呼吸道感染儿童补充维生素A以改善治疗结局
生物学、行为学和背景依据
急性下呼吸道感染（尤其是细支气管炎和肺炎,是急性下呼吸道感染中最严重的类型）是5岁以下儿童的主要死亡原因1, 2. 仅肺炎每年就导致180万婴幼儿死亡 3. 这些死亡大多数是可预防的，其多发生于资源匮乏的地区，并与贫困、卫生保健条件不足和营养不良密切相关。
研究表明，某些营养干预措施可有效减少急性下呼吸道感染病例数量以及与肺炎相关的潜在致死性后果。维生素A/视黄醇参与红细胞、淋巴细胞和抗体的生成、生长和分化4, 并维持上皮组织的完整。由于维生素A已被证实可有效预防麻疹相关肺炎5, 已开展了关于补充维生素A作为一项可能的干预措施用于加快急性下呼吸道感染的康复、降低疾病严重程度以及预防继发感染的研究6–11. 但研究结果完全不一致。有些作者报道认为补充维生素并无益处6, 12–15, 而另外一些作者认为此项干预只在特定人群（如低体重儿童16 或既往维生素A缺乏者17. 中产生正面效果。另有研究发现，补充维生素A可导致急性下呼吸道感染的发病率升高，这主要见于营养素摄入状况较佳的儿童16, 18.
维生素A缺乏的儿童发生呼吸道感染并因而死亡的风险似乎更高19. 既往维生素A缺乏可加重感染的发生，补充维生素A可使6~59月龄儿童的死亡风险降低约23%~30%20. 对于麻疹相关肺炎病例，大剂量补充维生素A具备明显的保护效应21, 22. 然而对于急性下呼吸道感染病例，无论是大剂量还是小剂量补充维生素A均未观察到类似效应。例如，有报道称小剂量补充维生素A可降低呼吸道感染风险16, 而大剂量补充维生素A则可产生负面效应23. 研究人员提出了数种理论，试图解释相关结果的适用范围及其可能的生物学机制。对于维生素A储量充足的儿童，大剂量补充维生素A可引起免疫调节功能的一过性受损。这可能导致儿童更易罹患感染性疾病18.
近期有两项系统综述阐述了补充维生素A在预防儿童呼吸道感染方面的作用，并得出结论认为：只有那些营养状况不佳的儿童才需要补充维生素A1. 研究结果提示，在提出补充维生素A的建议时，应充分考虑其剂量和潜在的不良反应1, 24. 过量补充维生素A可导致恶心、呕吐、食欲减退等毒副反应，从而进一步减少营养素的摄入。一项研究评估了中等剂量维生素A的效应，发现其在维生素A摄入量充足的儿童中可产生正面效应，且未报告副作用9. 迄今尚无研究探讨过此类正面效应是否与含维生素A食物的摄入增加相关。在卫生资源不足,急性下呼吸道感染高发的地区3,维生素A含量较高的食物（如肝、奶、乳酪、蛋类等动物制品）或强化食品的摄入量通常较低。在这种情况下,需要通过开展膳食多样化以及家庭自产食物规划,增加富含维生素A的食物（如芒果和木瓜）的摄入量24–26.
参考文献
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