Many factors are at work in successful fat-loss diets. Two of the more significant aspects involve total energy expenditure at rest, or resting metabolism, and the satiety value of the diet. Satiety refers to how effectively a diet quells appetite. Controlling appetite leads to taking in fewer calories, which results in faster and greater bodyfat losses.
Last year a group of Danish researchers showed that substituting protein for carbohydrates during a lowfat diet led to greater fat losses.1 This year the same researchers compared animal-protein, vegetable-protein and high-carbohydrate diets for their effect on 24-hour energy expenditure in 12 healthy but fat men.2 The men followed three diets for four days:
1) An animal-protein diet, consisting of 29 percent fat and 29 percent protein.
2) A soy, or vegetable-protein, diet, containing 29 percent fat and 28 percent protein.
3) A carbohydrate diet, containing 28 percent fat and 11 percent protein.
The remaining percentages in all three diets consisted of carbohydrates; for example, the carb diet contained 61 percent carbs. The authors measured 24-hour energy expenditure in special metabolic chambers before the diets began and on day 4. They found that replacing the same number of carb calories with either animal- or vegetable-protein sources resulted in a 3 percent higher 24-hour energy expenditure after four days. The increased energy burning occurred despite a lower caloric intake.
The higher-protein diets produce a greater thermogenic effect, or wastage of calories as heat. That effect is referred to as diet-induced thermogenesis (DIT). While the thermogenic effect isn’t spectacular, it does result in greater and more effective fat losses than what occurs when someone eats a higher-carb diet. The greater satiety value of protein’evident during this study’also makes dieting a bit easier due to greater appetite control.
In comparing the two sources of protein used in this study, animal (pork) and vegetable (soy), the authors also noted that animal protein led to a 2 percent higher 24-hour energy expenditure than the vegetable-protein diet. They suggest that protein is superior to carbs in producing DIT because the body has no effective capacity for storing protein as it does carbs (as glycogen in liver and muscle).
The thermogenic effect of protein may also relate to increased calorie use during protein synthesis reactions in the body. Protein metabolic excretion and the conversion of protein to carbs in the liver are also high energy processes that use up calories.
Protein absorption didn’t differ significantly between the animal- and vegetable-protein diets, though the diet contained 16.8 percent more fiber than the pork diet. Physical activity also tended to be lower during the soy diet than the pork and carb diets. The animal-protein diet may have a greater thermogenic effect than the soy diet because it contains a more balanced blend of amino acids. Past studies show that balanced amino acid blends produce greater thermogenic responses than poorer combinations of amino acids.
Thus, this study shows that eating a high-quality high-protein diet may be the most effective way to increase resting energy expenditure while controlling appetite during a fat-loss diet.
1 Skov, A.R., et al. (1999). A randomized trial of protein vs. carbohydrate in ad libitum fat-reduced diet for the treatment of obesity. Int J Obesity. 23:528-36.
2 Mikkelsen, P.B., et al. (2000). Effect of fat-reduced diets on 24-hour energy expenditure: comparisons between animal protein, vegetable protein and carbohydrate. Am J Clin Nutr. 72:1135-41.