Journal Title (Medline/Pubmed accepted abbreviation): Nutr J
Page numbers: 72
Summary of Background and Research Design
Background:Protein-enriched meal replacements (MR) have become increasingly popular among overweight subjects who wish to reduce their body weight. Although MRs are considered a safe and effective method for weight loss and weight management, there is growing concern that the long-term use of a high protein meal replacement may negatively affect liver and kidney function and reduce bone mineral density.
Hypothesis/purpose of study: This study’s aim was to propose an optimal MR strategy and to evaluate effects of long-term high protein MR compared with standard protein MR on liver and renal function and bone density in obese outpatients.
Subjects:Eighty-five obese men (23) and women (62), out of 100 initially enrolled, were randomly assigned to diets including either high protein (HP) or standard protein (SP) MR (mean ± standard deviation: age, 49.4 ± 11.0 yr; weight, 93.5 ± 14.0 kg for HP group and 92.7 ± 15.9 kg for SP group; BMI, 34.7 ± 6.8 kg/m2 for HP group and 34.3 ± 10.3 kg for SP group). During the 12-month trial, 15 additional subjects withdrew and only 70 outpatients completed the study.
Experimental design:Randomized, single-blinded, placebo-controlled clinical trial
Treatments and protocol:Subjects were equally randomized to 1 of 2 isocaloric MR diets: HP (providing 2.2 g protein/kg of lean body mass) or SP (providing 1.1 g protein/kg of lean body mass). Participants were advised to replace 1 of their meals and a snack with the MR daily for 12 weeks, and then continue with only 1 MR a day for an additional 40 weeks. All participants were also encouraged to exercise 30 minutes daily. Qualitative food logs were recorded and reviewed by a registered dietitian at week 2, months 1, 2, 3, 6, 9, and 12. During each visit, weight, height and BMI were assessed. Fasting blood samples for biochemical assessments of liver and renal function were collected at baseline and months 3, 6, and 12. Bone mineral density was measured via dual energy x-ray absorptiometry (DEXA) at baseline and at 12 months.
Summary of research findings:
- At 12 months, subjects in both the HP and SP groups experienced significant weight loss and BMI reduction compared with baseline (weight, P < .01; BMI, P not given).
- There were no significant differences in body weight and BMI changes between the 2 dietary groups.
- At 3 and 6 months, total blood cholesterol and low-density lipoprotein were significantly decreased only in the HP group (P < .05 for all), with no change in the SP group.
- At 3 months, triglyceride levels were significantly decreased only in the HP group (P < .05).
- At 6 months, high-density lipoprotein levels were significantly increased only in the HP group (P < .05).
- There was no significant difference between the 2 dietary groups for any of the lipid parameters.
- Blood AST, ALT, bilirubin, and alkaline phosphatase levels remained within the normal range during the study in both groups and did not significantly change.
- No significant differences were observed between the 2 dietary groups for renal function.
- At 12 months, total bone mineral density was not significantly different within or between the 2 dietary groups.
Interpretation of findings/Key practice applications:
This study has shown that adding protein to a MR diet, when used daily in overweight individuals for a prolonged period of time, may be beneficial for weight reduction and weight maintenance without apparent adverse effects on hepatic, renal, or bone health. However, research has suggested that high triglyceride levels or the presence of insulin resistance may influence the response to MR diets. Stratification by triglyceride level and measurement of insulin resistance was not done in this study. In addition, there was no weight loss difference between the protein-enriched MR compared with a standard MR, although any expected weight loss difference between the 2 MRs may have been overshadowed by the power of MR intervention itself (by simplifying weight-loss efforts). Future research should attempt to determine whether very high protein intake, exceeding the Institute of Medicine recommendations and popular with some athletes, could negatively affect bone density, liver function, or renal function.
The dropout rate for this study, as with many weight loss studies, was high, with 70% of randomized subjects completing the entire study. Also, the overall level of dietary protein intake (diet plus MR) was not reported for the two groups. Finally, more detailed information on body composition (eg, fat mass, fat-free mass) and site-specific DEXA results (eg, hip, wrist, lumbar spine) was not reported.
NOTE: There were some reported changes in the text of the Results section of the article (eg, serum cholesterol, urinary protein) that did not numerically match the data reported in Tables 2 and 4.