Grant William B
Sunlight, Nutrition, and Health Research Center; San Francisco, CA USA.
Dermatoendocrinol. 2012 Apr 1;4(2):198-202. doi: 10.4161/derm.20514.
Evidence continues to mount that vitamin D reduces the risk and mortality rates of many types of disease. However, evidence from prospective cohort studies is sometimes weaker than that from case-control and ecological studies. A suggested reason for this discrepancy is that, because serum levels of 25-hydroxyvitamin D [25(OH)D] change over time, a single 25(OH)D concentration measurement taken at study enrollment does not reliably indicate 25(OH)D concentration related to the health outcome. To evaluate this suggestion further, this paper plots results from 12 prospective cohort studies of all-cause mortality rate vs. follow-up time. The regression fit to the hazard ratio per 20-nmol/l increase in serum 25(OH)D concentration vs. time increased from 0.82 (95% CI, 0.67-1.02) for 6 y to 0.96 (95% CI, 0.90-1.01) for 14 y. The value extrapolated for zero follow-up time was 0.72 (95% CI, 0.50-1.03), giving a hazard ratio reduction 3.5 times higher than the standard result from the meta-analysis [0.92 (95% CI, 0.89-0.95)]. Using the example of the Vitamin D Pooling Project of Rarer Cancers, this paper also discusses follow-up time's effect in interpreting prospective cohort studies of cancer outcome. This paper recommends that meta-analyses of prospective cohort studies account for follow-up time and, if possible, that studies measure serum 25(OH)D concentration every 2-4 y.
越来越多的证据表明，维生素D可降低多种疾病的风险和死亡率。然而，前瞻性队列研究的证据有时比病例对照研究和生态学研究的证据更弱。造成这种差异的一个可能原因是，由于血清25-羟基维生素D [25(OH)D] 水平会随时间变化，在研究入组时进行的单次25(OH)D浓度测量并不能可靠地表明与健康结果相关的25(OH)D浓度。为了进一步评估这一观点，本文绘制了12项全因死亡率前瞻性队列研究的结果与随访时间的关系图。血清25(OH)D浓度每增加20 nmol/l时，危险比随时间的回归拟合从6年时的0.82（95%CI，0.67-1.02）增加到14年时的0.96（95%CI，0.90-1.01）。零随访时间外推值为0.72（95%CI，0.50-1.03），危险比降低幅度比荟萃分析的标准结果 [0.92（95%CI，0.89-0.95）] 高3.5倍。本文还以罕见癌症维生素D汇总项目为例，讨论了随访时间在解释癌症结局前瞻性队列研究中的作用。本文建议，前瞻性队列研究的荟萃分析应考虑随访时间，并且如果可能的话，研究应每2-4年测量一次血清25(OH)D浓度。
