This was a single-site, double-blind, randomised, controlled, crossover study, which aimed to compare the effects of conventional cow's milk (CON, containing both A1- and A2-types of beta-casein) versus milk free of A1-type beta-casein (A1PF milk) on gastrointestinal (GI) physiology and symptoms of digestive discomfort in non-regular milk drinkers with self-reported intolerance to conventional cow's milk. The study location was the Pennington Biomedical Research Center in Louisiana, USA. The study comprised a 2-week washout period, 2 weeks of milk consumption, crossover, a second 2-week washout period, and a second 2-week period of milk consumption. Eligible participants were stratified by sex to achieve an approximate 1:1 male to female ratio and were randomised to receive conventional cow's milk (sequence 1) or A1PF milk (sequence 2) during the first period of milk consumption. During the second period of milk consumption, participant groups underwent crossover to receive the alternate study milk.

Milk was consumed three times per day after meals, with 250 mL provided at each administration. During washout periods, participants were required to avoid all dairy products and use only rice milk to replace any normal dairy intake, and to avoid foods with very high fermentable oligosaccharides, disaccharides, monosaccharides, and polyols content, to avoid potential confounding of the symptoms experienced. Any medication, nutritional supplements, dairy products, acidophilus milk, or probiotics were prohibited during the study.

The study protocol and all related documentation were approved by the Pennington Biomedical Research Center Institution Review Board. The study was conducted in accordance with the principles of the Declaration of Helsinki and the Good Clinical Practice Guidelines of the International Conference on Harmonisation. All participants provided written informed consent prior to initiating any study procedures.

Inclusion Criteria:

• Male or female participants aged 18 to 68 years
• Irregularly consumed milk
• Had self-reported mild-to-moderate digestive discomfort after milk consumption
• Had normal electrocardiograms and blood pressure during quiet respiration

Exclusion Criteria:

• Known allergies to dairy products
• Severe milk intolerance
• A history of faecal impaction
• Having suffered from GI disorders (e.g., irritable bowel syndrome, colitis, ulcerative colitis, or coeliac disease)
• Currently being administered drugs for cardiovascular or metabolic disease
• Trying to lose weight by following a diet or exercise regimen
• Took any appetite or weight loss drugs for the previous three months