Regularly drinking milk may increase a woman’s risk of breast cancer up to 80 percent depending upon the amount consumed, according to a study by researchers at Loma University Health in California published in the International Journal of Epidemiology.
Even just a small amount—less than a cup of dairy milk a day—was associated with an increased risk of 30 percent. Cheese, yogurt, and soy foods like soymilk did not raise breast cancer risk.
Though the increases in risk are sharp, it’s important to keep them in perspective. If drinking one cup of milk a day increases your breast cancer risk by 50 percent, it does not mean you have a 50 percent chance of developing cancer. It means your risk is 50 percent higher than it would otherwise be.
Researchers suggest that hormones associated with dairy milk may be the culprit behind the cancer connection. They suggest women at increased risk for the disease find milk substitutes.
As endurance athletes, we’ve long been told that milk does a body good. Now a study of nearly 53,000 North American women published in the International Journal of Epidemiology calls that into question, at least for women. Regularly drinking milk appears to significantly increase a woman’s risk of breast cancer—up to 80 percent depending upon the amount consumed.
“Consuming as little as one-quarter to one-third cup of dairy milk per day was associated with an increased risk of breast cancer of 30 percent,” lead researcher Gary E. Fraser, Ph.D., of Loma Linda University Health in California said in a press release. “By drinking up to one cup per day, the associated risk went up to 50 percent, and for those drinking two to three cups per day, the risk increased further to 70 to 80 percent.”
That’s concerning, considering the U.S. Dietary Guidelines currently recommend getting two to three servings of dairy, which you could get through two to three cups of milk, per day.
The study, which is part of the Adventist Health Study-2(AHS-2), a long-term health study exploring the links between lifestyle, diet, and disease among the Seventh-day Adventist community, examined the dietary intakes of 52,795 women, average age of 57, who were cancer-free and followed them for nearly eight years. The goal was to examine relationship between soy and/or dairy consumption and breast cancer.
By the end of the study, 1,057 women had developed breast cancer. There were no clear associations between soy products and breast cancer risk. However, compared to women who drank little to no milk, those who consumed more dairy, particularly dairy milk, had a higher risk of breast cancer. Cheese and yogurt didn’t appear to increase risk, and it didn’t matter if the milk was full-fat, reduced fat, or skim.
The risk associated with dairy echoes findings of another recent AHS-2 report suggesting that vegans (who eat no animal products), but not lacto-ovo-vegetarians (who consume dairy and eggs), experienced less breast cancer than women who eat animal foods.
The researchers believe one possible reason for the link between breast cancer and dairy milk may be the sex hormone content of dairy milk. Breast cancer in women is a hormone-responsive cancer. Dairy cows are lactating, and often about 75 percent of the dairy herd is pregnant. So women drinking milk are exposed to those hormones.
Also, some research has found a connection between dairy and other animal protein consumption with higher levels of a hormone, insulin-like growth factor-1 (IGF-1), which may promote certain cancers.
This isn’t the first time research has found a link between dairy intake and breast cancer. The Nurses’ Health Study II in 2003 found that women who ate two or more servings of high-fat dairy products such as whole milk and butter every day had a higher risk of breast cancer before menopause than their counterparts who ate fewer servings of high-fat dairy.
But these findings also fly in the face of previous research, including a large analysis of data from more than 20 studies that found no link between consuming dairy products, including milk, and risk of breast cancer. Other research has even suggested that the calcium and vitamin D in milk may reduce a woman’s risk of the developing the disease before menopause.
However, those studies may not have found a link because they weren’t looking at the very lowest levels of dairy consumption, Fraser told Bicycling. This most recent study examined a wide range of dairy consumption from thousands of women who drank no milk at all to those who drank lesser and greater amounts. When the researchers compared non-milk drinkers to those who drank just a little, they saw a spike in breast cancer risk.
“The risk increased quickly,” Fraser told Bicycling. There was an especially strong rise in risk up to two-thirds of an 8-ounce cup of milk. But after that initial sharp rise, the risk still increased, but much severely. Other studies may have been looking at populations with higher consumption where you don’t see sharp differences, Fraser said.
It’s important to note that a strong correlation in a study like this doesn’t mean causation: The researchers can’t conclude milk causes breast cancer. But the connection was strong enough to give the researchers pause, because even when they took lifestyle, exercise, and other dietary factors into account, the milk-breast cancer connection remained.
“Our data very strongly suggests that the probability that the connection is a matter of chance is quite small,” Fraser told Bicycling.
Though the increases in risk are sharp, it’s important to keep them in perspective. If drinking a cup of milk a day increases your breast cancer risk by 50 percent, it does not mean you have a 50 percent chance of developing the disease. It means your risk is 50 percent higher than it would otherwise be.
That means if the average woman’s risk is 12 percent (based on the statistic that one in eight women develop breast cancer in their lives), that risk is a 50 percent increase over 12 percent or 18 percent.
Of course, any increase in cancer risk is worth considering, especially if you already have other risk factors for the disease.
“If you are at a relatively high risk because of family history or long-term use of hormones, I would be cautious about the use of dairy milk,” Fraser told Bicycling.
In fact, the study data predicted that women who substituted soymilk for dairy milk could significantly lower their risk. “This raises the possibility that dairy-alternate milks may be an optimal choice,” Fraser said in the release.