呼吸道感染儿童补充维生素D以改善治疗效果
生物学、行为学和背景依据
急性下呼吸道感染（主要是肺炎）是全球各区域5岁以下儿童死亡的主要原因。1 据估计,2007年全球约有900万儿童死亡，其中近20%（180万）死于肺炎。营养不良增加急性下呼吸道感染的严重程度和总患病率，同时也是重症呼吸道感染死亡率的重要决定因素。1–4.
较差的营养状况,连同落后的社会经济状况、种族、不完善的免疫接种、接触烟草、空气污染和其它基础慢性疾病或婴儿早产一起，被公认为儿童早期急性下呼吸道感染的易感因素。sup>5–7. 已经提出几项微量营养素干预措施用于保护和预防儿童发生急性下呼吸道感染。在许多地区，儿童维生素D缺乏与急性下呼吸道感染风险密切相关。8–13. 例如，研究人员发现，在埃塞俄比亚42%的住院肺炎患儿患有佝偻病，或严重维生素D缺乏症。14.
维生素D是一类脂溶性分子，是维持人体健康非常重要的微量营养素。维生素D2和维生素D3都可从膳食中获取15, 16 但含量相对较低。仅深海脂质鱼类(如鲑鱼、鲔鱼、沙丁鱼)或鱼肝油含有大量维生素D3。大部分维生素D3通过人体皮肤暴露于阳光中紫外线而合成。在某些国家，强化食品（如麦片、乳酪和牛奶）是儿童维生素D的重要来源，尽管这些食品中维生素D的含量低且不稳定。15, 16, 17. 成人中10%-20%的维生素D储量来自膳食,儿童中这一比例甚至更低。18.
在冬季，由于日照时间减少、日光照射角度变化、皮肤裸露面积减少，人体合成的维生素D自然会减少，成人和儿童中发生的急性下呼吸道感染也更频繁。19, 20 一般认为，维生素D在免疫系统调节过程中起重要作用,可能有助于防止感染，21, 22, 也有助于防止癌症、心血管疾病和自身免疫疾病（如1型糖尿病）23–26。补充维生素D可能降低上述病症以及急性下呼吸道感染等的发病率，以及减少这些病症引起的不良后果，同时也可降低全因死亡率27, 28。
迄今为止,关于治疗或预防各类急性下呼吸道感染的营养干预措施文献报道很少。在阿富汗开展的一项随机对照试验发现,1-36月龄的住院肺炎患儿补充大剂量维生素D3（结合抗生素治疗）后，的确降低了生活在维生素D高度缺乏地区儿童肺炎的复发。28.
仍需在不同地区开展进一步的研究以证实上述发现，尤其应对生活在富裕地区不存在维生素D缺乏的儿童开展研究。生活在高纬度地区、缺乏日光照射的儿童以及肤色较黑的儿童发生维生素D缺乏以及较严重类型的急性下呼吸道感染的风险最高29, 30。 不过,衡量儿童中维生素D是否充足的阈值和推荐的每日摄取量仍存在争论之中13, 31, 32。美国儿科学会近期建议,自出生后不久起每日补充400IU（国际单位）维生素D，并一直持续到儿童期和青春期22。在不能提供维生素D3补充的地区，建议多晒太阳以促进维生素D3合成的同时，必须注意不要过度暴露于紫外线照射33。 需要开展进一步研究以明确维生素D在儿童感染和免疫功能中的作用34。
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