The investigators will recruit 45 fluid milk avoiding adults to participate in a five-week double-blind, double-crossover study in which the physiological responses to increasing doses of lactose-free (A1-rich milk), A2 milk, and lactose-free A2 milk will be explored.

Dairy provides essential nutrients and dairy products and are among the best quality and bioavailable protein sources available to humans. Milk protein allergy is extremely rare outside of childhood, and lactose intolerance is highly variable among ethnicities. Nonetheless, many adults avoid fluid milk and dairy products unnecessarily. Recent studies suggest that dairy intolerant individuals are having gastrointestinal reactions specifically to the bioactive metabolites (specifically, beta-casomorphin-7[BCM-7]) of casein protein subtype A1, and consuming milk void of A1 and rich in A2 casein alleviates symptoms.

The investigators will recruit 45 fluid milk avoiding adults to participate in a five-week double-blind, double-crossover study in which physiological responses to increasing doses of lactose-free (A1-rich milk), A2 milk, and lactose-free A2 milk will be explored. Enrollment assessments will include a comprehensive metabolic panel, diet history, and medical history questionnaires. Stool samples will be collected at the beginning and end of each of the 3 test phases (6 total samples per participant). Each test phase will last 6 days long, during which participants will receive increased doses of casein every 2 days.

On Day 1, participants will receive 50mg casein/kg/bw (~125ml milk). On Day 3, participants will receive 100mg casein/kg/bw (~250ml milk). On Day 5, participants will received 200mg casein/kg/bw (~500ml milk). During test phases, breath hydrogen and methane will be measured every thirty minutes for three hours, and blood glucose will be measured every thirty minutes for two hours. Gastrointestinal symptoms will be assessed with visual analog scales for the following 24 hours. There will be a 7-10 day wash out period in between test phases.

Inclusion Criteria:

• Adults age 18+
• Currently avoid liquid milk
• Can read and speak English

Exclusion Criteria:

• Diagnosis of milk protein allergy
• Diagnosis of Crohn's Disease, Ulcerative Colitis, Celiac Disease, or Peptic Ulcer Disease or Gastroparesis
• Using antibiotic or antiviral medications within 30 days prior to study start
• Undergoing chemotherapy, or using any investigational drugs within 30 days prior to study start
• Pregnant or lactating