Independent Researchers Issue New Assessment of Possible Milk-Prostate Cancer Link

American Institute for Cancer Research
Tuesday, 26 February 2002

Conflicting Evidence Raises Legitimate Concerns, But Still No "Smoking Gun," says Cancer Institute

WASHINGTON, DC - The American Institute for Cancer Research (AICR) today issued a comprehensive analysis of recent scientific literature that has investigated possible associations between milk consumption and prostate cancer. The AICR review concludes that, to date, the evidence for a causal link between milk consumption and prostate cancer remains conflicted and unconvincing.

But the report acknowledges that one recent theory - which proposes a particular biochemical pathway that may factor in prostate cancer - is gaining support and demands serious scientific attention.

"There are huge gaps in our understanding," said Helen A. Norman, Ph.D., co-author of the AICR review. "The data from human studies simply do not agree, and the theoretical connections that have been suggested in some recent laboratory work are intriguing, but remain resolutely theoretical."

AICR issued the review in an attempt to introduce an independent, science-based perspective into an issue that has become muddied by an ongoing, vocal debate between dairy industry lobbyists who dismiss a milk-cancer connection and vegetarian advocacy groups who treat it as a scientific fact.

The AICR review, which examined data from over 100 previously published scientific papers, makes it clear that it is still far too early for either side to claim victory.

"From a purely scientific standpoint, it would be irresponsible to assert that drinking milk increases a person's risk for prostate cancer in particular," said Dr. Norman. "But it would be equally irresponsible to insist that the existence of such a link was impossible."

Diets High in Animal Foods Have Long Been Known to Carry Cancer Risks

Research has been showing for years that diets that are generally high in animal products tend to increase risk for many cancers, including prostate cancer. In fact, the 1997 AICR report called Food, Nutrition and the Prevention of Cancer: A Global Perspective, reviewed over 4,500 studies on diet and cancer and concluded that cancer risk decreases when diets downplay meat and dairy and instead center around vegetables, fruits, whole grains and beans.

But questions about the cancer risks associated with the consumption of dairy products in particular have attracted a great deal of attention in the years since publication of the 1997 AICR report.

Some of the studies that have investigated this question have compared the diets and rates of prostate cancer in different countries and regions. Others have tracked large groups of individuals over several years and compared the diets of subjects who eventually developed prostate cancer to those who did not. Still other human studies asked prostate cancer patients to recall the diets they consumed in the years leading up to diagnosis and compared these findings to a statistically matched group of individuals without prostate cancer.

Several of these studies found that consumption of dairy foods seems to coincide with increased risk for prostate cancer. For example, one recent well-publicized study published in the American Journal of Clinical Nutrition found consumption of dairy foods, and skim milk in particular, to be associated with increased risk.

However, in several other studies of similar size - some of which were carried out by the same authors as the aforementioned AJCN study - no association of any kind was found between dairy intake and prostate cancer risk.

Scientists Look to the Laboratory for "Smoking Gun"

The new AICR review cites the inconsistency of human data as one reason it is still too early to make a responsible judgment on the milk-prostate cancer question. Another, equally important reason is the lack of a "smoking gun" - a clear biochemical pathway that could explain how consumption of milk might actually influence the growth of prostate cancer cells. Without being able to demonstrate that such a mechanism exists, no amount of human studies could ever prove that milk consumption is linked to prostate cancer.

AICR's Norman stressed that human studies are only a blunt tool to investigate the enormous complexity of the diet. This is particularly true when studying how diet influences disease risk. Even when scientists attempt to account for the influences of all other possible dietary factors besides milk consumption, important limitations remain.

It's been shown, for example, that diets that are high in milk tend to be high in other animal products as well. They also tend to be higher in fat, higher in total calories, and lower in vegetables and fruits. All of these variables are believed to factor in cancer risk.

"That's why, when a study suggests that milk consumption coincides with a higher risk for prostate cancer, we cannot and should not immediately conclude that milk is a culprit," said Norman. "Human studies don't work that way - they aren't designed to show that A causes B. They can only show associations - that A and B tend to happen together. The fact that they happen together is an important finding, and it points us toward further research, but it's not the whole picture."

Scientists must turn to laboratory studies to demonstrate a causal mechanism, because such studies allow for a more focused investigation of diet and cancer risk in cells, tissues and in animals. In fact, scientists have so far proposed several theories to explain how the consumption of milk might plausibly affect cancer cells, and are now putting these hypotheses to the test in laboratories around the world.

"Laboratory work has suggested several possible reasons that milk and dairy products may factor in prostate cancer, but figuring out which of these reasons, if any, apply to humans is the work that lies ahead of us," said Dr. Norman.

