The randomized controlled trial will be preceded by a 2-week run-in period in which participants will consume 1 serving/day of reduced-fat dairy. Following the run-in period, adherent participants will be randomly assigned to one of the three treatment groups (n=20 per group) in a 1:1:1 allocation ratio. Stratified permuted block randomization will be used to ensure adequate balance among groups using a computer-generated list of random numbers. Due to the nature of the test products, participants cannot be blinded to the diet intervention. However, outcome assessors will be blinded to treatment allocation.
During the screening visit, and after obtaining informed consent, eight, height, waist circumference, blood pressure, and pulse will be measured. Standard blood and urine tests, an OGTT, chest X-ray, electrocardiogram, and a physical exam will be performed. In addition, volunteers will be interviewed regarding demographic information, medical and family history, dietary habits, and physical activity using questionnaires.
At the run-in visit, participants will retrieve their dairy products and a dietitian will instruct them on how to incorporate incorporating 1 serving of reduced-fat dairy products each day into their habitual diet. Participants will be asked to record their food intake for 3 consecutive days (including one weekend day). They will also receive an accelerometer and instructions for use on the same 3-days they record their food intake. Participants will maintain a daily record of consumed dairy products during the run-in periods and throughout the study.
At the baseline and 12-week intervention visits, participants will undergo a hyperinsulinemic-euglycemic clamp to assess systemic insulin sensitivity. In addition, body composition will be measured by dual-energy X-ray absorptiometry and resting energy expenditure by indirect calorimetry. A biopsy of subcutaneous adipose tissue from the thigh will be taken at fasting.
During the 12-week intervention, participants in the limited dairy group will be instructed to limit their intake of dairy products to ≤1 serving/d of dairy. Participants in the 2-3 dairy servings of reduced or regular fat dairy groups will be provided with the study products. Serving sizes will be 250 mL milk, 175 g yogurt, and 50 g cheese. The dietitian will instruct participants individually on how to incorporate the dairy products into their habitual diets by reducing the intake of other foods of equivalent caloric content and complementary nutrient density, to maintain stable body weight. No additional dietary changes will be recommended.
At the monthly follow-up visits, weight, blood pressure, and pulse will be measured. In addition, participants will complete questionnaires about their health and physical activity. At each visit, they will be asked to track their food intake for 3 consecutive days (including one weekend day) using a mobile application on their smartphone or, if not possible, in written food diaries. Additionally, a blood sample will be collected to measure fatty acid biomarkers of dairy intake. On the second monthly visit, they will also receive an accelerometer and instructions for use on the same 3-days they record their food intake for the 12-week visit.
At the final follow-up visit, participants' weight, blood pressure, and pulse will be measured, and they will complete questionnaires regarding their health and activity. They will undergo an oral glucose tolerance test, as well as blood and urine tests, as described for the first visit.
