Meghana D. Gadgil, MD, MPH, from the Department of Medicine, Johns Hopkins University School of Medicine, Baltimore, Maryland, and colleagues evaluated data from the randomized controlled crossover Optimal Macronutrient Intake Trial to Prevent Heart Disease (OmniHeart), which investigated the effect of 3 healthful diets of differing macronutrient composition on blood pressure and lipids, with weight held constant.
Dr. Gadgil stated that, "A diet higher in unsaturated fats, akin to the standard Mediterranean diet, is clinically meaningful and relevant as a means to reduce risk factors for cardiovascular disease in generally healthy adults…. We believe that our findings strengthen the case for the partial replacement of carbohydrates with unsaturated fats as means for diet-based prevention of cardiovascular disease."
The study population was made up of 164 adults aged 30 years and older with a systolic blood pressure of 120 to 159 mm Hg or a diastolic pressure of 80 to 99 mm Hg. The participants were randomly assigned to receive each of 3 different study diets for 6 weeks at a time, separated by 2 to 4 weeks during which they ate their own food.
The comparator diet (CARB) provided 58% of kilocalories from carbohydrates, 15% from protein, and 27% from fat; it resembled the Dietary Approaches to Stop Hypertension or DASH diet. A second study diet (PROT) replaced 10% of the carbohydrate calories with protein, so that protein content made up 25%, carbohydrates 48%, and fat 27%.
The third diet, UNSAT, replaced 10% of the carbohydrates with unsaturated fat, so that total fat made up 37%, carbohydrates 48%, and protein 15%. Most of the unsaturated fats added in the UNSAT diet consisted of monounsaturated fat from olive, canola, and safflower oils as well as nuts and seeds.
All 3 diets contained a small amount (6%) of saturated fat. The type of carbohydrate in each diet was similar, and each diet included more than 30 g fiber per day. All meals were prepared for the study participants, with calories adjusted to keep their body weights constant.
The participants had a mean age of 54 years and were 45% women and 55% black. A majority (79%) were overweight or obese, with a mean body mass index of 30.2 kg/m 2.
The primary study outcome was change in insulin sensitivity from baseline to the end of each 6-week diet period, as calculated with the quantitative insulin check index, or QUICKI.
After 6 weeks of the UNSAT diet the QUICKI increased by a mean of 0.005 (95% confidence interval, 0.000 – 0.009) more than the CARB diet (P = .04). In contrast, the PROT diet showed no significant difference compared with the CARB diet or the UNSAT diet.
Similar results were found when insulin sensitivity was assessed by the homeostasis model assessment of insulin resistance (HOMA-IR). Reported as I/HOMA, there was a significant improvement of 0.11 (95% confidence interval, 0.03 – 0.20) with the UNSAT diet compared with the CARB diet (P < .05), but this improvement was not seen for PROT compared with CARB or UNSAT compared with PROT.
Adjusting for feeding period did not change the results significantly.
Dr. Gadgil stated that, because weight loss has consistently been shown to improve insulin sensitivity, an important aspect of this study was that weight was kept constant. "It is often weight changes during a study of dietary composition that can dictate effects on parameters of cardiometabolic disease, and our study sought to isolate the effects of macronutrients themselves."
Further analysis by subgroup showed that the effect of the UNSAT diet on QUICKI was seen among the normal-weight and normoglycemic study participants, with trends toward significance in UNSAT compared with CARB diets for both groups (0.012 [P = .06] and 0.005 [P = .06], respectively), whereas there were no significant changes among those who were overweight and/or had prediabetes.
"It is likely that the choice of macronutrient to prevent insulin resistance and type 2 diabetes is less important than overall weight loss in the obese, whereas partial replacement of carbohydrates by unsaturated fat intake can mitigate risk in those of normal weight," Dr. Gadgil and colleagues write.
Professor Thomas Sanders, PhD, DSc, head of the Diabetes & Nutritional Sciences Division and professor of Nutrition & Dietetics, School of Medicine, King’s College London, United Kingdom, who coauthored a study that produced conflicting results, found fault with the data.
"The study is in fact a retrospective analysis based on the original OmniHeart trial, which was not designed to test the issue whether insulin sensitivity was affected…. The findings should be regarded with caution because adjustment has not been made for multiple outcomes," Dr. Sanders added.
Furthermore, he added, "the size effects are very small and therefore of uncertain clinical significance."
Diabetes Care. Published online December 5, 2012. Abstract