LAUSR

The consumption of dairy foods, both low-fat and full-fat, tends to be associated with better metabolic health and lower type 2 diabetes risk in most observational studies. We aimed to test the effects of diets rich in either low-fat or full-fat dairy products on glucose tolerance and its key determinants, compared to a diet limited in dairy. We hypothesized that diets rich in dairy, particularly full-fat dairy, would improve glucose homeostasis. Seventy-two men and women with the metabolic syndrome first completed a 4-week wash-in period during which they limited their dairy intake to no more than 3 servings of skim 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. Glucose tolerance and its key determinants (insulin sensitivity, beta-cell function, liver fat content, low-grade inflammation) were assessed before and after the 12-week intervention period. In primary per protocol analyses (n = 67), repeated measures analysis of variance revealed no treatment effect on glucose tolerance, as measured by the area-under-the-curve glucose in a 3-hour oral glucose tolerance test (P = 0.340). However, intervention effects were seen for the homeostasis model assessment insulin resistance index (P = 0.004 overall), and post hoc tests showed a significant increase in insulin resistance in the low-fat dairy group (mean ± SD + 0.8 ± 1.4, adjusted P = 0.03) and the full-fat dairy group (+ 1.3 ± 1.7, adjusted P = 0.003) compared to the limited dairy group (-0.2 ± 1.1). These effects were unchanged in secondary analyses adjusted for moderate changes in fat mass. In this 12-week dietary intervention study, we detected no evidence for our hypothesis that dairy consumption improves glucose tolerance. Unexpectedly, insulin resistance increased in individuals consuming diets rich in either low-fat or full-fat dairy products, compared to participants who limited their dairy intake. This work was supported by the National Dairy Council, Dairy Farmers of Canada, the Dutch Dairy Association (NZO), Dairy Australia, and the French Dairy Interbranch Organization (CNIEL).