Irritable Bowel Syndrome & Dairy Intolerances

 By Andrea Hardy, RD, a2 Milk™ Council Member

Have irritable bowel syndrome (IBS)? IBS affects up to 15% of the population, so it’s common! It’s a functional gut disorder, meaning while there is nothing structurally wrong with the gut, it’s not quite functioning the way it should. This can result in gas, bloating, abdominal pain, and changes in bowel habits like constipation or diarrhea.

 With IBS, food intolerances are common, which is why people often look towards food-first changes to relieve their IBS symptoms, and this often can include dairy. Dairy is one common trigger for IBS symptoms – but did you know that doesn’t mean you have to go dairy-free? Let’s talk about different components of dairy that contribute to IBS symptoms, and strategies to manage them so you can still enjoy dairy!

Difficulty Digesting A1 Protein in Dairy

A1 protein is a type of protein that naturally occurs in dairy. Cows produce either A1 or A2 protein depending on genetic variation. Since the dairy on our shelves comes from multiple cows and multiple farms, A1 protein can be found in almost all dairy here in Canada.

Since milk can often be a dietary trigger in IBS, researchers have been looking to understand more deeply why this is. While we know the sugar, lactose, can be a significant trigger for IBS symptoms, there are times where patients report that even lactose-free dairy can cause symptoms. This has researchers wondering – what other components in dairy might be contributing to IBS-like symptoms?

Because of the differences how A1 protein versus A2 protein are digested, researchers have speculated that A1 protein might be a culprit in digestive distress. They’ve gone on to conduct studies to test this theory and have found consumption of A1 protein to be associated with digestive symptoms like abdominal pain, bloating, and diarrhea.

In fact, in one study, researchers recruited participants who reported digestive symptoms after milk consumption. They performed a cross-over study, trialling the participants for a period of time on milk that only contained A1 protein, giving their guts a break from dairy, and then switching over to milk containing only A2 protein. All the while, the participants were unaware which milk they were drinking. What they found was – yes! A1 protein does appear to be a significant contributor to digestive distress. When consuming the A1 milk, participants reported greater levels of diarrhea, and with that diarrhea, increased abdominal pain1. This was corroborated in a similar study, where participants had a significant decrease in abdominal pain with a2 Milk™, in comparison with milk products that contained A1 protein2.

While the studies assessing digestive issues associated with A1 protein are relatively new and small, they confirm what dietitians like myself have seen in practice and suspected; when some patients who struggle with digesting dairy switch to a2 Milk, they often report that their digestive symptoms improve.

Where does a2 Milk™ fit with IBS?

As a dietitian, my IBS patients often say dairy is a major contributor to symptoms. Since lactose is a common culprit, we start there first and switch to lactose-free dairy. However, if patients find that their symptoms persist even after switching to lactose-free dairy – that’s when I suspect that A1 protein could be a contributor.

This is the perfect time to try a2 Milk™! a2 Milk™comes from cows that only produce A2 protein – meaning you’ll get none of the A1 protein found in most other milk on the shelves. It’s a great way to explore and see if it’s easier on your digestion. For those that miss dairy – this is a huge win; they get all the benefits of the protein, calcium and vitamin D found in milk, without the digestive distress.  

1.     Ho, S., Woodford, K., Kukuljan, S., & Pal, S. (2014). Comparative effects of A1 versus A2 beta-casein on gastrointestinal measures: a blinded randomised cross-over pilot study. European journal of clinical nutrition, 68(9), 994-1000.

2.     Ramakrishnan, M., Eaton, T. K., Sermet, O. M., & Savaiano, D. A. (2020). Milk Containing A2 β-Casein ONLY, as a Single Meal, Causes Fewer Symptoms of Lactose Intolerance than Milk Containing A1 and A2 β-Caseins in Subjects with Lactose Maldigestion and Intolerance: A Randomized, Double-Blind, Crossover Trial. Nutrients, 12(12), 3855.


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