Department of Cell Biology and Anatomy, Medical School, University of Tampere, Medisiinarinkatu 3, 33014 Tampere, Finland.
Proc Nutr Soc. 2012 Feb;71(1):90-7. doi: 10.1017/S0029665111003351. Epub 2011 Nov 25.
Vitamin D insufficiency is a global issue that has significant implications for health. The classical role of vitamin D in bone mineralisation is well known; vitamin D deficiency leads to rickets, osteomalacia or osteoporosis. The role of vitamin D in an immune system is less known. Vitamin D is not an actual vitamin but a secosteroid hormone produced in the skin from 7-dehydrocholesterol after exposure to sunlight UVB radiation. Nutrition and supplements are main sources of vitamin D in wintertime in northern countries as sunlight exposure is inadequate for the production. For activation vitamin D needs to be hydroxylated in liver to form 25-hydroxyvitamin D and in kidney to 1,25-dihydroxyvitamin D, the most active hormone in Ca absorption in the gut. For determination of vitamin D status serum 25-hydroxyvitamin D level, the major circulating form of the hormone is to be measured. Vitamin D regulates gene expression through binding with vitamin D receptors, which dimerises with retinoid X receptor. This complex binds to vitamin D-responsive elements inside the promoter regions of vitamin D-responsive genes. Vitamin D has a key role in innate immunity activation; the production of antimicrobial peptides (cathelicidin and defensins) following Toll-like receptor stimulation by pathogen lipopeptides is dependent on sufficient level of 25-hydroxyvitamin D. Clinically, there is evidence of the association of vitamin D insufficiency and respiratory tract infections. There is also some evidence of the prevention of infections by vitamin D supplementation. Randomised controlled trials are warranted to explore this preventive effect.
维生素 D 不足是一个全球性问题，对健康有重大影响。维生素 D 在骨骼矿化中的经典作用众所周知；维生素 D 缺乏会导致佝偻病、骨软化症或骨质疏松症。维生素 D 在免疫系统中的作用鲜为人知。维生素 D 不是一种真正的维生素，而是一种在皮肤中由 7-脱氢胆固醇在暴露于阳光紫外线 B 辐射后产生的甾体激素。在北方国家的冬季，营养和补充剂是维生素 D 的主要来源，因为阳光照射不足以产生维生素 D。为了激活，维生素 D 需要在肝脏中羟化为 25-羟维生素 D，在肾脏中羟化为 1,25-二羟维生素 D，这是肠道中钙吸收的最活跃激素。为了确定维生素 D 状态，需要测量血清 25-羟维生素 D 水平，这是激素的主要循环形式。维生素 D 通过与维生素 D 受体结合来调节基因表达，维生素 D 受体与视黄酸 X 受体二聚化。这种复合物结合到维生素 D 反应基因启动子区域内的维生素 D 反应元件中。维生素 D 在先天免疫激活中起着关键作用；在病原体脂肽刺激 Toll 样受体后，抗菌肽（cathelicidin 和 defensins）的产生依赖于足够水平的 25-羟维生素 D。临床上，有证据表明维生素 D 不足与呼吸道感染有关。也有一些证据表明维生素 D 补充可以预防感染。有必要进行随机对照试验来探索这种预防作用。
