A study done by Kaiser Permanente researchers suggests that women diagnosed with early-stage breast cancer who eat full-fat dairy products after diagnosis are more likely to die from breast cancer than women who eat low-fat dairy products after diagnosis.
The study was published online by the Journal of the National Cancer Institute on March 14, 2013. Read the abstract of “High- and Low-Fat Dairy Intake, Recurrence, and Mortality After Breast Cancer Diagnosis."
The hormone estrogen stimulates breast cell growth, including the growth of hormone-receptor-positive breast cancer cells. Estrogen is created and stored in fat cells. Many researchers believe that dairy products eaten in the United States and other Western countries have high levels of estrogen and progesterone in them because most of the milk is produced by pregnant cows. So it might be possible that low-fat dairy products have lower levels of estrogen and progesterone because most of the fat has been removed. This suggests that low-fat dairy products may be a better choice for women who’ve been diagnosed with breast cancer, especially hormone-receptor-positive breast cancer.
In this study, the researchers wanted to know if eating full-fat dairy products increased the risk of breast cancer recurrence (the cancer coming back), as well as the risk of dying from breast cancer. For this study, full-fat dairy products included:
- whole milk
- condensed or evaporated milk
- ice cream
The researchers looked at nearly 1,900 women who had been diagnosed with early-stage breast cancer from 1997 to 2000. Most of the women were from northern California. When the study started, the researchers sent the women a 120-question survey, including questions on:
- how often they ate dairy products during the previous year
- how large the serving size was
- the type of dairy products eaten
- whether the dairy product was full-fat, 2%, 1%, or non-fat/skim
Six years later, the researchers sent the same survey to all the women. About 1,500 women returned the second survey. The women were followed for about 12 years.
Low-fat milk and butter were the dairy foods the women reported eating most often. Overall, the women said they ate about 0.8 servings per day of low-fat dairy products and about 0.5 servings per day of full-fat dairy products.
During follow-up, 349 women had a breast cancer recurrence; 372 of the women died from any cause and 189 of those women died specifically from breast cancer.
The researchers found that women who reported eating one or more servings per day of full-fat dairy products had a 64% higher risk of dying from any cause and a 49% higher risk of dying from breast cancer compared to women who ate fewer servings per day of full-fat dairy products or women who ate low-fat dairy products.
There was no association between the amount of low-fat dairy products eaten and breast cancer survival. There also was no association between the risk of recurrence and the amount of full-fat dairy products eaten.
While the results of this study are troubling, it’s important to keep several things in mind.
First, the women in the study had to remember exactly how much and what type of dairy products they ate for the entire previous year. Unless you keep a detailed food diary, this is extremely difficult to do. So, it’s likely that the amount and type of dairy products eaten was misreported.
Second, it’s not clear if the researchers looked at what else the women were eating or whether the women’s weight and exercise habits were considered in the research. Being overweight and a lack of exercise are two factors that can lead to worse breast cancer outcomes and increase the risk of breast cancer recurrence.
Third, we don’t know the hormone-receptor status of the breast cancers in the study. If a woman was diagnosed with hormone-receptor-positive breast cancer, it’s likely that she would be prescribed hormonal therapy medicine for at least 5 years after surgery to reduce the risk of the cancer coming back. Did some (or many) of the women quit taking hormonal therapy early? If so, this also might affect the results of the study.
So while this study shows an association between full-fat dairy products and worse breast cancer outcomes, it doesn’t prove the first causes the second. Much more research is needed on the potential link between full-fat dairy products and breast cancer.
If you’ve been diagnosed with early-stage breast cancer, it makes sense to do everything you can to minimize your risk of recurrence and improve your chances of survival, including:
- eating a healthy diet that’s low in processed foods and sugar
- avoiding alcohol
- maintaining a healthy weight
- exercising daily
- not smoking
- staying on track with any medicines you’re taking to reduce the risk of recurrence
To learn more about breast cancer risk factors, including recurrence risk factors, visit the Breastcancer.org Lower Your Risk section.