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What's the Evidence?
Why Should Milk Increase Prostate Cancer Risk?
Don't Throw Out Your Milk Just Yet
The Bottom Line

Milk is good for you, right? In general, it is. Sure, you can overdo it: 6 servings a day of whole milk has 900 calories, 48 grams of fat and 1,800 milligrams of calcium. Depending on the rest of your diet, that might be too much of a good thing.

But, in general, moderate intake of low-fat or skim milk is an excellent drink choice, especially if it is replacing highly sweetened drinks, like soda, that have little nutritional value.

There may be other "risks" associated with milk intake. There is a possible increased risk of prostate cancer among milk drinkers.

What's the Evidence?

The notion that milk intake is linked to prostate cancer comes from studies that look at the diets of men with prostate cancer and compare them to the diets of similar men who don't have prostate cancer. Some of these studies have concluded that the risk of prostate cancer rises with increasing milk intake:

A study of 43,000 Japanese men published in 2008 found that increasing intake of dairy products, including milk, was associated with an increased risk of prostate cancer.
A study among people in western Australia found that a "western diet" (including high fat milk, red meat and fried fish) was associated with an increased risk of prostate cancer when compared with other healthier diets.

One review published in July 2009 by researchers in Brazil did not find a convincing link between prostate cancer risk and milk intake. But the researchers suggested that the issue deserved more study. They also found that milk intake might reduce the risk of type 2 diabetes and obesity.

Why Should Milk Increase Prostate Cancer Risk?

One theory is that hormones in milk (including estrogen and "insulin like growth factor") might trigger prostate cancer or stimulate cancer cells to grow.

Another theory links cancer risk to milk's effects on vitamin D regulation. Milk is high in calcium. This reduces conversion of vitamin D into its most potent form (1,25 dihydroxyvitamin D). This form of vitamin D may have a protective effect against cancer. With less of this form of the vitamin, cancer risk may increase. However, this remains an unproven theory. And, a study published in July 2009 found that calcium intake did not lower 1,25 dihydroxyvitamin D levels.

Don't Throw Out Your Milk Just Yet

Given some evidence linking milk to prostate cancer, why do nutritionists consider milk and other dairy products part of a healthy diet?

There are several reasons:

While some studies suggest a link, others do not. For example, a study published in 2008 that combined data from 45 previously published studies found no association between milk intake and the risk of prostate cancer.

Studies that link a particular food or drink to a specific disease are notoriously prone to error. A prominent example of this was the idea that hormone replacement therapy (such as estrogen) reduced heart disease risk. The conclusion was based largely on studies in which researchers look back in time and compare features of people who already have a disease with other people who do not. We now know from better studies that hormone replacement treatment actually increases the risk of heart disease.

Even if milk did increase the risk for prostate cancer, the increase might be small and be far outweighed by the health benefits associated with drinking milk. For example, milk is an excellent source of protein and calcium. These are vital nutrients that the average western diet is short on.

In fact, most nutritionists do not warn of the dangers of milk because they don't believe the concern is needed.

The Bottom Line

If you are a man who likes to drink milk, I find no compelling scientific evidence to discourage you from doing so.

Then again, it's a good idea to keep an open mind. Future research could shed new light on the prostate cancer and milk connection. Whether we’re talking about milk or another dietary choice, we need studies to accurately estimate the risks of increasing intake as well as the risks of reducing it.

So, for now, consider milk a healthy food choice, as long as you stick with low-fat or nonfat versions and don't overdo it.

Robert H. Shmerling, M.D. is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.

Does Milk Cause Prostate Cancer?