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Several large studies have explored the connection between dairy and calcium and prostate cancer and have turned up variable results. The data, so far, has been considered limited but suggestive.Sean Kilpatrick/The Canadian Press

Leslie Beck, a Toronto-based private practice dietitian, is director of food and nutrition at Medcan. Follow her on Twitter @LeslieBeckRD

Rates of prostate cancer vary considerably worldwide, suggesting that environmental factors, such as diet, influence the risk of this common cancer. Among dietary factors, dairy and calcium have been considered possible risk factors.

Several large studies have explored the connection between dairy and calcium and prostate cancer and have turned up variable results. The data, so far, has been considered limited but suggestive.

Now new findings, published this month in the American Journal of Clinical Nutrition, add weight to the association between dairy – but not calcium – and increased risk of prostate cancer.

The latest research

For the study, researchers from Loma Linda University in California asked 28,737 men about their diets and then followed them to see who developed prostate cancer. All were initially free of cancer.

Participants lived in the United States and Canada and adhered to various types of diets, including vegans who avoid dairy, lacto-ovo vegetarians who consume about half the typical North American dairy intake and non-vegetarians who consume dairy at typical levels (e.g., about one cup of milk a day).

By the end of a follow-up period of nearly eight years, 1,254 prostate cancer cases were reported.

Men who consumed the most total dairy (one and three-quarter cups of milk a day) versus the least (one half-cup of milk a week) had a 27 per cent higher risk of total prostate cancer (advanced and non-advanced).

Compared with the group of men who never consumed dairy, high dairy consumers were 60 per cent more likely to develop prostate cancer.

To arrive at these results, other potential risk factors including family history of prostate cancer, body weight, calorie intake, alcohol intake, smoking status and physical activity level were accounted for.

The increase in prostate cancer risk was driven by both low-fat and full-fat milk. The researchers didn’t find strong evidence that yogurt or cheese influenced prostate cancer risk.

The findings also didn’t show a connection between calcium intake, whether from diet or supplements, and prostate cancer risk.

What “risk” really means

These observed increases in risk do not mean that a man’s personal, or absolute, risk is 27 or 60 per cent higher. Rather these risks indicate the likelihood of prostate cancer occurring in one group of men compared with another group.

In North America men have, on average, a 12.5 per cent risk of developing prostate cancer at some point in their lives. A 27 or 60 per cent increased risk, then, raises a man’s absolute risk to 15.8 or 20 per cent, respectively.

Strengths, caveats

The study can be credited for its large sample of participants diverse in age, race, geographical location and socioeconomic status.

As well, participants’ wide range of dairy and calcium intakes allowed researchers to examine associations with prostate cancer at low intakes and also at more typical intakes.

There were limitations. Diet information was collected only once at the start of the study so any changes over time weren’t captured. It’s possible that participants changed their dairy intake over the course of the study.

As well, the study was observational and, as such, it uncovered associations; it did not prove a cause-and-effect relationship.

How milk might harm

There are plausible ways in which dairy might increase prostate cancer risk.

Previous research has found that higher dairy intakes raise levels of insulin-like growth factor I, a hormone thought to promote prostate cancer.

Dairy milk, which comes from lactating pregnant cows, also contains other hormones which may influence the development of prostate cancer.

Should men give up cow’s milk?

This new study did not conclude that drinking milk directly causes prostate cancer.

It’s also possible that some other unknown factor, one that is closely tied to dairy, is associated with the observed increased cancer risk. This needs to be sorted out.

Even so, according to Dr. Gary Fraser, the study’s principal investigator and professor at Loma Linda University, “I do think it would be prudent for men with a family history of prostate cancer or other risk factors to turn to plant milks”.

Don’t lose sight, though, of other lifestyle choices that can help lower the risk of prostate cancer. Obesity and a lack exercise, for example, have been shown to increase the risk of aggressive prostate cancer.

Focus on maintaining a healthy weight, eating healthy diet plentiful in colourful vegetables and fruits and getting regular exercise.

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