High-fat dairy may increase risk of death from breast cancer relapse

After a patient has been diagnosed and treated with breast cancer, the types of food she consumes may have a significant impact on her long-term survival.

A new study has revealed women who consume higher rates of high-fat dairy products after a breast cancer diagnosis have a much greater chance of dying from recurrence of the disease years later.  The research, conducted by scientists at Kaiser Permanente in Oakland, Calif., is published in the Journal of the National Cancer Institute.

The idea to examine breast cancer prognosis in relation to dairy consumption stems from the way milk and dairy are produced in Western culture.

“The way milk and dairy are produced in the western world, through breeding of cows and modification of feed, cows are able to give milk at the same time as being pregnant,” lead author Dr. Candyce Kroenke, staff scientist with the Kaiser Permanente Division of Research, told FoxNews.com. “This leads to higher estrogen rates in the fats of the milk.”

Kroenke said while estrogen resides in the fats of milk regardless, pregnant cows have higher estrogen rates, therefore the milk they produce have much higher rates of estrogen stored in the fat.  In nature, pregnant cows are not capable of producing milk until after their calves are born.

While high-fat dairy products made in Western society are generally high in estrogen, dairy products in which the fat was removed – such as skim milk or 1 and 2 percent milk – had effectively lower estrogen levels.

For women who have been diagnosed with breast cancer, higher amounts of estrogen in the body can trigger relapse of the disease.  Therefore, Kroenke and her team theorized that the more high-fat dairy a woman consumed, the higher her chances for breast cancer relapse – and ultimately death.

To test their hypothesis, the researchers followed a cohort of 1,893 women who had been diagnosed with breast cancer approximately two years prior to the beginning of the study.   Each participant was given a food-frequency questionnaire, and 1,513 were given a follow-up questionnaire six years later. Overall, the participants were followed for an average of 12 years.

In the questionnaire, the women were asked about their dairy intake – such as milk, yogurt, butter and cheese.  They were also asked what kinds of milk they consumed, whether it was high-fat, 1 or 2 percent, or non-fat.  With the answers of those questions, the researchers were able to categorize each woman into either a high-fat dairy group or a low-fat dairy group.

“In terms of total dairy intake, we didn’t find a strong association overall,” Kroenke said.  “But for women who consumed high levels of high-fat dairy, they had an elevated risk of breast cancer death and death from all causes, as well as non-breast cancer-related deaths.  For the non-breast cancer-related deaths, the women died primarily of cardiovascular disease.”

Of the sample of women, 349 had breast cancer recurrence and 372 died of any cause, 189 of them from breast cancer.  This translated to a 50 percent elevated risk of dying from breast cancer for women who consumed high-fat dairy.

With such a substantial association established, Kroenke noted a very clear take-away message from the research.

“Overall, one of the potential opportunities is to shift away from using high-fat dairy towards using lower fat dairy sources,” Kroenke said.  “Also, women with breast cancer can reduce their dairy intake, or people can shift to more plant-based milks.  Those are some of the recommendations.”

Another way to combat this problem is for dairy production in Western society to return to a more natural process – more as a consequence of pregnancy rather than genetically designed to occur during pregnancy, Kroenke said.  However, the current dairy practices have been put in place to increase profits as much as possible.

"In a different food environment, high fat dairy may not be as much of an issue,” Kroenke said.