• Vitamin D deficiency is increasing and people with dark skin living in the Northern hemisphere are particularly affected.
• Deficiency is increasingly being linked to various diseases including breast cancer.
• A study has found that Black and Hispanic women who have adequate vitamin D levels are 21% less likely to develop breast cancer than those who are deficient.

Nearly two-thirds of Black women and over a third of Hispanic women in the United States are thought to be deficient in vitamin D.

Black and Hispanic people are more likely to be deficient in vitamin D than white people due to higher levels of melanin in the skin. This means less vitamin D is created when exposed to sunlight. This phenomenon becomes more pronounced at higher latitudes as the amount of sunlight available throughout the year is lower than it is nearer the equator.

This disparity in the levels of vitamin D deficiency could be contributing to some health inequalities, according to a recent paper in the Journal of Nutrition. This is because low vitamin D levels are increasingly found to be correlated with many health conditions including some autoimmune diseases and cancers.

However, this is an area of some controversy. It has not been proven, for example, that vitamin D deficiency can explain the increased risk of severe COVID-19 experienced by Black and Asian people.

While white women are slightly more likely to develop breast cancer than Black women, Black women have the highest rate of mortality from breast cancer in the U.S., at 31%. Health inequities and discrimination likely play a key role in this disparity.

The causes of breast cancer are multifactorial, including genetic predisposition, age, and lifestyle. Some studies suggest vitamin D levels may also affect the likelihood of developing breast cancer.

In this context, a recent longitudinal study published in the journal Cancer delves deeper into the link between vitamin D levels and the risk of breast cancer in Black and Hispanic women.

The study examined Black and Hispanic women who had a sister who had been diagnosed with breast cancer. Women who have a sister who has had breast cancer are considered twice as likely to be diagnosed as somebody without a close relative with the disease.

The Sister Study

Using data collected for the Sister Study, a breast cancer study that follows the sisters of people who have had breast cancer, researchers identified 1,300 Black women and 562 Hispanic and Latina women for their analysis.

Researchers analyzed the data, which included at least one blood sample for vitamin D levels, as well as answers to annual questionnaires about health, lifestyle, and background. This included whether or not they had been diagnosed with breast cancer.

Lead author Dr. Katie O’Brien who leads the Sister Study at the National Institute of Environmental Health Sciences explained to Medical News Today in an email: “We used the most sensitive test available – liquid chromatography-tandem mass spectrometry – which was able to differentiate between the different types of vitamin D.”

“Although we summed them all together as total vitamin D for this analysis, we may wish to think about the individual types (25(OH)D₃, 25(OH)D₂ and epi-25(OH)D₃) in future work.”

Of the women followed over a mean of 9.2 years, 290 Black participants and 125 Hispanic and Latina participants developed breast cancer.

Analysis of this group compared to the women who did not develop breast cancer showed that having sufficient levels of total circulating 25(OH)D concentrations in their blood (defined as under 20ng/mL) were 21% more likely to develop breast cancer.

This effect was stronger among non-Black Hispanic and Latina women who were 48% less likely to develop breast cancer if they had sufficient levels of vitamin D, compared to a reduction of 11% for Black women.

Dr. O’Brien said: “While we did our best to measure and control for other factors that might have contributed to this association, we cannot determine from these results whether there is a causal relationship between vitamin D and breast cancer.”

The authors of the study pointed out that one limitation of the study was that they had only measured vitamin D levels at one particular time point.

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