New Theory Gaining Scientific Support

One theory is that the fat content in milk influences the body's hormones, which in turn speed up cell proliferation. Any time cellular reproduction occurs at a faster rate than normal, the chances increase that something could go wrong, such as the kind of random mutation that could lead to cancer. Some in vitro ("test tube") studies have found higher prostate cancer risk to be associated with high blood levels of certain hormones and hormone-like "growth factors," but these findings have been controversial.

Another theory has been attracting considerable attention, particularly in the years following the 1997 publication of the AICR report. It involves emerging knowledge about how two components of milk - vitamin D and calcium - seem to interact.

This particular theory did not start in the laboratory, but grew out of research comparing prostate cancer mortality rates in different regions of the US. Scientists found that mortality rates from prostate cancer were lower in regions exposed to more sunlight (the Southwest) and higher in areas with less sunlight (the Northeast). They proposed that the active form of vitamin D, which comes from the vitamin D synthesized in skin exposed to sunlight, may take on some kind of protective role against prostate cancer in the human body.

In subsequent laboratory studies (both in vitro and in animal models), this active form of vitamin D was shown to inhibit the growth of prostate cancer cells and to encourage normal (i.e., non-cancerous) cellular differentiation.

Sunlight is the chief source of this possibly protective form of vitamin D. The only dietary sources that contain significant amounts of vitamin D are fatty fish and fortified dairy products.

But scientists have singled out dairy products for further study because they contain another component - calcium - that has been shown in some recent studies to lower the level of cancer-protective vitamin D in the blood.

Evidence Mounts, but Relevance to Humans Remains in Question

According to the AICR review, that laboratory finding has been supported by some recent human studies, which have found that countries whose diets are high in calcium tend to have higher rates of prostate cancer mortality. Once again, however, other studies of similar design and scope have found no such association.

Scientists are now investigating whether vitamin D may play a role in regulating how well the body absorbs calcium. And indeed, there is growing scientific support for such a mechanism. Some laboratory studies have shown that when calcium levels are very low, the body speeds up production of vitamin D in an attempt to compensate.

Studies have also found that when calcium intake is very high, the body seems to suppress production of vitamin D in an attempt to keep from absorbing too much calcium. One recent study investigating the vitamin D-calcium hypothesis concluded that calcium only interferes with vitamin D production when subjects are getting more than 2,000 milligrams of calcium per day - more than twice the amount recommended by the USDA.

More importantly, no study conducted to date has determined whether reducing the amount of dietary calcium might affect blood levels of active vitamin D. This is the all-important question, according to Dr. Norman.

Scientific Jury Still Out, But Small Amounts of Dairy Seem OK

"Future studies need to examine whether this proposed mechanism factors in the real world. Before we can categorically state that vitamin D and calcium influence human prostate cancer risk, we have to be able to accurately distinguish the role of dietary vitamin D from the vitamin D our bodies make when exposed to sunlight."

The new AICR review concludes that more research needs to be done before any changes are made to dietary recommendations for dairy products. More human and laboratory studies need to define the role of other dietary factors that may influence prostate cancer risk, such as fat and total calories. Further support for the vitamin D-calcium hypothesis can be secured with studies that determine how changing the amount of dairy sources in the diet influence blood concentrations of these nutrients.

AICR's Dr. Norman added that milk and other dairy products provide many important nutrients that are required by the body. She believes that making healthy forms of dairy like nonfat or lowfat milk and yogurt a small part of a plant-based diet seems warranted by current research. But she adds that the public should be aware that researchers are watching this question very closely.

"The science that has been conducted so far can offer no assurances either way," she says. "The only thing we can say for certain right now is that nothing is certain."

Does Milk Cause Prostate Cancer?

Evidence for a possible milk-prostate cancer link has inspired a hot debate between dairy industry lobbyists and vegetarian advocacy groups.

To provide an objective, science-based perspective, AICR has issued an independent review of scientific literature that has dealt with the possible connection between milk and prostate cancer.

The AICR review concludes:

There is currently insufficient evidence to either prove or dismiss a possible connection between milk consumption and prostate cancer.

There is growing, if inconsistent, laboratory evidence for a possible biochemical connection that demands further scientific attention.

Because dairy foods provide important nutrients, making healthy options such as lowfat or nonfat milk and yogurt A SMALL PART of a diet centered around vegetables, fruits, whole grains and beans seems warranted by current research.

The AICR review, Is Milk a Risk Factor for Prostate Cancer? An Assessment of the Literature, was authored by AICR researchers Helen A. Norman, Ph.D., and Ritva Butrum, Ph.D.

For more information, or to contact American Institute for Cancer Research, see their website at: www.aicr.org