Current dietary recommendations include 0.8 g/kg/day of protein to meet metabolic requirements of nitrogen and amino acids; however, a growing body of data has identified positive changes in weight, body composition, blood pressure, and metabolic markers with increased intake of protein. Related to increased protein intake, research has been directed toward various sources of protein and composition responsible for specific metabolic responses. Therefore, more research is needed to evaluate the effective use of protein to improve body weight and composition, and perceptions consumers have regarding the role of dietary protein in weight control/maintenance.
The focus of this dissertation was two-fold, 1) to evaluate the effectiveness of increased protein intake for promoting weight loss and improving body composition in a controlled weight loss study, and 2) to evaluate the use of the practice of "eating more protein" to prevent weight gain among midlife women. Additionally, the activity of the Renin-Angiotensin system (RAS) was evaluated to assess related metabolic effects with protein intake.
The first study described the effect of three weight loss diets on body composition, blood pressure, and RAS metabolites. Midlife participants were randomly assigned a control diet (15% protein), a mixed protein diet (30% protein), and a whey protein diet (15% mixed, 15% whey) condition for a 5 month period. Total body weight and fat loss between groups was not significantly different, but a trend toward greater body weight and fat loss was observed with the whey protein diet. No differences in RAS metabolites were observed between diets, but a statistically significant decrease in systolic blood pressure was observed with the whey protein diet. These results confirm that reduction of energy intake is the primary effective step in weight loss, but secondary effects of regional fat loss and decreased blood pressure may be achieved with a high protein diet containing whey protein.
The second study described survey results of a national panel of midlife women regarding weight maintenance practices and weight self-efficacy. In this cross-sectional survey, "eating more protein" was identified as the fourth most common practice used to prevent weight gain. Self-reported weight loss over 2 years was associated with reporting the use of the eating more protein practice. Although those who gained and lost weight reported similar weight maintenance practices, those who lost weight had significantly higher Weight Efficacy Lifestyle scores than those who gained weight. Educating individuals on the best use of protein to encourage successful weight maintenance may enhance the results.
In conclusion, while many metabolic effects have been identified with increased protein intake, the best use practices for weight maintenance and weight loss continue to be a significant research topic. Increased protein intake has been associated with increased satiety and insulin sensitivity, and decreased blood pressure. Whey protein intake in a high protein weight loss plan may further result in regional fat loss and decreased blood pressure, but the specific mechanisms have not been determined. Among a national sample, midlife women reported eating more protein to maintain weight, and high self-efficacy was associated with successful weight maintenance.