Milk does a body good, right? Right?

While that’s a catchy ad slogan, some recent research calls into question the conventional “milk builds strong bones” wisdom.

For one thing, if milk really is important for bone health, you would think that people with a lactose intolerance would suffer from higher rates of osteoporosis—or brittle, weak bones. But the evidence linking lactose intolerance to bone weakness is mixed.

One study, appearing in a 2014 issue of BMJ, found heavy milk consumption (three or more glasses a day) was associated with a greater risk for bone fractures in women, but not in men. The BMJ study also found a correlation between milk consumption and markers of oxidative stress and inflammation.

All this research dovetails with some online claims—mostly on health blogs—that milk is an acid-producing food, and so could promote inflammation and bone breakdown. 

Don’t toss out your dairy just yet...

For one thing, those “milk produces acid” claims are bogus.

Milk actually decreases urinary acid after digestion, according to a study. So the idea that milk ups your body’s acid, which in turn harms your bones, doesn’t hold up.

Some experts also question the findings of that BMJ study. To begin with, it suggests only an association between heavy milk consumption and some bone issues. It doesn't prove that drinking milk is the cause of those issues. 

“If you look at the bulk of the literature, most have shown daily dairy intake both improves bone mineral density and prevents loss of bone density over time,” says Dr Shivani Sahni.

Sahni has overseen several studies on dairy consumption and bone health. Along with improving bone mineral density, “dairy consumption also prevents osteoporotic fractures—particularly hip fractures,” she says.

Milk may be especially good for the bones of older women, according to a study published this year in the American Journal of Clinical Nutrition. That study found post-menopausal women who consumed dairy every day—the equivalent of 600mL of milk, or 300mL of yogurt—had significantly stronger bones than women who consumed less dairy.

Strong bones—as well as healthy muscles—require protein and calcium, says that study's author, Dr René Rizzoli. While other foods contain protein and calcium, few can match dairy when it comes to packing these nutrients into an affordable package, he says.

Dairy offers more than just protein and calcium.

While she agrees protein and calcium are important for strong bones—and that dairy is an excellent source of both—Sahni says dairy also contains potassium and magnesium, both of which may also help protect against bone weakness and fractures. 

She also mentions the probiotic component of fermented dairy foods like yogurt. “These might affect the [gut] microbiome, and how calcium is absorbed,” she explains. Again, more research is needed. But it’s possible that dairy yogurt may be especially good for your bones.

So how much dairy should you be eating? Assuming you’re not lactose intolerant, Sahni says she recommends three servings—roughly three cups—of dairy per day for adults.

“There is some controversy—I won’t deny that,” Sahni says. “But looking at all the existing research, I think dairy intake is protective against fractures and bone weakness.”