Holick Michael F, Chen Tai C, Lu Zhiren, Sauter Edward
Department of Medicine, Section of Endocrinology, Nutrition, and Diabetes, Vitamin D, Skin, and Bone Research Laboratory, Boston University Medical Center, Boston, Massachusetts, USA.
J Bone Miner Res. 2007 Dec;22 Suppl 2:V28-33. doi: 10.1359/jbmr.07s211.
Throughout evolution, exposure to sunlight and the photosynthesis of vitamin D(3) in the skin has been critically important for the evolution of land vertebrates. During exposure to sunlight, the solar UVB photons with energies 290-315 nm are absorbed by 7-dehydrocholesterol in the skin and converted to previtamin D(3). Previtamin D(3) undergoes a rapid transformation within the plasma membrane to vitamin D(3). Excessive exposure to sunlight will not result in vitamin D intoxication because both previtamin D(3) and vitamin D(3) are photolyzed to several noncalcemic photoproducts. During the winter at latitudes above approximately 35 degrees , there is minimal, if any, previtamin D(3) production in the skin. Altitude also has a significant effect on vitamin D(3) production. At 27 degrees N in November, very little ( approximately 0.5%) previtamin D(3) synthesis was detected in Agra (169 m) and Katmandu (1400 m). There was an approximately 2- and 4-fold increase in previtamin D(3) production at approximately 3400 m and at Everest base camp (5300 m), respectively. Increased skin pigmentation, application of a sunscreen, aging, and clothing have a dramatic effect on previtamin D(3) production in the skin. It is estimated that exposure in a bathing suit to 1 minimal erythemal dose (MED) is equivalent to ingesting between 10,000 and 25,000 IU of vitamin D(2). The importance of sunlight for providing most humans with their vitamin D requirement is well documented by the seasonal variation in circulating levels of 25-hydroxyvitamin D [25(OH)D]. Vitamin D deficiency [i.e., 25(OH)D < 20 ng/ml] is common in both children and adults worldwide. Exposure to lamps that produce UVB radiation is an excellent source for producing vitamin D(3) in the skin and is especially efficacious in patients with fat malabsorption syndromes. The major cause of vitamin D deficiency globally is an underappreciation of sunlight's role in providing humans with their vitamin D(3) requirement. Very few foods naturally contain vitamin D, and those that do have a very variable vitamin D content. Recently it was observed that wild caught salmon had between 75% and 90% more vitamin D(3) compared with farmed salmon. The associations regarding increased risk of common deadly cancers, autoimmune diseases, infectious diseases, and cardiovascular disease with living at higher latitudes and being prone to vitamin D deficiency should alert all health care professionals about the importance of vitamin D for overall health and well being.
在整个进化过程中，暴露于阳光以及皮肤中维生素D(3)的合成对陆地脊椎动物的进化至关重要。暴露于阳光下时，能量为290 - 315纳米的太阳中波紫外线光子被皮肤中的7 - 脱氢胆固醇吸收，并转化为维生素D原(3)。维生素D原(3)在质膜内迅速转化为维生素D(3)。过度暴露于阳光不会导致维生素D中毒，因为维生素D原(3)和维生素D(3)都会光解为几种无钙活性的光产物。在冬季，纬度高于约35度的地区，皮肤中即使有维生素D原(3)生成，量也极少。海拔高度对维生素D(3)的生成也有显著影响。11月在北纬27度时，在阿格拉（169米）和加德满都（1400米）检测到的维生素D原(3)合成量非常少（约0.5%）。在约3400米和珠穆朗玛峰大本营（5300米），维生素D原(3)的生成量分别增加了约2倍和4倍。皮肤色素沉着增加、涂抹防晒霜、衰老和衣物穿着对皮肤中维生素D原(3)的生成有显著影响。据估计，穿着泳衣暴露于1个最小红斑剂量（MED）的阳光相当于摄入10,000至25,000国际单位的维生素D(2)。25 - 羟基维生素D [25(OH)D]循环水平的季节性变化充分证明了阳光对大多数人满足维生素D需求的重要性。维生素D缺乏症[即25(OH)D < 20纳克/毫升]在全球儿童和成人中都很常见。暴露于产生中波紫外线辐射的灯具是皮肤中产生维生素D(3)的极佳来源，对脂肪吸收不良综合征患者尤其有效。全球维生素D缺乏的主要原因是对阳光在满足人类维生素D(3)需求方面作用的认识不足。很少有食物天然含有维生素D，而且即使含有，其维生素D含量也差异很大。最近观察到，野生捕获的三文鱼比养殖三文鱼的维生素D(3)含量多百分之75至90。关于生活在高纬度地区且易患维生素D缺乏症会增加患常见致命癌症、自身免疫性疾病、传染病和心血管疾病风险的关联，应提醒所有医护人员注意维生素D对整体健康和幸福的重要性。
