more muscular while scientists insist that no more than 0.8 g of protein per pound of bodyweight can be used even by hard-training lifters. It may be that greater fat and water loss gives the illusion of more muscle by way of improvements in cuts and definition.
One study found that protein caused three times the rise in body temperature relative to carbohydrates. Other studies suggest it's about double. In any event, the superior thermogenic property of protein versus carbs or fat is the reason dietitians used to call the thermic effect of food "specific dynamic action." It was thought to be specific to protein alone.
Althoughprotein is thermogenic, its slimming effect may also be due to its ability to curb appetite; protein is very satisfying for the number of calories it contains. One group of researchers tested a variety of common foods and found that those rich in protein (and/or water and fiber) but low in fat were best at leaving subjects feeling full longer than other foods on a calorie-per-calorie basis. For example, a 240-calorie serving of fish was almost five times as satisfying as a 240-calorie serving of a croissant (the fish reduced the number of calories eaten voluntarily over the next five hours by a factor of five compared to the croissant). Croissants are mostly carbs and fat, and fish is mostly protein and water, so you can see how powerful protein and water are as appetite suppressants. Solid foods high in water and fiber, such as fruits and vegetables, also scored extremely well. Ironically, most of ephedrine and caffeine's fat-loss benefit is similarly due to appetite suppression, so your friend's comparison of protein to ephedrine and caffeine is surprisingly apt.