The study, carried out by Kaiser Permanente researchers and published in the Journal of the National Cancer Institute, is one of the first of its kind to identify a link between high fat dairy consumption and poorer breast cancer survival.
Other studies have found associations between certain eating habits and breast cancer risk. Researchers from Michigan State University reported in 2010 that a high-fat diet during puberty can increase breast cancer risk later in life, even among females who never become overweight.
Many dairy products contain elevated estrogen levels - high exposure to estrogen may increase the likelihood of breast cancer, according to previous research.
Estrogen levels are highest in fatty dairy products, compared to low-fat dairy.
The investigators analyzed a group of women who were diagnosed with early-stage, invasive breast cancer between 1997 and 2000. Most of the them (83 percent) were from Kaiser Permanente's Northern California region.
According to the lead author of the study, Candyce H. Kroenke, ScD, MPH, patients who had a high-fat dairy diet had "higher breast cancer mortality as well as higher all-cause mortality and higher non-breast cancer mortality."
"Specifically, women consuming one or more servings per day of high-fat dairy had a 64 percent higher risk of dying from any cause and a 49 percent increased risk of dying from their breast cancer during the follow-up period."
High fat dairy products include:
- Mature and/or hard cheese
- Evaporated milk
- Ice cream
- Whole milk
- Yogurt (made from full fat milk)
Senior author of the study, Bette J. Caan, DrPH, research scientist with the Kaiser Permanente Division of Research, said: "High-fat dairy is generally not recommended as part of a healthy diet. Switching to low-fat dairy is an easy thing to modify."
1,893 women participated in the study two years after they were diagnosed with cancer. They answered a food-frequency questionnaire, 1,513 of them answered a follow up questionnaire 6 years later.
They asked the women how often they ate dairy foods such as milk, cheese, yogurt etc., as well as the portion sizes.
349 of the women in the study had a recurrence of breast cancer, of whom 189 died.
The investigators found that a high-fat dairy diet was linked to a greater breast cancer death rate - no association was found in low-fat dairy products though.
Dr. Kutner concluded:
"Women have been clamoring for this type of information. They're asking us, 'Tell me what I should eat?' With this information, we can be more specific about recommending low-fat dairy products."