According to most available research, women with dense breasts have at least double the risk of breast cancer compared to those with mostly fatty breasts. Dense breasts have a higher proportion of fibroglandular tissue to fat than normal breasts. Such fibroglandular tissue appears white on mammograms.
However, tumors and other abnormalities also appear white, causing them to be more difficult to detect on the mammograms of dense breasts. Both breast MRIs and ultrasounds can improve the detection of breast cancer in such women. High breast density is fairly common—one 2016 study estimated that more than 40% of U.S. women aged 40 to 74 have relatively dense breasts.
Using hormone replacement therapy (HRT), also known as menopausal hormone therapy, increases breast density and amplifies the heightened risk associated with dense breasts. Breast density can be modified by diet, giving women an opportunity to influence this breast cancer risk factor.
Perhaps surprisingly, while high breast density increases the risk of breast cancer and of subsequent breast cancer in the opposite breast (contralateral breast cancer), it does not appear to be associated with increased risk of distant metastasis. For example, breast cancer survivors with high breast density who undergo lumpectomy do not have an increased risk of death from breast cancer (as long as they also have radiotherapy). In fact, low breast density has been linked to poor breast cancer prognosis.

Having dense breasts increases risk of breast cancer

Having dense breasts increases the risk of both main histological types of invasive breast cancer, ductal and lobular. Dense breasts are associated with hormone receptor positive (ER+/PR+) and mixed hormone receptor (ER+/PR- and ER-/PR+) breast cancer. Studies have reported conflicting results for hormone receptor negative (ER-/PR-) tumors. One study found that percent mammographic density appears to be inversely associated with ER expression but might correlate positively with PR expression. Another study reported that the link between density and breast cancer risk appears to be stronger in ER- than in ER+ tumors.
Women with dense breasts have a higher rate of second primary breast cancer (a new tumor that arises after treatment and that does not appear to be directly related to the first breast cancer).
Women with a decrease in breast density over time have been shown to have decreased breast cancer risk compared to women whose breast density remains stable. On the other hand, an increase in breast density is associated with increased risk. Use of medications such as metformin to treat type 2 diabetes appears to reduce breast density, whereas taking insulin might increase it.

Dense breasts can start early in life

Dense breasts run in families. High birth weight and childhood height, known risk factors for breast cancer, are associated with dense breasts. Also, older mothers (over 39 years of age) are more likely to have daughters with dense breasts.
Mammographic density has been found to be higher for women with first degree relatives with breast cancer.

Environmental and dietary factory influence breast density

Breast density is not fixed; it can be increased or reduced by certain food groups and environmental exposures. Several studies have reported that high calorie diets, diets high in sugar and sweetened foods, and diets high in saturated fat (such as that found in full-fat milk, butter, and cheese, as well as beef) are associated with higher breast density. On the other hand, high intake of omega-3 fats has been linked to lower breast density in both animal and human studies.
Alcohol consumption is also associated with increased breast density. The association is stronger in women with a family history of breast cancer. In fact, alcohol increases breast density most among women who are already at high risk of breast cancer due to other factors. Olive oil and white meat consumption were associated with lower breast density in one study. Consuming foods high in carotenoids (e.g., carrots, pumpkins, saffron) has been reported to reduce the risk of breast cancer among women with high breast density, although not all studies are in agreement.
Exposure to cadmium can increase breast density. Dietary sources of cadmium include shellfish, rice, flaxseed, escargot, sunflower seeds, and dried apricots from certain geographic regions. Exposure to certain other endocrine disruptors found primarily in food packaging and personal care products can also increase breast density. Animal experiments have demonstrated that prenatal exposure to bisphenol A (BPA), phthalates, and endocrine disrupting pesticides results in greater mammary gland density. Exposure to air pollution has also been linked to increased breast density.
Hormone replacement therapy, particularly combined estrogen plus progestin HRT, heightens the risk of breast cancer associated with dense breasts among postmenopausal women.
Women with dense breasts who also have benign breast disease are at particularly high risk for future breast cancer. One 2021 study reported that women with benign breast disease, as well as high breast density, had three times the risk of breast cancer compared to those without benign breast disease and with breasts having mostly non-dense tissue with some areas of higher density.

Breast cancer treatment reduces breast density

Chemotherapy reduces breast density, especially among younger women. Radiation treatment significantly reduces the risk of local recurrence among breast cancer patients with dense breasts who are treated with lumpectomy. One study found that 40% of women with high breast density who did not receive radiation therapy experienced local breast cancer recurrence by the 10-year mark. Aromatase inhibitor treatment also reduces breast density. In fact, changes in breast density during the first year of aromatase inhibitor or tamoxifen treatment can determine whether such endocrine treatment is working.
Below are links to recent studies on this topic. For a more complete list of studies, please click on breast density.