The thermogenic effect of food (TEF) is the energy required for the body’s digestion, absorption, and disposal of ingested nutrients. The magnitude of TEF depends on the composition of the food consumed. Of the three types of food, the TEF can be broadly summarized as follows:
- Carbohydrates: 5 to 15 % of the energy consumed
- Fats: at most 5 to 15 % of the energy consumed
- Protein: 20 to 35% of the energy consumed
Thus, metabolic processing of protein requires the greatest expenditure of energy, with estimates ranging as high as 35% of the derived total energy being needed for the initial protein breakdown. Dietary fat, on the other hand, is very easily processed, and when turned into body fat, there is very little thermogenic effect, perhaps only 2 or 3%. Finally, the amount of energy required to process carbohydrates falls between that of protein and fat. It is also known that the thermogenic effect of food is increased by both aerobic exercise of sufficient intensity and duration and also by anaerobic weight training.
Upon seeing the above breakdown of the TEF figures for the various classes of foods, one can see why a figure of 10% is generally used to account for the thermogenic effect of food. Thus, for instance if you want to replace 1000 calories burned through activity, you need to eat 10% more, or 1100 calories, to account for the TEF effect. If you eat 1000 calories, 10% of these will be burned off by the thermogenic effect, leaving only 900 calories effectively available for use.
Some diets have been advocated as being effective for fat and weight loss, based on their relatively high proportion of protein. This includes both the Atkins Diet, and some more recent “metabolic diets” such as “Metabolic Cooking” – http://metaboliccookingrevealed.com .
In a thorough meta-analysis of high protein diets, and its effects on weight loss, available at http://www.jacn.org/content/23/5/373.long , (Journal of the American College of Nutrition, 2004), entitled “The Effects of High Protein Diets on Thermogenesis, Satiety and Weight Loss: A Critical Review,” it was concluded that
“Our review suggests that higher protein diets may significantly increase total weight lost and possibly percentage of fat lost when compared to a lower protein diet in the short term. Possible mechanisms include an increased satiety and decreased subsequent energy intake with higher protein diets. All 5 investigations that utilized an ad lib intake found significantly increased weight lost with the higher protein regimens in the short term studies (6 months or less).”
The final conclusion of this review is “Although the optimal amount and sources of protein cannot be determined at this time, the weight of evidence suggests that in dietary practice, it may be beneficial to partially replace refined carbohydrate with protein sources that are low in saturated fat.”
One can thus conclude that diets that include a larger than usual amount of protein may be successful due to the inherent thermogenic effect of protein. Indeed, a diet like Metabolic Cooking can explain its success, at least in the short term, as due to this thermogenic effect of protein. These findings may explain the relatively successful “Paleo Diet,” otherwise called the Cave Man diet.