This is a randomized controlled feeding trial designed to investigate whether the consumption of a diet rich in low-fat dairy vs. full-fat dairy vs. a limited dairy diet differentially affects glucose homeostasis. The researchers will also begin an investigation into the mechanisms by which dairy may affect glucose tolerance and its determinants. The investigators will study 60 men and women with the metabolic syndrome who will consume diets differing in their type and content of dairy foods, in a parallel-design randomized controlled trial consisting of a 4-week wash-in diet period and a 12-week dietary intervention period.

Subjects will be randomized using block randomization stratified by gender and insulin resistance (low insulin resistance vs. high insulin resistance or manifest diabetes) to one of three diet groups, which they will follow for 12 weeks: the limited dairy diet, the low-fat dairy diet, or the full-fat dairy diet. During the dietary intervention, participants will be provided with specific amounts and types of dairy products by the Human Nutrition Lab (HNL) at Fred Hutchinson Cancer Research Center (FHCRC). In the limited dairy diet arm, participants will be asked not to consume any dairy, other than three servings of nonfat milk per week, which will be provided. In the two dairy diet arms, participants will be asked to consume 3.3 servings per day of either nonfat/low-fat or full-fat milk, yogurt, and cheese. Participants will be asked to consume all of the dairy products they receive, not to consume any other dairy products for 12 weeks, and to continue consuming their habitual diet ad libitum (i.e., to eat only when hungry, and to stop eating when comfortably satiated).

Prior to randomization, subjects will complete a wash-in diet period of 4 weeks during which they will be asked to consume the limited dairy diet (i.e., consume 3 servings of nonfat milk per week, and not consume any other dairy products). In the third week of the wash-in diet period, subjects will also complete their first of two 5-day controlled feeding periods (i.e., consume study dairy products alongside a provided standard American diet) to measure ad libitum energy intake. During the last week of the wash-in diet period, participants will be admitted to clinic for a baseline visit (clinic visit #1). After clinic visit #1, subjects will be randomized to one of the three study arms, as outlined above. They will follow their randomly assigned study diet for the next 12 weeks. In the second week of the main intervention period, subjects will complete their second 5-day controlled feeding period to again measure ad libitum energy intake, this time on the specific intervention diet they had been randomized to. In the last week of the 12-week diet phase, subjects will be admitted for the follow-up clinic visit (clinic visit #2). At both clinic visits, the researchers will collect fasting blood; measure body weight and height, waist and hip circumference, and blood pressure; conduct a 3-hour FS-OGTT to assess glucose tolerance, insulin sensitivity, and pancreatic beta-cell function; conduct a whole-body DEXA scan to assess body fat mass, lean mass, and body fat distribution; and an abdominal MRI scan to assess liver triglyceride content and the ratio of intra-abdominal to subcutaneous adipose tissue. Subjects will also complete five Food Frequency Questionnaires (FFQ's) and five unannounced 24-hour dietary recalls during the study to assess dietary intakes.

The primary analysis will be a per protocol analysis that will include at least 60 participants (at least 20 in each intervention arm) who comply with all study procedures per protocol. The investigators anticipate enrolling up to 72 participants to achieve this goal. In a secondary analysis, the researchers will analyze the impact of the dietary intervention on all enrolled participants, including those who dropped out, were excluded, or non-compliant with the study protocol, in an intent-to-treat (ITT) analysis that will be reported and interpreted together with the per protocol analysis.