Breast Density, Weight Gain Both Risk Factors for Breast Cancer


In research presented at the annual meeting of the American Association for Cancer Research, two studies have added to the knowledge of how body changes over time can affect a woman’s risk for being diagnosed with breast cancer.

The first study, conducted at the Mayo Clinic in Minnesota, has confirmed that having dense breasts are a strong risk factor for breast cancer, but that the decline in density that occurs over time decreases that risk. Researchers followed 19.924 women over the age of 35 without a history of breast cancer and looked at breast density changes and cancer diagnoses over time.

According to the BI-RADS (Breast Imaging Reporting and Data System) staging categories for breast density, a very dense breast (level 4) has greater than 75% fibroglandular tissue. Glandular tissue is contained in the duct system that allows breasts to produce milk, and shows up on a mammogram as white. Breasts that are the least dense (level 1) are almost entirely fat, with less than 25% glandular and connective tissue. These show up as gray or black on the mammographic image.

The women in the study who went on to develop breast cancer were more likely to have been in category 4. Those who decreased in one BI-RAD level or more over the study period of six years were at a 28% reduced risk.


Estrogen levels correlate with breast density. The higher the estrogen, the greater the breast cancer risk, says Dr. Joanne Mortimer, medical oncologist and director of the women’s cancer programs at the City of Hope Comprehensive Cancer Center in California. Factors that increase estrogen levels include hormone replacement therapy, which previous research has found to increase the risk by about 24%. Other factors include pregnancy and lactation, weight loss, and inflammation.

Weight changes can affect the proportion of glandular tissue and fatty tissue in the breast. A separate study presented at the conference found that gaining a pound or two each year after the age of 20 increases a woman’s chances of developing postmenopausal breast cancer.

The study, conducted by the National Cancer Institute, found that women who had a normal BMI at age 20, but increased at least 5 BMI units by the time they reached age 50, had an 88% increased risk of developing postmenopausal breast cancer, compared with women who reported a stable BMI.

"Weight gain is a major risk factor for breast cancer," and could play as much of a role as other known risk factors, like family history of cancer, or the age at first menstruation or childbirth, says study coauthor Regina Ziegler, an epidemiologist at NCI.

Excess fat increases the level of estrogen in the body, which is thought to fuel the growth of tumors, and as above, can increase breast density.