Population studies have uncovered differences in the chemopreventive activities of carotenoids according to breast cancer type or other individual characteristics. A Scandinavian study found that dietary (but not supplemental) beta-carotene has a protective effect against lobular breast cancer in postmenopausal women. Another European study reported that high intake of beta-carotene is protective against breast cancer in postmenopausal women using hormone replacement therapy (HRT). The same study also found that dietary beta-carotene is associated with lowered risk of breast cancer in postmenopausal women with high alcohol consumption. A U.S. study reported that high consumption of carotenoids may reduce the risk of premenopausal but not postmenopausal breast cancer, particularly among smokers.
Anti-cancer mechanisms of action of carotenoids
The biological processes by which carotenoids inhibit breast cancer are far from understood. However, studies have described several mechanisms of action which probably contribute to this effect. Estrogen exposure is a major risk factor for breast cancer. On the other hand, estrogens are known to be beneficial for bone health. Researchers have demonstrated that carotenoids inhibit estrogen signaling in breast cancer cells while enhancing estrogen signaling in bone cells.
A prospective study of women in the Nurses' Health Study reported that high circulating carotenoid levels can offset some of the increased breast cancer risk associated with high breast density. High breast density has been reported to be a powerful indicator of increased breast cancer risk. In the study, circulating carotenoids were not found to be associated with mammographic density. However, among women in the highest third of mammographic density, total circulating carotenoids were associated with a 50% lower risk of breast cancer.
Another study demonstrated that vitamin A, or retinol, was able to reverse the early stages of the cancer development process at the cell level. Certain carotenoids such as beta-carotene are efficiently converted into retinol. The study used a laboratory model of breast cancer progression developed by the authors. Cultivating normal breast cells causes the cells to polarize and spontaneously differentiate tubular structures. When such cells are transformed in carcinogenesis, the cells form tubules and spherical masses. The authors demonstrated that retinol was able to re-differentiate such transformed cells.
Multidrug resistance (MDR) proteins are present in a majority of human tumors and are an important cause of eventual treatment failure. A study that investigated the effects of carotenoids on the activity of the MDR-1 gene reported that lycopene and zeaxanthin induced apoptosis (programmed cell death) in triple negative breast cancer cells made resistant to chemotherapy through MDR-1.
Several studies have reported that beta-carotene is made more bioavailable when combined with olive oil or black pepper. Dietary carotenoids also inhibit estrogen signaling of both estradiol and the soy isoflavone genistein, thereby reducing their harmful effect in hormone-dependent breast cancer.
Beta-carotene and other carotenoid supplements
Generally speaking, studies of the effect of carotenoid supplements have reported no effect or a small harmful effect on breast cancer risk and prognosis. For example, a study that included women with early-stage breast cancer in the Kaiser Permanente Northern California Cancer Registry reported that frequent use of combination multiple carotenoids was associated with double the risk of death from breast cancer compared to no such use. The same study found no such relationship for beta-carotene or lycopene alone. The women were enrolled in the study approximately two years after diagnosis. However, note that lycopene supplements should be avoided because some studies have found increased risk of breast cancer specifically associated with this carotenoid.
Latest research reports reduction in breast cancer risk for several common carotenoids
The new study referenced at the beginning of this news article was designed to investigate the associations between carotenoid intake and risk of breast cancer by hormone receptor status. To conduct the study, the authors pooled data from 18 prospective population studies. The data included 1,028,438 participants. A total of 33,380 invasive breast cancers were diagnosed during follow up. High intakes of beta-carotene, alpha-carotene, and lutein/zeaxanthin each were found to be associated with 13% to 16% reduced risk of estrogen receptor negative (ER-) breast cancer: beta-carotene (13% reduced risk), alpha-carotene (16%), and lutein/zeaxanthin (13%). No associations were found with the risk of estrogen receptor positive (ER+) breast cancer.
Reductions in risks of both ER- and ER+ breast cancer were found for high intake of beta-cryptoxanthin, but the results were not statistically significant. Lycopene intake had no significant associations. Nonsignificant associations were observed for progesterone receptor positive (PR+) and PR- breast cancer. The authors conclude that intakes of beta-carotene, alpha-carotene, and lutein/zeaxanthin are inversely associated with risk of ER-, but not ER+, breast cancer. However, the authors warn that the results need to be interpreted with caution since it is possible that the observed associations are due to other components of the same food sources.
Please see our article on how to optimize your breast cancer diet for information on what to eat during all stages of treatment and recovery.