Comparison of the impact of bovine milk β-casein variants on digestive comfort in females self-reporting dairy intolerance:a randomized controlled trial

Background: Lactose malabsorption is one of the major causes of digestive discomfort following dairy ingestion, although more recently the role of the A1 type of the bovine β-casein protein has been proposed. Objective: Therefore, we examined whether there are distinct symptoms of digestive discomfort due to either lactose or differing β-casein types. Design: Women (n=40, 25.2 ± 0.5 years) with self-reported varying levels of dairy tolerance underwent a 50 g lactose challenge to confirm lactose intolerance. On the basis of lactose malabsorption and associated digestive discomfort, participants were classified as either lactose intolerant (LI, self-reported intolerant and lactose intolerant, n=10), non-lactose dairy intolerant (NLDI, n=20, self-reported intolerant and lactose tolerant) or dairy tolerant (DT, n=10, self-reported tolerant and lactose tolerant). In a double-blinded randomised sequence, participants consumed 750 mL UHT treated milks, including; conventional milk (CON, containing both A1 and A2 β-casein), milk exclusively containing A2 type β-casein (A2M) or lactose-free conventional milk (LF-CON). Subjective digestive symptoms and breath hydrogen were recorded regularly over 3 hours, and further ad hoc digestive symptoms over 12 hours. Results: Participants identified as NLDI experienced rapid onset of transient abdominal distension, bloating, and flatulence, in the absence of increased breath hydrogen, irrespective of milk type. Contrastingly, those identified as LI experienced prolonged digestive discomfort with the CON milk. In these LI subjects, A2M reduced nausea, faecal urgency and attenuated the rise in breath hydrogen over the first 3 hours, relative to CON milk. Conclusions: In this study, self-reported dairy intolerance, without lactose malabsorption (NLDI), is characterised by early onset digestive discomfort following milk ingestion, irrespective of either the lactose content or β-casein types. For individuals with LI, lactose malabsorption and digestive comfort with lactose-containing milks was improved with milk containing exclusively A2 β-casein.