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Abstract
Background: Dairy is often considered an important part of a healthful diet. However, questions remain as to the type of dairy foods that best promote cardiometabolic health. In the observational literature, there is particularly strong and consistent evidence linking yogurt and biomarkers of full-fat dairy food intake with a reduced risk of type 2 diabetes. Further, emerging evidence challenges the long-standing view that full-fat dairy foods promote cardiovascular disease through the lipid altering affects of its high saturated fat content. Rather, evidence indicates that dairy fat may not negatively impact CVD risk, particularly when consumed in whole foods with a complex matrix. However, trials investigating the effect of dairy foods on cardiometabolic health rarely include full-fat or fermented dairy foods in their interventions and often tested only one type of dairy food. Therefore, we aimed to compare the effects of diets rich in a wide variety of either low-fat or full-fat dairy foods, or a diet limited in dairy on glucose tolerance and its determinants as well as cardiovascular disease risk factors.
Methods: This was a parallel-design randomized controlled trial set at an urban research center in the northwestern United States. Seventy-two men and women with metabolic syndrome completed a 4-week wash-in period, limiting dairy intake to ≤ 3 servings of nonfat milk per week. Participants were then randomized to either continue the limited dairy diet, or switch to a diet containing 3.3 servings per day of either low-fat or full-fat milk, yogurt, and cheese for 12-weeks (n=24 per group). Main outcome measures included glucose tolerance as measured by the area-under-the-curve glucose during a standardized 180-minute oral glucose tolerance test (primary), insulin sensitivity, pancreatic β-cell function, systemic inflammation, liver fat content, body weight and composition, a comprehensive fasting lipid profile, and blood pressure.
Results: In the primary per-protocol analysis (n=67), there was no treatment effect on glucose tolerance (p=0.340, overall repeated measures analysis of variance). However, both the low-fat and full-fat dairy diets decreased the Matsuda insulin sensitivity index (p=0.012 overall) and increased the homeostatic model assessment of insulin resistance (p=0.005 overall) as compared to the limited dairy group. Two measures of pancreatic β-cell function, insulinogenic index and glucose sensitivity did not change differentially. Dairy consumption also resulted in weight gain (p=0.006 overall), with an increase in the full-fat dairy compared to the limited dairy diet, and the low-fat dairy diet falling in-between. Intervention effects on insulin sensitivity were not, or only mildly, attenuated by adjusting for changes in adiposity. No intervention effects were seen in measures of liver fat content or biomarkers of systemic inflammation. Further, there were no intervention effects on fasting plasma total-, low-density lipoprotein-, and high-density lipoprotein-cholesterol; triglycerides; free fatty acids; or cholesterol content in 38 isolated lipoprotein fractions (p>0.1 for all variables). There was also no intervention effect on diastolic blood pressure. There was a significant intervention effect for systolic blood pressure (p=0.045), with a trend for a decrease in the low-fat dairy diet compared to the limited dairy diet in post hoc testing after adjustment for multiple testing. Results from the intent-to-treat analysis (n=72) were consistent with those of the per-protocol analysis.
Conclusions: Contrary to our hypothesis, consuming 3.3 servings per day of low-fat dairy or an equivalent amount of full-fat dairy, did not improve glucose tolerance in men and women with the metabolic syndrome. Both dairy diets similarly decreased insulin sensitivity through mechanisms largely unrelated to changes in body weight, fat mass, liver fat content, or biomarkers of systemic inflammation. Additionally, dairy fat, when consumed as part of mixed complex whole foods such as milk, yogurt, and cheese, did not adversely impact the serum lipid profile. Overall, our findings indicate that lower dairy intake may be beneficial in individuals with the metabolic syndrome and that full-fat dairy products do not differ from low-fat dairy products in regards to their impact on cardiometabolic health.
