For a number of years now there has been a growing body of research (including data from randomized controlled clinical trials) that has been suggesting that diets higher in protein may aid weight loss and weight control. Other researchers have suggested increasing dietary protein may not only enhance satiety but also boost metabolic rate after meals due to its higher specific dynamic action. However, alcohol has an even greater specific dynamic action than does protein, so if boosting metabolic rate was the key to creating a negative energy balance, then alcohol would likely work even better than protein. When researchers examined the impact of feeding people diets rich in either fat, protein, carbohydrate, or alcohol, they concluded: "Despite differences in substrate metabolism and hormone concentrations, satiety and ad libitum energy intake were not significantly different between meals. Our data, therefore, do not support the proposed relation between the macronutrient oxidation hierarchy and the satiety hierarchy."
A serious flaw in any argument that simply consuming a higher % protein diet with less carbohydrate and/or fat) will somehow aid weight loss and control is the fact that total dietary protein intake in the USA has changed little since the 1960s when about 3/4 of American adults were normal weight. As can be seen from Figure 1., protein intake has changed very little over the past half decade in the USA. However, today about 3/4 of American adults have become overweight or obese, even though the average American is still consuming about the same amount of protein as s/he was back in the 1960s. Since the total protein content and % protein in the typical American diet has changed very little over the past 50+ years while the % of overweight and obese Americans have increased markedly, it seems likely that something other than a change in the amount of protein or % of calories from protein must be largely responsible for the growing obesity epidemic in the USA. Indeed, in much of Asia the consumption of meat and protein has actually increased quite a bit over the last several decades and yet this greater intake of meat and somewhat higher % protein diet has been associated with markedly increasing rates of type 2 DM and overweight/obesity throughout Asia. So it is debatable whether or not increasing dietary protein can prevent weight gain. Dr. Stin Soenen and colleagues at Maastricht University in the Netherlands did a study that suggested perhaps an inadequate protein content on a weight loss diet may contribute to greater risk of regaining lost body weight during the weight maintenance phase of a weight control program. Dr. Soenen concluded that: "Based on the comparison of a number of reliable studies, we conclude that a normal protein diet of 0.8 g/kg B.Wt./day is sufficient for body weight management, whereas a high protein diet of 1.2 g?/kg B.Wt./day is necessary for preservation of resting energy and a stronger initial sparing effect of fat free mass and lowering of diastolic blood pressure."
Let's assume then that increasing the % dietary protein (especially during the weight loss phase of a weight control program) can have some beneficial effects on sparing the loss of lean body mass, reducing hunger, and maintaining metabolic rate. The question then becomes how best to maintain adequate dietary protein intake, especially when calorie intake is being restricted. This may necessitate increasing the % protein calories in order to prevent protein intake falling below the adequate RDA level of 0.8g/Kg B.Wt./day. However, doing so by increasing the intake of animal protein may adversely impact blood lipids and elevate metabolic markers increasingly associated with more morbidity and mortality such as elevated IGF-1, 3-HIB, Hcy, AGEs, HAAs, PCHs, and TMAO. This takes us to a recent study conducted by Anne Raben and associates at the University of Copenhagen, Denmark. They fed 43 healthy normal weight young men 3 test meals in a crossover, randomized clinical trial. Two of the 3 experimental diets were high in protein [each of the two high protein (HP) diets had 19% calories as protein]. In one, the protein came largely from meat (HP-M) and in the second the extra protein came largely from beans (HP-B). The 3rd diet was low in protein (LP) and with just 9% of calories and most of the protein from beans (LP-B). All 3 diets had 28% fat calories and the two HP diets had 53% carbohydrate calories. The LP diet also got most of its protein from beans but got 63% of its calories from carbohydrate. All 3 test meals had the same calorie density and the same calorie content. However, since real foods were used, the fiber content of the 3 test meals varied from 6g/100g on the HP-M meal to 10g/100g on the LP-B meal and 25g/100g on the HP-B meal. All subjects were fed the same pre-test meal the night before and were allowed nothing but water thereafter. The following morning they consumed one of the 3 breakfast meals in random order. Subjective appetite sensations after the 3 meals did not differ. Interestingly, the subjects rated the LP-B meal just as palatable as the HP-M meal with veal & pork. And the subjects rated the LP-B just as satiating as the HP-M meal, even though its protein content was only 9% vs 19% for the HP-M meal. By contrast, the HP-B meal was rated slightly less palatable than the two other test meals but it was significantly more satiating than the LP-B and HP-M meals even after adjusting for its somewhat reduced palatability score. And when the subjects were offered a standard lunch meal and instructed to eat as much as they wanted, they consumed 12% fewer calories 3 hours after the HP-B and 13% fewer calories compared to the LP-B meal. This objective measure of satiety confirmed subjective reports of satiation after consuming the 3 test meals. Ad libitum calorie intake after the LP-B and HP-M meals was not significantly different on the HP-M compared to the LP-B meal. This suggests that the 40% reduction in fiber on the HP-M compared to the LP-B meal roughly negated whatever increase in satiety the doubling of the protein content of the HP-M compared to the LP-B meal provided.
Bottom Line: The data from this study suggests that if increasing satiety per calorie is the goal, that goal is likely to be met by increasing dietary fiber than by increasing dietary protein. If increasing dietary protein does boost satiety as some (but not all) studies suggest, then it appears to this reviewer the better way to increase dietary protein content would be with higher fiber and high protein foods like beans and spinach and perhaps egg whites and nonfat dairy products rather than with meat, fatty dairy products, and whole eggs.
By James J. Kenney, PhD, FACN
 [Raben A., Agerholm-Larsen A., Flint A. et. al. Meals with similar energy densities but rich in protein, fat, carbohydrate, or alcohol have different effects on energy expenditure and substrate metabolism but not on appetite and energy intake. Am J Clin Nutr 2003; 77:91-100
Stephanie Ronco has been editing for Food and Health Communications since 2011. She graduated from Colorado College magna cum laude with distinction in Comparative Literature. She was elected a member of Phi Beta Kappa in 2008.