“I've looked at fractures during the last 25 years. I've been puzzled by the question because there has again and again been a tendency of a higher risk of fracture with a higher intake of milk,” said the study's lead author Karl Michaelsson, a professor at Uppsala University.
The study, published in the British Medical Journal, utilized data from two large, long-term Swedish studies of adult men and women, which asked about their dietary habits -- how much and what types of milk and dairy products they consumed.
Women who consumed three or more glasses of milk a day had a higher risk of fracture and a higher risk of death. Men who drank three or more glasses of milk a day had a slightly higher risk of death -- mostly associated with cardiovascular death -- compared to those who drank less than one glass a day. And there was no reduced risk of fracture as milk consumption increased.
Michaelsson's team also analyzed data from the two studies about the levels of a biological marker of stress in some of the participants.
In both men and women, the amount of milk they consumed was also associated with higher levels of a biological stress marker -- oxidative stress -- which has been associated with aging, cancer and cardiovascular disease.
Other studies have observed a similar -- and seemingly paradoxical -- relationship between the consumption of calcium-rich milk and bone fractures.
But why milk consumption might be associated with mortality is a much more difficult question to answer.
Notably, the association between fractures or mortality and dairy consumption was not seen with derivatives of milk, such as cheese, yogurt, sour milk, and other fermented products. In fact, in Michaelsson's analysis, each serving of cheese or fermented milk products reduced rates of mortality and hip fractures by 10-15 percent.
According to Michaelsson, the results suggest that more research is needed to understand whether a specific component of milk -- a sugar called D-galactose-- might play a role in his findings.
D-galactose is known to induce aging in animals, and is linked to increased oxidative stress and inflammation.
“With fermented cheese the level of galactose is very low. It’s not as high as it is in milk,” Michaelsson said. “Yogurt also has the probiotic effect of bacteria.”
The study is just the latest in a long string of evidence calling into question the health benefits of one of the most popular beverages nature has to offer.
In addition to questions about the benefits of milk consumption for preventing hip fractures and preserving bone density, recent research has linked dairy to ailments, such as acne. Not to mention that about 65 percent of the world can’t digest the lactose found in generous quantities in milk -- and to a lesser extent in certain cheeses and yogurts -- after infancy.
Recently, Americans -- and indeed much of the world -- have been coming down from their milk high for some time.
Since the 1970s, milk consumption in the United States has dropped from about 1.5 cups a day to about 0.8 cups a day today.
With evidence of health benefits of milk eroding with each passing year, some physicians have called into question government recommendations that most adults and adolescents consume up to three cups of milk a day.
In children, encouraging milk consumption through the National School Lunch Program often takes the form of sugar-sweetened chocolate milk, which has sugar content similar to soda, points out David Ludwig, a Harvard professor of nutrition.
In a 2013 paper Ludwig co-wrote, he suggested that there is not enough scientific evidence to support federal milk consumption recommendations.
And in fact, he added, there is more evidence that humans -- who only recently began consuming milk with the domestication of large animals -- don’t need it at all.
“Until very very recently, from an evolutionary perspective, humans would have consumed no milk products at all and would have consumed calcium from other sources,” Ludwig said. “Populations that drink no milk at all have perfectly fine bones.
“Which isn’t to say that milk is necessarily unhelpful, we just lack a good evidence base for the recommendation that we consume such high levels of it.”
Milk, by design, encourages rapid growth in animals through the naturally occurring presence of hormones, which Ludwig suggests could be deleterious to humans over the course of their life span.
“In some situations growth is a good thing, but in others it's not,” Ludwig explained. “To be experiencing life-long over-stimulation of growth pathways could in theory increase risk for cancer.”
Despite these concerns, there is still insufficient, definitive clinical evidence to make recommendations about whether people should continue to consume milk, Michaelsson noted. Still, he's aware that his research casts doubt on the benefits of long-held beliefs about milk.
“This is how we expand our knowledge,” Michaelsson said. “But I'm fully aware that many aren't keen to hear these results.”