Forty-eight studies, with 50 outcome comparisons, were included in the review. Fifteen RCTs (n=187) assessed thermogenesis, 20 RCTs (n=361) assessed satiety and subsequent energy intake, and 15 RCTs (n=720) assessed weight and fat loss.
A number of additional studies and reviews that assessed renal function, blood lipids and cardiovascular disease risk were also discussed, but it was unclear whether these formed part of the systematic review; these results were not considered in this abstract.
Thermogenesis (15 studies).
The duration of most studies was 6 to 7 hours (range: 2 to 36). Ten of the RCTs were crossover studies. Only 2 studies involved more than 15 participants. All 6 studies that assessed the thermic effect of food as a percentage of ingested energy reported a greater energy expenditure for the higher protein versus the lower protein diet. All 3 studies reporting the thermic effect in kilojoules reported a significantly greater effect for higher protein versus higher fat or higher carbohydrate meals. All 6 studies that used a variety of other measures to assess the thermic effect reported increased effect for higher protein diets.
Satiety (14 studies).
Most of the studies were of a crossover design. The duration of the studies ranged from 2 minutes to 15 days. Eleven of the 14 studies that compared high protein with at least one other macronutrient reported that a protein pre-load significantly increased subjective ratings of satiety; the remaining 3 studies reported no difference between higher and lower protein pre-loads.
Subsequent energy intake (15 studies).
Eight of the 15 studies reported a significant decrease in energy intake after a higher versus a lower protein pre-load; the other 7 studies reported no significant differences between higher and lower protein pre-loads. Total weight loss (15 studies).
The duration of the studies ranged from 7 days to 1 year. Most used a fixed energy intake. Seven of the 15 studies reported a significantly greater weight loss with a higher protein diet. Five of these studies were of longer duration (6 months or more) and two were of shorter duration (few weeks).
Eight studies reported no significant difference in weight loss between higher versus lower protein diets. Four of these studies were of short duration (10 weeks or less) and had small sample sizes (n=6 to 35); the other four were of longer duration (12 weeks or more).
Total fat loss (10 studies).
Three of the 10 studies reported a statistically significant greater fat loss with higher versus lower protein diets.