A2 beta casein milk and digestive comfort in people
What it found
In 45 people who said they were intolerant to milk, drinking milk with both A1 and A2 beta casein led to worse gut symptoms, slower digestion and slightly slower thinking than milk with only A2 beta casein.
Milk with only A2 beta casein did not worsen these symptoms.
What they found
Studies in people
Gut symptoms
When people drank milk with both beta casein types, their bloating, wind and stomach rumbling scores were worse than at baseline. When they drank milk with only A2 beta casein, these symptoms did not get worse.
Transit time
Using a smart pill, the authors found that milk with both beta casein types slowed colonic transit by 6.6 hours and whole gut transit by 6.3 hours compared with milk with only A2 beta casein. Small bowel transit did not differ.
Inflammation markers
Blood levels of IL-4, IgG, IgE and IgG1 were higher after drinking milk with both beta casein types than after milk with only A2 beta casein. Small bowel inflammation improved in 36.4% of people after switching from the mixed milk to the A2-only milk, compared with 11.1% after switching the other way.
Gut bacteria products
Fecal levels of acetic acid, butanoic acid and total short-chain fatty acids were lower after drinking milk with both beta casein types than after milk with only A2 beta casein.
Thinking speed
On a computer test of information processing, people were slower and made more errors after drinking milk with both beta casein types than after milk with only A2 beta casein. The authors describe these as small but significant changes.
Lactose intolerance subgroup
About half the people were confirmed lactose intolerant by a urine test. Both groups had worse gut symptoms on the mixed milk, but the worsening tended to be greater in the lactose intolerant group. The A2-only milk did not worsen symptoms in either group.
What the authors conclude
“These results suggest that the exacerbation of gastrointestinal symptoms associated with milk in lactose intolerant subjects may be related to A1 β-casein rather than lactose per se.”
Also in their conclusions
- They conclude that consumption of milk containing A1 beta casein in addition to A2 beta casein worsens gastrointestinal symptoms, increases gastrointestinal transit time, increases serum inflammation markers, lowers total fecal short-chain fatty acid content, and slows cognitive processing speed and decreases processing accuracy compared with baseline values.
- They state that consumption of milk containing only A2 beta casein did not adversely affect these variables, indicating that the changes observed with milk containing both beta casein types were attributable to the presence of A1 beta casein.
- They suggest that the exacerbation of gastrointestinal symptoms associated with milk in lactose intolerant subjects may be related to A1 beta casein rather than lactose per se.
How it was done
The authors wanted to test whether some milk intolerance is caused by A1 beta casein rather than lactose. They noted that digesting A1 beta casein can produce a peptide called beta-casomorphin-7, which has been linked to gut effects.
They ran a double-blind, randomised, crossover trial in 45 Han Chinese adults aged 25 to 68 who reported intolerance to regular milk. Each person drank 250 ml of milk after two meals a day for 14 days, first one type then the other, with 14-day washout periods before and between. The milks were identical except one had only A2 beta casein and the other had both A1 and A2 beta casein in a 40:60 ratio.
What it can’t tell you
- The smart pill was not used at baseline, so the authors could not tell whether the A2-only milk changed transit time beyond the effect of washing out dairy.
- Each milk was drunk for only 2 weeks, which may be too short to show changes in some markers or in local inflammation.
- The study focused on gut symptoms, so it cannot show other effects of these milks.
Who paid
- Authors work at
- Huadong Hospital affiliated to Fudan University, China; Xin Hua Hospital affiliated to Shanghai Jiao Tong University School of Medicine, China; Endoscopic Center, China
The paper
- Title
- Effects of milk containing only A2 beta casein versus milk containing both A1 and A2 beta casein proteins on gastrointestinal physiology, symptoms of discomfort, and cognitive behavior of people with self-reported intolerance to traditional cows' milk
- Type
- Randomised controlled trial
- Evidence
- Studies in people
- Summarised from
- Full text
- Licence
- CC BY. Words in quotation marks are the authors’; the rest is our summary.
- Cite
- Jianqin S, Leiming X, Lu X, Yelland GW, Ni J, Clarke AJ (2016). Effects of milk containing only A2 beta casein versus milk containing both A1 and A2 beta casein proteins on gastrointestinal physiology, symptoms of discomfort, and cognitive behavior of people with self-reported intolerance to traditional cows' milk. Nutrition journal. doi:10.1186/s12937-016-0147-zFree full textPubMed 27039383DOI
Summary written 26 Sep 2026. Check it against the paper before it changes what you eat. How we summarise papers · Report an error