光照不足会导致非裔美国人缺乏维生素D

African-American men living in low sunlight areas are more likely to experience vitamin D deficiency than European-American men living in the same environment. Researchers believe that these findings should change recommendations for daily intake of vitamin D. "This study shows that across the board vitamin D recommendations just won't work for everybody," said Adam B. Murphy, M.D., M.B.A., clinical instructor in the department of urology（泌尿学） at Northwestern University Feinberg School of Medicine, who reported the study at the Fourth AACR Conference on The Science of Cancer Health Disparities, held Sept. 18-21, 2011, in Washington, D.C.

"With so many diseases linked to low levels of vitamin D, we should have more stratified（分层的） recommendations to consider groups within the population instead of making monolithic（整体的） suggestions," he added.

Researchers evaluated the marker for vitamin D -- the 25 hydroxyvitamin D level (25-OH D level) -- in 492 men aged 40 to 79 years who lived in Chicago, a low ultraviolet radiation (UVR) part of the country. Of that group, 93 percent of African-American men and 69.7 percent of European-American men were vitamin D deficient; having 25-OH D levels of less than 30 ng/mL.

Results showed that vitamin D levels were low in African American men, those with lower income and those with higher body mass index. Low sunlight exposure is a known factor in lower levels of vitamin D, but researchers found that African American men still had lower levels of vitamin D in sunnier seasons.

Murphy attributes low vitamin D levels to the composition of African-American skin, which contains more of the pigment melanin（黑色素） than lighter skin. When UVR light hits the skin cells, it reacts with the molecule 7-dehydrocholesterol to begin the production of vitamin D, which is then further processed by the body to make active vitamin D. In African-Americans, though, melanin blocks UVR rays from being absorbed, thus reducing the amount of vitamin D naturally produced.

Vitamin D deficiency has been linked to multiple diseases, including breast cancer, prostate cancer, diabetes, rheumatoid arthritis（风湿性关节炎） and multiple sclerosis, which is why Murphy believes it is essential to adjust recommendations to reflect differences between African Americans and European-Americans.

"Because we have a lot of special populations in the United States -- people who have darker skin, people who cover their skin for religious reasons and people who live in poor sunlight environments -- there shouldn't be uniform vitamin D recommendations for the entire population," he said.