There is mounting evidence that vitamin D levels may play a key role in the development of breast cancer (BC) among Black women. Black women are more often vitamin D deficient than their White counterparts, and this has been attributed to their darker skin pigmentation, lactose intolerance, lack of supplementation, and vitamin D receptor gene polymorphism. A recent case-control study lends further support to these findings.

In this largest investigation to systematically evaluate vitamin D exposure from multiple sources including diet, supplements, and sun exposure and also to examine calcium intake in relation to risk of BC subtypes, researchers reported on data from two studies that are part of the Women’s Circle of Health Study (WCHS). WCHS and WCHS2 are case-control studies designed to identify risk factors for BC among Black women aged 20 to 75 years who have recently been diagnosed with ductal carcinoma in situ or invasive BC. 

Women were recruited n 2002 to 2008 from the metropolitan New York City hospitals and in 2006-2012 from New Jersey through the New Jersey State Cancer Registry for these studies. Sociodemographic data and information on reproductive behaviors, lifestyle factors, and a family history of cancer were gathered during in-person interviews. This information was coupled with hormone receptor and human epidermal growth factor receptor (HER2) status. Dietary intake was assessed using either the Fred Hutchison Cancer Research Center Food Frequency Questionnaire (FFQ) or a modified National Cancer Institute Block FFQ. 

Unfortunately, only the WCHS2 study assessed supplemental intake of vitamin D and calcium. A total of 1724 cases—709 cases from the WCHS 2002-2012 study and 1015 cases from the WCHS2 2012-2017—made up the study population. The majority of women in both trials were estrogen receptor positive [ER+] (68.5% and 71.6%, respectively), and about 20% of cases in both studies were triple negative [TNBC] (i.e., estrogen receptor-negative [ER-], progesterone receptor-negative [PR-] and HER2-receptor negative) for a total of 1213 ER+, 511 ER-, and 335 TNBC cases. TNBC, which has a poorer prognosis, is more common in Black women. Approximately 70% were in stage I/II at the time of diagnosis, and their mean age was 51.9 years in WCHS and 54.8 years in WCHS2. The controls were significantly younger in the WCHS study, with a mean age of 50.4 years.  

Between 22% and 26% of postmenopausal women had previously received hormone therapy compared with about 18% to 22% in the controls, which was not statistically significant. About 15% had never given birth and of those who had had a child, about 40% had breast fed; these trends were similar in all case and control groups. The majority of women (61%-73% in the cases and 57%-68% in the controls) had used oral contraceptives, again not different. Interestingly, only about 15% to 19% of cases had a first-degree relative with BC compared with about 11% to 12% of controls; this difference between cases and controls was significant in the WCHS2 trial. One-third of women with BC in both trials had a history of benign breast disease, and this was significantly higher than controls, in whom the prevalence rate was between 23% and 28%. 

About 60% of all women had never smoked. Few women (13%-23% for cases and 12%-21% for controls) engaged in any vigorous activity. Sunscreen use was only queried about in WCHS2 and was found to be used either occasionally or usually/always by about 40% of women; the rest of the women did not use sunscreen. Alcohol use was practically nonexistent at about 3% or less across the board. Obesity was prevalent with a mean BMI of 30.6 for cases (31.0 for controls; NS) in the WCHS trial and a mean BMI of 31.5 for both cases and controls in the WCHS2 trial. 

Supplemental vitamin D was only asked about in WCHS2 and was not different between groups with an average of 1751 IU/day and 1679 IU/day consumed in the cases and controls, respectively. The vast majority of women in both the cases and controls in the WCHS2 trial used vitamin D supplementation (~60% in both groups) either in the form of a multivitamin (~80% in both groups) or single supplement use or in combination with calcium (56% in cases and 47% in controls; this difference was significant). Dietary vitamin D was not associated with the risk of cancer subtypes in the two studies. However, the use of supplemental vitamin D in a dose of <800 IU significantly reduced the risk of ER+ BC (odds ratio [OR] = 0.68, CI, 0.50-0.94) and TNBC (OR = 0.58, CI, 0.35-0.94) compared with nonusers. This effect on TNBC is very promising since treating the disease is difficult. However, higher doses of supplemental vitamin D were not found to be beneficial. Calcium intake did not appear to play a role in BC risk. 

The investigators also found that more sun exposure, defined by daylight hours spent outdoors, was associated with a decreased risk of developing ER+ BC, ER- BC, and TNBC, with the benefit being greater in the later cancer subtype. 

Investigators concluded that a moderate intake of supplemental vitamin D (<800 IU/day) was associated with decreased risk of developing BC subtypes, especially TNBC. This information is vital for pharmacists as they are in the ideal position to counsel Black women to have their vitamin D levels checked and to recommend appropriate supplementation. 

The content contained in this article is for informational purposes only. The content is not intended to be a substitute for professional advice. Reliance on any information provided in this article is solely at your own risk.

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