If dairy products make you feel icky, you could be tempted to write off milk—or go for lactose-free products—forever. But lactose might not be behind your problems.
Some people have trouble digesting lactose, a sugar in milk. When the body can’t break lactose down, people with lactose intolerance experience bloating, gas, diarrhea, and more gastrointestinal issues. But research suggests that some people’s symptoms might come from a protein in milk, not the sugar.
About a third of the protein in milk comes from beta-casein, which has two main forms: A1 and A2. Most cows’ milk contains both, but some has just A2. Genetic tests can figure out whether a cow will naturally produce the A1 protein. (Check out our guide to plant-based milks.)
The A1 and A2 proteins are identical in all but one of their 209 amino acids, says registered dietitian Bonnie Johnson, vice president of scientific affairs for The a2 Milk Company, which sells milk from cows that only produce the A2 protein. “In A1, the amino acid that’s different is easier for the body to digest,” she says. Your body can break down A1—but not A2—into a protein fragment called beta-casomorphin-7 (BCM-7), which could cause inflammation. The GI symptoms would look similar to lactose intolerance, and might cause a skin rash.
People who drank milk with both the A1 and A2 proteins for two weeks had more digestive discomfort and more inflammation than those who drank milk with only A2, found a small study in Nutrition Journal that was funded by The a2 Milk Company. Both types of milk had the same amount of lactose, but the results held true for participants with lactose intolerance.
The A1 protein might make it harder for lactose intolerant people to digest the milk sugar, says Johnson. The inflammation from BCM-7 starts high up in the gut, making digestion even tougher as your body tries to break down the sugar. “If you can stop inflammation up top, you can make the environment more hospitable for breaking down lactose later on,” says Johnson.
Don’t assume avoiding A1 will solve all your digestive woes though, says registered dietitian nutritionist Angela Lemond, spokesperson for the Academy of Nutrition and Dietetics. Studies have already disputed claims that the A1 protein can raise risk of heart disease, Type 1 diabetes, and autism. The only human studies linking A1 to digestive problems have been very small, and the results aren’t conclusive, she says.
Still, if dairy gives you tummy issues, consider trying an A2-only milk or goat milk, which never has A1. “If they continue to have symptoms, it would likely be from the lactose because goat milk has just as much lactose as cow’s milk,” says Johnson.
Keep in mind that lactose intolerance doesn’t mean you have to give up run-of-the-mill cow’s milk either, says Lemond. “That is a common misconception,” she says. “Most people can digest a small amount, and you can actually slowly work up to more if you start slow.” Next, discover which foods have more protein than an egg.