Varying protein source and quantity do not significantly improve weight loss, fat loss, or satiety in reduced energy diets among midlife adults.

We hypothesized that a whey protein diet would result in greater weight loss and improved body composition compared with standard weight loss diets. Weight change, body composition, and renin-angiotensin aldosterone system activity in midlife adults were compared between diet groups. Eighteen subjects enrolled in a 5-month study of 8-week controlled food intake followed by 12-weeks ad libitum intake. Subjects were randomized to 1 of 3 treatment groups: control diet (CD) (55% carbohydrate/15% protein/30% fat), mixed protein (40% carbohydrate/30% protein/30% fat), or whey protein (WP) (40% carbohydrate/15% mixed protein/15% whey protein/30% fat). Measurements included weight, metabolic measures, body composition by dual-energy x-ray absorptiometry, and resting energy expenditure. No statistically significant differences in total weight loss or total fat loss were observed between treatments; however, a trend toward greater total weight loss (P = .08) and total fat loss (P = .09) was observed in the WP group compared with the CD group. Fat loss in the leg and gynoid regions was greater (P < .05) in the WP group than the CD group. No renin-angiotensin aldosterone system-mediated response was observed, but a decrease in systolic blood pressure was significantly greater (P < .05) in the WP group compared with the CD group. In summary, increased whey protein intake did not result in statistically significant differences in weight loss or in total fat loss, but significant differences in regional fat loss and in decreased blood pressure were observed in the WP group.