There are a few things we can say for sure about milk: It is a concentrated source of calcium. It also contains protein and other nutrients. And it elicits strong opinions, even among scientists who study it, about how much we need. With recent studies suggesting that milk may be less important for our health than it has long been thought to be, many consumers are experiencing new bouts of indecision in the dairy aisle. Three glasses a day or none?
Here's a rundown of some of the latest research, pro and con, to help you decide.
Pro: Calcium loader
Calcium is an essential nutrient for all of our cells, and if we don't consume enough, our bones release some of their stored cache to maintain precise levels in the blood. That means that bones are the first to suffer when we get too little of the mineral. "Calcium and bone health is one of the few things that would be evaluated as grade-A level evidence in nutrition, which means that there are multiple meta-analyses showing benefits to bone from calcium," says nutrition scientist Connie Weaver, director of the Women's Global Health Institute at Purdue University in Indiana. But calcium is one of several nutrients that many Americans don't get enough of, according to national survey data. And even though lots of foods contain calcium, milk is a particularly efficient way to get it. To get the same 300 milligrams of calcium that comes in a glass of milk, according to the U.S. Department of Agriculture, you'd need to eat 10 cups of spinach or 4.5 servings of broccoli. Milk's calcium also comes in a readily usable form. Some plant sources, including spinach and beans, contain inhibitors that interfere with the body's ability to absorb the calcium they contain.
Pro: Nutrient mix
In addition to calcium, milk contains potassium, magnesium, phosphorous, protein and more. It's a similar mix of ingredients that makes up our bones, Weaver says, making dairy a useful way to support bone health. A 2011 study that modeled data from the self-reported diets of 16,000 Americans found that people who ate little dairy, even if they incorporated substitutes like calcium-fortified soy milk, took in lower levels of all sorts of nutrients, including vitamin A, riboflavin, vitamin B12, potassium and vitamin D. "Milk contains all the building blocks you need to make new tissue," says Dr. Richard Heaney, an endocrinologist who specializes in bone research and clinical nutrition at Creighton University in Omaha. "Dozens and dozens of studies have shown that adults who get less than two-thirds of the recommended intake of calcium are technically deficient in four to six other key nutrients."
Con: Calcium overload?
Studies have not been able to nail down how much calcium we need. "If you have low calcium intake, you will have weak bones and a higher risk of fractures," says Karl Michaëlsson, a medical epidemiologist at Uppsala University in Sweden. "But the tricky thing is, what level of intake is not good for you? We don't know that really."
Recommendations vary widely, from the World Health Organization's suggested minimum of 400 to 500 milligrams of calcium each day to 1,000 mg for healthy adults in the U.S. (more for older women and teenagers). And newer research is challenging the entrenched American belief that we need to load up on calcium to support strong bones. In a 2011 study that analyzed data on nearly 200,000 middle-aged and older men and women from six other studies, researchers found no relationship between calcium intake and risk of fracture, regardless of whether people drank 30 servings of a milk each week or just two. "Calcium is very important, and we absolutely do need some," says co-author Walter Willett, chair of the nutrition department at the Harvard School of Public Health. But our bodies adapt to what they get. "If we're low, we absorb a much higher percentage. If we're high, we greatly reduce the percentage we absorb."
Con: Death by milk?
Too much dairy may even cause harm, according to some research. In a study last fall, Michaëlsson and colleagues analyzed Swedish national health data on 60,000 women and found that women who drank at least three glasses of milk a day were twice as likely to die during the study period than were women who drank less than one glass a day, Michaëlsson says. The heavy milk-drinkers also had a 60% higher risk of hip fracture and about a 15% higher risk of any kind of fracture.
Because the study was based on observations, not trials, it is possible that people who started out with a higher risk of fracture or death were choosing to drink more milk, but the researchers looked for evidence of that and didn't find any. Instead, they found that a milk sugar called galactose can cause inflammation and stress on the cellular level, which might in turn lead to health woes. Cheese and yogurt have lower levels of galactose than milk does, and the study found that people who ate more fermented milk products had lower rates of fracture and death. "My recommendation is that people should not change their dietary habits because of this single study," Michaëlsson says. "Even if I say that, I have cut down on milk consumption myself. I have not eliminated it. But I eat more yogurt."
Pro: Milk for your heart?
When New Zealand researchers reported in 2010 that people who took calcium supplements had a 30% higher risk of myocardial infraction than people who didn't, the concerns that were raised extended to the calcium in milk. Since then, nobody has been able to replicate the link, says Weaver. In a not-yet-published study on pigs, her team also failed to find any mechanism to explain a connection between calcium and cardiovascular disease.
Some studies, in fact, show a slight benefit for the heart from consuming dairy products. Yogurt, in particular, has been linked to lower risks of weight gain, diabetes and cardiovascular disease, Willett says, though that may be because of its protective bacteria. Also still unclear is whether we should condemn or embrace milk fat. Some studies now suggest that people who eat full-fat dairy products stay leaner than those who stick with skim. "Fat is still a work in progress," Weaver says. "We used to discourage all saturated fats and animal fats in general. More recently, the research is saying that maybe milk fat doesn't belong lumped together with other animal fats."
Up for grabs: Cancer risks
Multiple studies have linked calcium, whether in milk or supplements, to lower risks of colorectal cancer. But dairy consumption has also been implicated in slightly elevated levels of IGF-1, a protein that can contribute to cancer growth. Last year, a meta-analysis of 32 studies found a relationship between a high consumption of dairy products and increased risk of prostate cancer. Studies on other kinds of cancer are ongoing but not yet conclusive. In the meantime, as with most nutrition debates, moderation might be the best bet.