African-Americans are 25 percent more likely to die from cancer than white Americans are, and the reasons are numerous, including lower socio-economic status, poorer access to health care, and the cancer diagnosis coming at later, more deadly stages.
Still, health experts say these factors cannot fully explain the extent of disparities in survival for the most common cancers, such as breast, lung, colon and prostate cancers.
A paper published in the current issue of the journal Dermato-Endocrinology points the finger at a seemingly obvious but overlooked culprit: the sun.
The researchers' theory is that, in northern latitudes, the dark skin of African-Americans cannot absorb enough sunlight to generate adequate amounts of vitamin D, which is often called the "sunshine vitamin." The body uses ultraviolet rays from the sun to manufacture vitamin D in the inner layers of the skin.
Vitamin D is needed for strong bones; doctors nearly 100 years ago associated a lack of adequate sun exposure with rickets among child laborers, exemplified by bowed legs. Recent studies also have shown that low levels of vitamin D in the blood seem to contribute to a weak immune system and a host of diseases, such as cancer and multiple sclerosis. [Infographic: The Power of Vitamin D]
This lack of vitamin D could completely fill in the health disparity gap for cancer survival between white and black Americans, the researchers said.
Previous work by geneticist Rick Kittles at the University of Chicago suggests that upwards of 75 percent of African-Americans are deficient in vitamin D. Kittles says that African-Americans living north of the 37th parallel — just about anyplace north of central California, Texas, Tennessee or North Carolina — will have difficulty through most of the year absorbing enough sunlight to make vitamin D, because of the low angle of the rays reaching the Earth's surface.
Given this largely established fact, researchers Alan Peiris of East Tennessee State University and William Grant of the Sunlight, Nutrition and Health Research Center in San Francisco set out to look for a correlation between vitamin D and cancer death disparities. (In past research, Grant and a colleague suggested low levels of ultraviolet-B rays in Austria, paired with Mozart's nocturnal habits, may have led to vitamin D deficiency in the composer, who died at the age of 35.)
What they found in the new study is preliminary but warrants further investigation, they said. Relying solely on a scientific literature review, the researchers found that low vitamin D is independently associated with each of the cancer types for which an unexplained health disparity exists between African-Americans and white Americans.
Specifically, they found lingering disparities for 13 types of cancer after accounting for socioeconomic status, stage at diagnosis, and treatment: bladder, breast, colon, endometrial, lung, ovarian, pancreatic, prostate, rectal, testicular, and vaginal cancer; Hodgkin's lymphoma; and melanoma. For each one, there is a vitamin-D connection.
Few scientific studies have directly explored the link between cancer deaths and low vitamin D levels in African-Americans, though. One study published in the journal Cancer in 2011 indeed found that vitamin D deficiency contributes to excess African-American mortality from colon cancer. A Harvard study published in Cancer Epidemiology, Biomarkers and Prevention in 2006 found that African-Americans who are at risk for low vitamin D also had a higher risk for cancer death, particularly for digestive-system cancers.
The paucity of studies makes this a ripe topic for exploration, said Grant. If low vitamin D is the cause of this disparity in cancer deaths, thousands of lives could be saved annually by encouraging African-Americans to take a daily vitamin D supplement in the range of 1,000 to 4,000 IUs, he said. [9 Good Sources of Vitamin D]
Peiris added that monitoring vitamin D levels should be routine. The issue becomes critical given that passive exposure from the sun simply is not enough for millions of African-Americans living in northern cities such as Washington, New York and Detroit. Air pollution filters sunlight, too; and many African-American children stay indoor for long hours, sometime over concerns of neighborhood safety.
Obtaining enough vitamin D through food, regardless of one's skin tone, can be difficult. Sources include fatty fish such as salmon, mackerel and wild catfish. At least the sources, aside from cod liver oil, tend to be tasty.
Christopher Wanjek is the author of the books "Bad Medicine" and "Food At Work." His column, Bad Medicine, appears regularly on LiveScience.
Live Science newsletter
Stay up to date on the latest science news by signing up for our Essentials newsletter.
Christopher Wanjek is a Live Science contributor and a health and science writer. He is the author of three science books: Spacefarers (2020), Food at Work (2005) and Bad Medicine (2003). His "Food at Work" book and project, concerning workers' health, safety and productivity, was commissioned by the U.N.'s International Labor Organization. For Live Science, Christopher covers public health, nutrition and biology, and he has written extensively for The Washington Post and Sky & Telescope among others, as well as for the NASA Goddard Space Flight Center, where he was a senior writer. Christopher holds a Master of Health degree from Harvard School of Public Health and a degree in journalism from Temple University.