The existence of health disparities in U.S. subpopulations is well-known and health disparities are a major focus of the U.S. 10-year plan for public health, Healthy People 2020. Healthy People, however, assumes that disparities in the health of different subpopulations results only from disparities in social factors, such as socioeconomic status. Biological factors, including vitamin D status, have never been considered relevant to this problem by nutrition or public health authorities in the U.S. Healthy People 2020 has over 1,200 goals related to public health, including goals related to reducing sun exposure, but does not mention vitamin D anywhere in the document.
However, it has been repeatedly shown that at the latitude of the U.S., those with darker skin have lower vitamin D levels than those with lighter skin. And we’ve known why since the 1930s – light-colored skin requires less exposure to ultraviolet light to produce the same amount of vitamin D as darker skin. Moreover, the specific diseases and conditions associated with skin-color based health disparities are typically the same diseases and conditions associated with poor vitamin D status.
I am co-author of a study investigating the relationship between health, skin color, and vitamin D in the U.S. population that was published in May:
We used data from 12,505 subjects (3,402 non-Hispanic blacks, 3,143 Mexican-Americans, and 5,960 non-Hispanic whites) 13 years or older, from the U.S. National Health and Nutrition Examination Survey (NHANES) surveys completed in 2003 through 2006. We analyzed the data with statistical software specialized for complex surveys, which makes the results nationally representative – the results reflect the actual values in the U.S. population for these groups during the years included in the study. The following graph shows the distribution of 25-hydroxyvitamin D (25(OH)D) levels in these three subpopulations, plus an estimate of the distribution in traditionally-living dark-skinned people near the equator (data from the study by Luxwolda and colleagues).
We measured health disparities using single-question self-rated health. Non-Hispanic blacks and Mexican-Americans rate their health lower than non-Hispanic whites do. The difference is a direct measurement of health disparities. However, when vitamin D status and other characteristics, including socioeconomic status, are controlled for, the disparities in how the subpopulations answer the self-rated health question disappear for non-Hispanic blacks and are greatly reduced for Mexican-Americans.
The paper concludes: “This study suggests that the disparities by skin color in the vitamin D status and in the health of the US population are related to each other. Treating groups at risk for insufficient vitamin D – including all people of color living in the United States – with inexpensive vitamin D supplements may be a viable strategy for reducing health disparities and lowering the cost of health care. Our findings suggest that US public health authorities may never eliminate health disparities without attending to the skin-color related disparities in vitamin D nutriture.”