Older Icelandic men who remember chugging a lot of milk in their teens are three times as likely to be diagnosed with advanced prostate cancer as more-moderate milk drinkers, researchers have found.
That makes them wonder whether the years around puberty, during which the prostate matures, could be a time of heightened vulnerability for the gland.
"We believe that our data are indeed solid and provide important evidence for the role of adolescence as a 'sensitive period' for prostate cancer development," Johanna Torfadottir, a nutrition scientist and a graduate student at the University of Iceland, told Reuters Health by email.
"However, we remain cautious in our interpretation," she added. "Causal inferences are not made on one study alone, thus more studies are needed to confirm our findings and also to explore possible mechanism behind this association."
So far, she added, the two studies on prostate cancer and milk intake in adolescents have come to mixed conclusions -- one found milk lovers seemed to be somewhat protected against the disease, while the other found no link at all.
But both studies were small and couldn't distinguish between advanced and early-stage tumors, Torfadottir said.
By contrast, Iceland offers the perfect "natural experiment," she and her colleagues write in the American Journal of Epidemiology.
The country had little infrastructure in the early part of the 20th century, so people in rural areas tended to live off the land. That included lots of milk from farm animals in central regions of the island, whereas the drink was scarce in seaside villages.
For their study, the researchers used data from more than 2,200 men born between 1907 and 1937. These men had been part of a medical study started in the 1960s and, in the early 2000s, had answered questions about their diet in early and mid-life as part of another study.
Among 463 men who recalled drinking milk less than once a day in their teens, one percent developed advanced prostate cancer or died of the disease over a quarter century of follow-up.
That figure was three percent among the more than 1,800 men who said they drank milk at least daily in adolescence.
The gap couldn't be explained by how often people had gone to the doctor for check-ups, their education or other foods they ate, such as fish or meat.
How much milk men drank had no connection to their risk of early-stage tumors, however. And intake in midlife -- the age group most other studies have focused on -- didn't seem to matter either.
Torfadottir said there are several physiological mechanisms that might, in principle, explain the link between she found. But at this point, all of them remain speculative.
"From these data alone we cannot recommend that teenage boys should chance their dietary habits," she said. "We are only looking at the risk of one disease, prostate cancer, and obviously risks of other conditions, e.g. bone health, need to be considered."
PREMATURE TO SAY 'CAUSES'
Dr. Matthew Cooperberg, a urologist at the University of California, San Francisco, agreed.
"It would be premature to say that drinking milk causes prostate cancer," he told Reuters Health. "You can talk about association, but it is hard to prove causality."
He added that people shouldn't be wary of drinking milk.
"There are plenty of health benefits from drinking milk in adolescence," Cooperberg said.
Torfadottir also ventured a bit of nutritional advice, noting that it's "important to have a balanced diet and moderate milk consumption is a part of that."