<International Circulation>: You contributed to one of the studies related to mortality and you showed greater mortality amongst Caucasians who had vitamin D deficiency due to stroke， but this increased mortality was not evident among African-Americans?
《国际循环》：您参与的一项关于死亡率的研究显示由卒中所致维生素D缺乏的白种人其死亡率是增加的，但是这种死亡率的增加并没有在非裔美国人中得到证实，您认为是什么原因呢？
Dr Michos: One of the problems with the NHANES Study is that they did not collect incident events like heart attacks and strokes that were not fatal. NHANES was merged with the National Death Index. I only had information about deaths， not non-fatal events. We first had a paper where we looked at all-cause mortality， and we showed that if you were in the bottom quartile (levels <18 mg/dl)， you had a 26% increased risk of all-cause mortality over nine years of follow-up after we had adjusted for extensive covariant adjustments and physical activity and socio-economic status. I was really interested in stroke and racial differences because we know that blacks have lower vitamin D levels than Whites due to the increased melanin in their skin and blacks also seem to have a higher stroke rate. This is not fully explained by traditional factors such as hypertension and diabetes， so I was concerned that low vitamin D levels may explain some of the excess hazard of stroke in blacks. We were only able to look at fatal stroke with the added limitation of being based on death certificate coding which has some problems with misclassification. In the overall population， we saw that low vitamin D levels do predict stroke and when we tried to break it down by race， we saw the signal in whites， but it was not significant in blacks. The question is whether it was just a power issue， because when we tested for an interaction between the two races， there was no significant racial interaction. It may have just been a power issue in that there were not that many fatal strokes diagnosed on death certificates. There were only sixty cases of fatal strokes amongst blacks. It may have been underpowered to look at that racial difference.
Dr Michos: 你说的是NHANES研究。这个研究的问题是他们并没有搜集发生的事件像非致命性心脏病发作和卒中。NHANES研究和全国死亡指数相一致。我只获得了死亡的资料，而不是非致命性事件。我们在第一篇文章中观察了全因死亡率，并且我们的结果显示如果你在四分位数以下(维生素D水平 <18)，在9年的随访后我们对数据经过大量的协变量调整和体育活动以及社会地位的矫正，发现这类患者全因死亡率的风险增加26%。我对卒中和种族差异非常感兴趣，因为我们知道由于皮肤中的黑色素更多，黑人比白人的维生素D水平更低，并且黑人似乎发生卒中的几率更高。这种现象不能被传统因素如高血压和糖尿病所完全解释，因此我认为低维生素D水平可能部分解释为何黑人卒中风险更高。由于我们只能研究基于死亡证书编码的致命性卒中，这就会有错误分类的可能。在总体人群中，我们发现低维生素D水平确实能预测卒中，但是当我们试着按不同的种族来重新分析，就会发现这种预测在白人中依然存在而在黑人中并不明显。问题在于这个现象是不是一个重要的结果，因为当我们研究两个种族之间的关系时，发现它们之间并没有明显的种族联系。当在黑人中只有60例致死性卒中而在死亡证书的诊断上没有那么多的时候这就可能是个重要的问题。可能是我们对种族差异的研究还不够透彻。
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