Journal Title (Medline/Pubmed accepted abbreviation): Phys Sportsmed.
Summary of background and research design
Numerous clinical weight loss trials have been published recently that utilize a relatively high proportion of dietary protein (HP) compared to traditional higher carbohydrate (HC) diets. This approach provides benefits on regulation of blood glucose and insulin levels, preservation of lean tissue mass, and prevents reduction in resting energy expenditure. There are studies that have combined this dietary approach with moderate physical activity, although the authors claim there are few such studies that have employed resistance-based exercise training. Additionally, there is some evidence that individuals with triglyceride levels above median levels experience better weight loss on an HP diet. The authors chose to adopt this method to stratify participants according to clinical status to evaluate potential differences in dietary efficacy.
This study enrolled middle-aged (~45 yrs), moderately obese (BMI ~34) women into a 10-wk weight loss trial. Participants were assigned to consume either a HP or HC diet based on their responses to a carbohydrate tolerance questionnaire. Under the guidance of a dietitian, both groups were instructed to consume 1200 kcal / day for week 1, and 1600 kcal / day for the remaining 9 weeks. The guidelines for HC diet were 55% CHO, 15% protein, and 30% fat for the full 10 weeks. The guidelines for the HP diet were 7% CHO, 63% protein, and 30% fat during the 1st week, and 15% CHO, 55% protein, and 30% fat for the remaining 9 weeks. All subjects participated in the Curves exercise program (45 minutes of circuit-style resistance training) for 3 days / week for the duration of the study.
Additionally, the subjects were stratified according to pre-test HOMA-IR values, which is a measurement of insulin resistance.
Summary of research findings
193 participants fell below the designated HOMA threshold of 3.5, while 28 participants fell above it. 92 participants followed the HC diet, while 129 followed the HP diet. Not surprisingly, the HP participants were unable to meet the level of prescribed carbohydrate restriction, but there were significant differences between groups in carbohydrate proportion (37% vs. 51%) and protein proportion (28% vs. 19%) of the diet. The HP group lost significantly more weight and fat mass than the HC group.
Mean triglycerides, glucose, and insulin levels were all significantly higher in the high HOMA-IR group. Participants in the HP group experienced a stronger reduction in blood glucose. The participants in the HP group with high HOMA-IR experienced the greatest reduction in blood glucose.
Interpretation of findings/Key practice applications
The program was successful in promoting weight loss among this population over 10 weeks, and consistent with other research, the HP diet was more efficacious than the HC diet in promoting weight loss. Among individuals with apparent insulin resistance, the results suggest that individuals in this population may gain an additional, minor benefit from adhering to the HP diet.
The study was limited by the fact that only 13% of participants were in the high HOMA-IR group, so statistical differences among groups of drastically different size may not represent the true population characteristics. Additionally, the participants were not randomly assigned to diet treatment, and the rationale for assigning individuals based on questionnaire responses was not provided. The prescribed HP was not particularly realistic for this population (7-15% CHO for 10 weeks), although free-living diets based on this prescription still yielded greater weight loss and fat loss than the HC prescription.
A key limitation of this study is its short duration (10 weeks, or 2.5 months). A longer trial would have been better able to assess the sustainability of each diet pattern in conjunction with exercise training.