CORRESPONDENCE: LETTER TO THE EDITOR
Reply
Joachim H. Ix, MD, MAS* and
Michael H. Criqui, MD, MPH
* Division of Nephrology-Hypertension, Department of Medicine, University of California San Diego and Veterans Affairs San Diego Healthcare System, 3350 La Jolla Village Drive, Mail Code 111-H, San Diego, California 92161 (Email: joeix{at}hotmail.com).
It is possible that vitamin D status could represent an unmeasured confounder explaining the higher prevalence of peripheral arterial disease (PAD) among African Americans (1). Vitamin D levels were not available in our study, so we cannot directly confirm or refute this hypothesis. However, to explain the residual 1.9-fold higher odds of PAD among African Americans compared with that in Caucasians, any unmeasured confounder would need to be strongly associated with both African-American race and PAD. Prior studies have demonstrated that African Americans have lower serum 25 (OH) vitamin D levels (2,3). Until recently, the association of vitamin D with PAD was unknown.
Published the same month as our study, Melamed et al. (4) reported that lower vitamin D levels were independently associated with PAD among 4,839 National Health and Nutrition Examination Survey participants (approximately 500 were African American). While the authors did not directly evaluate whether the association of African-American race with PAD was attenuated with statistical adjustment for vitamin D, they did report subgroup analysis stratified by race. Here, they report that each 10 ng/ml lower vitamin D was associated with an odds ratio of 1.11 (95% confidence interval: 0.81 to 1.53) for PAD among African Americans, while the same comparison provided an odds ratio of 1.39 (95% confidence interval: 1.16 to 1.66) among Caucasians. Thus, the association was weaker in African Americans. This suggests, but does not prove, that vitamin D is unlikely to account for the higher prevalence of PAD among African Americans in isolation. However, the p values for interaction by race were not statistically significant in this study, so the weaker association in African Americans may reflect differences by chance, and therefore, definitive evaluation of this question requires future study.
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References
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- Ix JH, Allison MA, Denenberg JO, Cushman M, Criqui MH. Novel cardiovascular risk factors do not completely explain the higher prevalence of peripheral arterial disease among African Americans. The San Diego Population study. J Am Coll Cardiol 2008;51:2347-2354.[Abstract/Free Full Text]
- Kant AK, Graubard BI. Ethnicity is an independent correlate of biomarkers of micronutrient intake and status in American adults J Nutr 2007;137:2456-2463.[Abstract/Free Full Text]
- de Boer IH, Tinker LF, Connelly S, et al. Calcium plus vitamin D supplementation and the risk of incident diabetes in the Women's Health Initiative Diabetes Care 2008;31:701-707.[Abstract/Free Full Text]
- Melamed ML, Muntner P, Michos ED, et al. Serum 25-hydroxyvitamin D levels and the prevalence of peripheral arterial disease: results from NHANES 2001 to 2004 Arterioscler Thromb Vasc Biol 2008;28:1179-1185.[Abstract/Free Full Text]
Related Article
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Lower Vitamin D Status May Explain the Higher Prevalence of Peripheral Arterial Disease Among African Americans
- William B. Grant
J. Am. Coll. Cardiol. 2008 52: 1432.
[Full Text]
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