Relax Everyone, Drinking Milk Isn’t Going to Kill You

Do some science that has a counterintuitive result, and the media will milk it for all it's worth. In this case, researchers in Sweden surveyed more than 45,000 men and 60,000 women about how much milk they drink and observed their health over the next 10 and 20 years respectively. The researchers concluded that people who drank more milk were more likely to break bones and were more likely to die earlier.

If that last bit has your bullshit detector pinging loudly, you're not wrong. But unfortunately, much of the popular media has been deaf to this signal. Since the study's publication last week in the British Medical Journal, the internet has worked itself into a tizzy about milk's marketing lies.

"The idea that milk prevents broken bones is an udder sham," Vox declared. "Is Milk Actually Bad for You?" Cosmo inquired. While the study itself has some merit, its conclusions are not nearly as cut-and-dry as some have made them out to be. And they're not inspiring any revisions to dietary guidelines anytime soon.

The study has a few things working in its favor. Its sample size is very large and, because it's in Sweden, is ethnically fairly homogeneous, which helps eliminate any ethnic differences to milk that could muddy results, such as lactose intolerance. (This means that the results are more likely to be accurate if you're Swedish, at the very least.) The authors monitored patients' health over a long period, and they were diligent about surveying study participants about the types of food they ate, distinguishing between milk and other dairy products.

But, for this study, that's where the good parts end. Large cohort studies like these are great at finding strong correlations between two factors, like milk consumption and the frequency of broken bones. But they're not good at establishing what exactly causes what.

For example, there are a lot of factors that can affect bone density and corollary factors like the frequency and severity of broken bones, such as how much people exercise (a previous study by one of the same authors correlated a sedentary lifestyle with more hip fractures), genetic predisposition, whether they take vitamins, and adolescent diseases that may have slowed the initial buildup of bone mass.

Milk consumption is just one of these, and without keeping track of every single one (which is impossible), the study authors cannot conclusively say that milk was the force behind more broken bones.

There are some deeper issues. The study authors never clearly set out what nutrient in milk they were looking at in particular—was it calcium, which is supposed to strengthen bones, or D-galactose, a sugar that reportedly weakens them? How about vitamin D, phosphorous, and magnesium, all of which are important in bone health and found in milk?

Calcium can be found in lots of other food sources, like vegetables and legumes, and study participants who consume the same amount of milk but different amounts of these other foods might have changed the study's outcome. The study authors just try to track all of the nutritional components, which impossible to do with any accuracy for thousands of people over the course of decades, especially when the tracking is done by self-reported surveys.

The idea of correlating milk consumption and mortality isn't as crazy as it may seem. Dairy products have a lot of saturated fats, which has been implicated as a factor for cardiovascular disease (although that finding has also been questioned lately). And about a quarter of all deaths in the US each year are due to cardiovascular disease, so there's reason to be concerned.

While the study authors surveyed subjects for their body mass indexes or and whether or not they smoke, they didn't ask about past cardiovascular events, if they have diabetes, or their cholesterol levels, which could most certainly impact their mortality. If people have these other conditions, they may be more or less likely to drink a lot of milk. The presence of cardiovascular disease, as commenter Chris Labos notes, stand a pretty good chance of confounding the authors' outcomes.

In the end, a study like this isn't enough to change dietary recommendations or clinical practices. It's one of severa lstudies that have come out in the past few years questioning milk's relationship to bone health, but other new studies maintain that milk is good for bones. The last literature review, published in 2000, found that 53 percent percent of studies showed no significant effect, 42 percent showed a favorable effect (milk is good for bones), and 5 percent showed that milk detracted from bone health. These numbers may not be the same if the review were conducted today, but it provides a baseline.

Until the evidence is more conclusive, there's nothing wrong with drinking a glass or two of milk per day. The study authors themselves agree. "The results should, however, be interpreted cautiously given the observational design of our study," they write. "The findings merit independent replication before they can be used for dietary recommendations." At least we can take their word on that.