Out with the old … In with the new …

According to a Reuters report on December 30, “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.” This report is based on an epidemiological study by Torfadottir et al. which looked at diet and risk for prostate cancer among Icelandic men born between  1907 and 1935.

The authors are very clear, however, that it is too early to draw specific conclusions from this large study, and this opinion is confirmed by other commentators.

So what did Torfadottir and her colleagues actually show?

They looked at data from 8,894 male participants in a larger study, of whom 2,268 had reported milk consumption levels in their early, mid-, and current lifetimes. These men were all followed for prostate cancer diagnosis and mortality from study entry (between 1967 and 1987) through 2009. Their findings can be summarized as follows:

  • The average (mean) follow-up from study entry was 24.3 years.
  • 1,123 men were diagnosed with prostate cancer.
    • 371/1,123 were diagnosed with advanced disease (stage III or higher or prostate cancer-specific death).
  • Compared with residency in the urban area of Reykjavik,
    • Rural residency in the first 20 years of life was marginally associated with increased risk of advanced prostate cancer (hazard ratio [HR] = 1.29)
    • Rural residency among men born before 1920 showed a higher risk for advanced prostate cancer (HR = 1.64)
  • Among 463 men who recalled drinking milk less than once a day in their teens, only 1 percent developed advanced prostate cancer or died of the disease.
  • Among >1,800 men who said they drank milk at least daily in adolescence, 3 percent developed advanced prostate cancer.
  • Daily (as compared to less than daily) milk consumption in adolescence, but not in mid-life or currently, was associated with a 3.2-fold increase in risk of advanced prostate cancer.

The authors conclude that their data “suggest that frequent milk intake in adolescence increases risk of advanced prostate cancer.”

Now there have long been suspicions that diet and other environmental factors in childhood and early adulthood may have implications for risk of later prostate cancer. This study certainly appears to confirm such a possibility. However, whether milk is the critical factor here or whether other aspects of rural life in Iceland in the early 20th Century may be relevant is hard to know.

The research team suggests that frequency of medical check-ups, education, and other dietary factors (such as fish or meat consumption) had little impact on prostate cancer risk. However, rural life in Iceland in the early 20th Century was still isolated: ” .. 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 [milk] was scarce in seaside villages.”

It is clear that in this study there was an association between milk consumption and risk for advanced prostate cancer. Whether there is a cause and an effect is much harder to determine, however. Torfadottir herself is quoted by Reuters are stating that it is “important to have a balanced diet and moderate milk consumption is a part of that.”

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