By Neal D. Barnard, M.D. -- PCRM's "Good Medicine" -- Winter, 1997

When researchers examine the differences in breast cancer rates in various countries, a surprising factor lurks in the background. In addition to the factors already under suspicion -- dietary fat, alcohol, hormone treatments, and chemical exposures -- several studies have implicated milk and other dairy products as possible contributors to breast cancer risk. It is just not the grease dripping out of a cheese pizza that is under scrutiny. Even skim milk is implicated.

Jessica Outwater of Princeton University looked into why milk might cause cancer. In her research at PCRM, she found that cow's milk is veritable cocktail of cancer-causing chemicals. Her report, published in Medical Hypotheses in December, explains these surprising, potentially lifesaving findings.

The First Clues

Milk is designed by nature to help infants grow. Human milk brings an infant to the stage where he or she can eat solid food. Cow's milk nurtures a baby calf until he or she is big enough to graze. Just as an old-fashioned choke adjusts the gasoline mixture to help an automobile get started, mother's milk helps a tiny body to grow rapidly. And just as a car's choke is harmful at highway speeds, it may be that the growth factors in milk can be risky for adults, perhaps even encouraging the growth of cancer cells.

Many human population studies have shown that dairy product use correlates with breast cancer rates. One interesting example comes from Seventh-day Adventists. Nearly all Adventists avoid tobacco, alcohol, and caffeine, and are generally health conscious. But about half are vegetarians and half are not. As you would expect, the vegetarians have much lower rates of many diseases, including some forms of cancer. But breast cancer rates are about the same for the both Adventist group -- vegetarians and non-vegetarians.

These vegetarians, however, are not generally vegans. While they avoid the animal fat in burgers and fried chicken, they will get much of it back in a cheese casserole. When breast cancer rates among Adventists are compared to dairy product consumption, a pattern emerges: the more dairy a woman consumes, the higher her risk.

Most other population studies show the same pattern. The higher the dairy product consumption, the higher the breast cancer risk. In some of these studies, the higher risk remains even when the effect of fat is removed, suggesting that the animal fat in milk may not be the only problem. Rather estrogens, chemical contaminants, and a growth-promoting peptide called IGF-I are the prime suspects in breast cancer investigations.

Estrogens in Milk

Farmers impregnate dairy cattle every year because a pregnant cow produces more milk. (When the calves are born, needless to say, the females join the dairy herd; males end up on the veal counter.) A pregnant cow produces extra estrogens that end up in her milk. Farmers actually look for these estrogens in milk as a sign that the impregnation was successful.

Excess estrogen is well-known for making breast cancer cells multiply, which is why doctors avoid prescribing estrogen supplements to cancer patients. Drugs that counter estrogen's actions, such as tamoxifen, are important in breast cancer treatment.

A liter of milk contains 4 to 14 nanograms of 17-b-estradiol. Whether these hormone traces have biological effects remain unclear. In addition, the fat in milk -- like fat in any food -- rapidly causes excess estrogen to be produced in a woman's body. The effect is rapid. Within a few weeks of increasing or decreasing the fat content of the diet, the estrogen level in the blood stream is readjusted higher or lower. Milk also has no fiber at all, and fiber is part of nature's way of eliminating excess estrogens.

IGF-I

Of even greater concern is a compound called insulin-like growth factor, IGF-I. As its name indicates, IGF-I stimulates growth in a child's body. The amount of IGF-I declines as years go by.

Unfortunately, IGF-I not only encourages growth of normal cells; it also encourages breast cancer cells to multiply. Mixed with cancer cells in the test tube, it causes them to reproduce; IGF-I is even more potent in this regard than estrogens. A little IGF-I goes a long way. Growth-promoting effects occur at concentrations of just one microgram per liter. IGF-I may also be able to cause normal cells to transform into cancer cells.

There are about 30 micrograms of IGF-I in a liter of cow's milk, although the amount varies with the stage of pregnancy. It is identical to human IGF-I and is not destroyed by the process of pasteurization.

Little is known as to the extent to which humans absorb IGF-I from cow's milk. While it was once thought that protein fragments were completely broken apart during digestion, it is now known that proteins and peptides are often absorbed intact. In fact, several different proteins from cow's milk are known to pass from the digestive tract into the blood stream and even into the breast issue of women who drink milk. Similar compounds, such as epidermal growth factor, are not destroyed by stomach acid and are apparently absorbed, suggesting that the same is true of IGF-I.

IGF-I is a normal part of mother's milk and of infants' diets prior to weaning. However, milk consumption after the age of weaning means prolonged intake of IGF-I.

If IGF-I is a problem, bovine growth hormone (BGH) will make it worse. BGH is used by some dairy farmers to increase milk production. BGH-treated cows produce two to four times more IGF-I, with a corresponding increase of the peptide in milk.

When the Food and Drug Administration approved BGH for use, it was aware of its tendency to increase IGF-I concentration, but approved the hormone anyway because IGF-I did not seem to cause a major effect on the body weight of rats. The experiments, however, had little relevance to humans.

As for BGH itself, traces of it are found in cow's milk even after pasteurization. Needless to say, financial interests overwhelmed both science and good sense when BGH was approved. BGH manufacturer Monsanto made payments to the American Dietetic Association and the American Medical Association, both of which issued favorable statements about BGH on the same day.

Organochlorines

Because pesticides and industrial chemicals tend to dissolve into fat, they end up in the mammary gland's fatty tissues and easily pass into milk. This is true for human breast milk and also for cow's milk. When three carcinogens found in Israeli milk (DDT, a-BHC, and g-BHC) were banned in that country, breast cancer deaths dropped. While this may be a mere coincidence, evidence for a casual relationship comes from the fact that organochlorines have estrogen-like effects. Moreover, the tissues surrounding human breast cancers have been found to have higher concentrations of organochlorines than other tissues.

We have looked at the link between milk and cancer of the ovary, which appears to result form a breakdown product of the milk sugar, galactose. Other parts of dairy products may exert damaging effects to other parts of the body.

It may be that the weaning process has an important biological function -- that of stopping the exposure to compounds that help during infancy but are dangerous on long-term exposure.

Healthy Calcium Balance

With all the criticism milk has earned for its artery-clogging fat and sensitizing proteins, the dairy industry rests its case on one last selling point: calcium. Yet that supposed benefit is suspect as well.

True, milk contains calcium. But only 30 percent of it is absorbed by the human body, less than for typical green leafy vegetables. In fact, green vegetables and beans provide plenty of calcium, along with vitamins, fiber, complex carbohydrates, and essential fatty acids that milk lacks.

Surprisingly, population studies show that a high calcium intake does not insure against osteoporosis. Countries with a high calcium intake, such as Sweden or Finland, tend to have much higher fracture rates than Asian countries where milk is not commonly consumed.

The most important step in maintaining calcium balance is to stop calcium losses caused by these five factors:

Animal protein. Eliminating animal proteins from your diet can cut your calcium losses in half.

Excess salt. Cutting your sodium intake in half can reduce the daily calcium requirement by about 160 milligrams.

Caffeine. If you have more than two cups of coffee per day, drink decaf.

Tobacco. Smokers increase their hip fracture risk by over 40 percent.

Lack of exercise. Sedentary people lose bone tissues.

Don't forget vitamin D, which is important for healthy bones. Ten minutes of summer sun on the face, hands, and arms two or three times per week produces all the vitamin D you'll need. For those who get infrequent sun exposure, any common daily multivitamin provides adequate vitamin D.