Dairy Fat and Fermentation Study

A randomized-controlled cross-over trial will be conducted to compare the effect of two servings per day of full-fat and fat-free milk, and full-fat and fat-free yogurt (hence forth referred to as dairy food items) on the gut microbiome, fecal and serum metabolome, and determine their relation to cardiometabolic risk factors (CMRF) and functional pathways associated with the differences identified. Participants will be provided with 2 servings per day of the test dairy products and requested not to consume additional bovine (cow) milk or yogurt during each 3-week diet phase.They will be allowed to use dairy substitutes such as plant-based milk alternatives and consume other types of dairy products (e.g. cheese).

The study consists of four diet phases: fat-free milk phase, full-fat milk phase, fat-free yogurt phase and full-fat yogurt phase. Each diet phase is 3 weeks in duration, separated by a 2 week break. Participants will be provided with 2 servings of a dairy based food item per day specific to their assigned phase and asked to consume them at two separate times during the day - morning and evening. Weight, waist circumference and blood pressure will be measured, blood and stool samples will be collected, and participants will complete two 24-hr food recall questionnaires during each phase.

Inclusion criteria

• Age >50 years
• Men and postmenopausal women
• BMI >25 and <35 kg/m2
• No milk allergy, lactose intolerance or adherence to vegan or other dietary pattern that excludes dairy products
• Non-smoker
• Usual pattern of bowel movements at least every other day.

Exclusion criteria

• Women and men <50 years
• Women, pre- or peri-menopausal
• BMI <25 and >35 kg/m2
• Current smokers or former smoker who quit < 6 months
• Use of nicotine replacement products within last 6 months
• Replacement or gender affirming hormonal therapy use
• Weight gain of >15 pounds within last 6 months
• Infrequent bowel movements
• History of anemia within past 2 years or confirmation of anemia at first study visit
• Blood donation within last 2 months
• Untreated hypertension
• Oral antibiotics use within 2 weeks of study entry
• Steroid use (including dental prophylaxis use within 2 wks prior to or during study, except non-prescription topical and nasal steroids, e. g. Flonase)
• Use of lipid lowering medications
• Chronic use of aspirin, non-steroidal anti-inflammatory medications (NSAIDS), laxatives, anti-diarrheal medication
• Regular use (>2 times per week) of acid lowering medication: antacids, proton pump inhibitors, or H2 Blockers.
• Use of dietary supplements other than multi-vitamins, including pre- and probiotics, within 2 months of study entry
• Milk allergy, lactose intolerance or adherence to vegan or other dietary patterns that exclude dairy products
• Colonoscopy within 3 weeks of study entry or during study period
• Diagnosis of malabsorption, inflammatory bowel disease (ulcerative colitis or Crohn's Disease), cirrhosis, history of gastrointestinal resection (other than appendectomy), chronic pancreatitis or any other chronic GI disease that in the opinion of the study physician would alter the study results
• Chronic heart, liver, thyroid, renal or kidney disease
• Type I or type II diabetes
• Alcohol consumption >7 drinks/week for women and >14 drinks/week for men
• Inadequate refrigerator storage capacity to store 21 8-oz containers of milk or yogurt
• Participation in other dietary intervention research study during the same time
• No social security number
• Not willing to commit to adhering to the protocol