A2 milk appears to be the better choice for digestive comfort, especially if regular milk bothers your stomach. The two types of milk differ by a single amino acid in one protein, but that tiny change affects how your body breaks the protein down and what it releases during digestion. The science is still evolving on bigger health claims, but the digestive evidence is fairly consistent.
What Makes A1 and A2 Milk Different
All cow’s milk contains a protein called beta-casein, and it comes in two main forms: A1 and A2. The difference between them is one amino acid at position 67 of the protein chain. A2 beta-casein has proline at that spot, while A1 has histidine. That single swap changes the shape of the protein and, more importantly, changes what happens when your digestive enzymes go to work on it.
When you digest A1 beta-casein, enzymes can clip the protein at that position 67 site, releasing a small fragment called beta-casomorphin-7 (BCM-7). The proline in A2 beta-casein resists that same cut, so far less BCM-7 gets released. BCM-7 is an opioid-like peptide, meaning it can interact with receptors in your gut and potentially slow digestion, trigger inflammation, or cause discomfort. This peptide is the central reason researchers think A1 and A2 milk may affect the body differently.
How They Affect Digestion
The strongest evidence favoring A2 milk comes from digestive studies. A randomized, double-blind crossover trial with 40 people who experienced gastrointestinal discomfort after drinking milk found that A2 milk caused significantly less abdominal pain, less fecal urgency, and fewer stomach rumbling sounds compared to regular (A1/A2) milk. A2 milk also lowered fecal calprotectin, a marker of intestinal inflammation, particularly in male participants.
The results weren’t perfectly one-sided. A2 milk was associated with slightly more bloating and looser stools in the same trial. Still, the overall conclusion was that A2 milk is an effective and safe alternative for people who get digestive discomfort from regular milk.
One particularly interesting finding: regular A1-containing milk appears to slow transit through the digestive tract. A clinical trial reported that participants drinking A1 milk had significantly longer colonic and overall gut transit times compared to those drinking A2 milk. Slower transit is linked to constipation and general discomfort. Researchers in that study suggested the digestive trouble people blame on lactose might actually be caused by A1 beta-casein, not lactose itself.
It Might Not Be Lactose Intolerance
This is probably the most useful takeaway for anyone reading this article. Many people who feel lousy after drinking milk assume they’re lactose intolerant, but multiple trials across New Zealand, China, Australia, and the United States have found that A1 beta-casein correlates with the same symptoms people attribute to lactose: cramping, bloating, irregular bowel movements. Some of these people don’t meet the diagnostic criteria for lactose intolerance at all.
If you’ve cut out milk because it upsets your stomach, A2 milk may be worth trying before you give up on dairy entirely. It contains the same amount of lactose as regular milk, so if your symptoms improve, A1 protein was likely the culprit, not the sugar.
Inflammation and Chronic Disease Claims
You’ll find bold claims online linking A1 milk to heart disease, type 1 diabetes, and systemic inflammation. The actual evidence is much weaker than the headlines suggest.
On inflammation, a randomized crossover trial measuring C-reactive protein (a standard blood marker of inflammation) and calprotectin found no significant increase during any milk intervention period, whether participants drank A1 or A2 milk. Only weak, non-significant hints of cytokine changes appeared. In short, neither type of milk caused measurable systemic inflammation in that study.
The link between A1 milk and type 1 diabetes is based largely on population-level correlations and a handful of small studies. One found that children with type 1 diabetes were about five times more likely to have consumed high amounts of milk (over 540 ml per day) compared to controls, but studies investigating this association overall have yielded conflicting results. No clear causal mechanism has been confirmed in humans, and no major health organization currently recommends avoiding A1 milk to prevent diabetes or heart disease.
Which Cows Produce Which Milk
Most conventional milk in the U.S. comes from Holstein cows, which make up about 92% of the dairy herd. Holstein milk contains roughly 66% A1 beta-casein, making it predominantly A1. Guernsey cows, by contrast, produce milk that is about 90% A2 beta-casein. Jersey cows also tend toward higher A2 levels, though not as dramatically as Guernseys.
Commercially sold “A2 milk” comes from cows that have been genetically tested to confirm they carry two copies of the A2 gene, meaning their milk contains no A1 beta-casein at all. Goat milk, sheep milk, and human breast milk are also naturally A2.
The Practical Tradeoffs
A2 milk typically costs more, sometimes 50% to 100% more per half gallon than regular milk. Nutritionally, there’s no difference. A2 milk has the same calories, fat, calcium, vitamin D, and lactose as conventional milk. The only change is the type of beta-casein protein.
If you digest regular milk without any issues, there’s no strong scientific reason to switch. The inflammation and chronic disease data doesn’t support paying a premium for prevention. But if drinking milk gives you cramps, urgency, bloating, or irregular stools, A2 milk is a reasonable experiment. The digestive benefits are the one area where clinical trials consistently show a meaningful difference.