March 2021 Issue

Ask the Expert: A1 vs A2 Milk Proteins
By Toby Amidor, MS, RD, CDN, FAND
Today’s Dietitian
Vol. 23, No. 3, P. 10

Q: My clients have been asking about A1 vs A2 proteins in cow’s milk. One says she’s heard that A1 may pose health risks. Is there any evidence to back this up?

A: There have been several claims that the A1 protein in milk may have health consequences including inflammation, cancer, and gastrointestinal (GI) discomfort in some individuals. A2 milk, which comes from cows that produce milk lacking A1 protein, has been touted to avoid these concerns and be more easily digested. Much of the A1 research still is in its infancy but doesn’t indicate serious health risks.

What Is A1?
More than 95% of the proteins in cow’s milk comprise casein and whey, with casein proteins being the most abundant. The four main types of casein are alpha S1-, alpha S2-, beta-, and kappa-casein, and the A1 and A2 proteins are variants within beta-casein—so cow’s milk naturally contains both A1 and A2 proteins. The primary difference between the A1 and A2 variants is the amino acid sequence, which impacts how the proteins are broken down during digestion.

The theory behind the A1 variant and inflammation is that when digested, the A1 releases a greater amount (approximately three times more than A2) of beta-casomorphin 7 (BCM-7), a seven–amino acid peptide that has been speculated to induce inflammation and be linked to type 1 diabetes, heart disease, infant death, autism, and digestive problems.1-3 This article focuses on the research surrounding A1’s relationship with inflammation, cancer, and GI distress.

The Research
Dennis Savaiano, PhD, Virginia Claypool Meredith Professor of Nutrition Science at Purdue University in West Lafayette, Indiana, who’s also associate director of community health partnerships and director of the Clinical Research Center at Purdue’s Clinical and Translational Sciences Institute, says that A1’s potential association with inflammation “needs more work” and that he’s unaware of any correlation between A1 and cancer.

More research has been conducted on the relationship between A1 milk and GI health. A 2014 eight-week crossover study with 41 participants between the ages of 19 and 68 were randomized into two groups. One received cow’s milk containing A1 protein, while the other consumed milk containing only the A2 variant, for two weeks, with each trial separated by a two-week washout period where rice milk was substituted for cow’s milk.4

Participants were asked to record stool consistency using the Bristol stool scale (BSS) and symptoms of bloating, abdominal pain, and flatulence. The results found that A1 milk was associated with significantly higher stool consistency values (indicating softer stools) compared with A2 milk, and there was a significant positive association between abdominal pain and stool consistency in subjects consuming A1 milk but not A2 milk. Researchers posited that the significantly higher BSS values in subjects consuming the A1 variant compared with the A2 variant may be due to proinflammatory factors of A1.4

A 2016 study also looked at A2 milk compared with regular milk, which contains both A1 and A2 proteins, regarding symptoms of GI discomfort and found that participants consuming regular milk experienced significantly greater digestive discomfort symptoms post consumption.5

While these studies suggest a relationship between the A1 protein and inflammation and GI concerns, more research is needed to understand the health implications of A1.

Recommendations for Clients
While there may be an association between A1 and inflammation-related GI concerns, RDs can communicate that there’s no evidence A1 causes cancer. And while clients may connect GI distress they experience with milk containing A1 protein, these symptoms may have other causes, such as lactose intolerance. Practitioners may recommend clients track their symptoms after consuming cow’s milk to see whether it may be strongly associated with their GI distress, followed by a test to determine whether lactose intolerance is present. If lactose intolerance is present, RDs can counsel them based on lactose intolerance guidelines.6 Conversely, if the GI distress isn’t due to lactose intolerance, dietitians can recommend A2 milk or fortified soymilk. A2 milk is free of the A1 protein and has the same nutrient profile as conventional cow’s milk, and fortified soymilk is recommended by the 2020–2025 Dietary Guidelines for Americans as an appropriate dairy substitute, making both nutritious choices.

— Toby Amidor, MS, RD, CDN, FAND, is the founder of Toby Amidor Nutrition ( and a Wall Street Journal best-selling author. Her cookbooks include The Best 3-Ingredient Cookbook, The Best Rotisserie Chicken Cookbook, The Create-Your-Plate Diabetes Cookbook, Smart Meal Prep for Beginners, The Easy 5-Ingredient Healthy Cookbook, The Healthy Meal Prep Cookbook, and The Greek Yogurt Kitchen. She’s a nutrition expert for and a contributor to U.S. News Eat + Run and other national outlets.


1. Kaskous S. A1- and A2-milk and their effect on human health. J Food Eng Technol. 2020;9(1):15-21.

2. Sodhi M, Mukesh M, Kataria RS, Mishra BP, Joshii BK. Milk proteins and human health: A1/A2 milk hypothesis. Indian J Endocrinol Metab. 2012;16(5):856.

3. Asledottir T, Le TT, Petrat-Melin B, et al. Identification of bioactive peptides and quantification of β-casomorphin-7 from bovine β-casein A1, A2 and I after ex vivo gastrointestinal digestion. Int Dairy J. 2017;71:98-106.

4. Ho S, Woodford K, Kukuljan S, Pal S. Comparative effects of A1 versus A2 beta-casein on gastrointestinal measures: a blinded randomized cross-over pilot study. Eur J Clin Nutr. 2014;68(9):994-1000.

5. Jianqin S, Leiming X, Lu X, Gregory WY, Jiayi NI, Clarke AJ. 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 cow’s milk. Nutr J. 2016;15:35.

6. Brown-Riggs C. Digestive wellness: lactose intolerance and nutrient disparities. Todays Dietitian. 2015;17(12):16.