Department of Nutritional Sciences, University of Wisconsin-Madison, Madison, WI 53706, USA.
Exp Biol Med (Maywood). 2021 May;246(9):1045-1053. doi: 10.1177/1535370221992731. Epub 2021 Mar 25.
Vitamin A is a fat-soluble vitamin involved in essential functions including growth, immunity, reproduction, and vision. The vitamin A Dietary Reference Intakes (DRIs) for North Americans suggested that a minimally acceptable total liver vitamin A reserve (TLR) is 0.07 µmol/g, which is not explicitly expressed as a vitamin A deficiency cutoff. The Biomarkers of Nutrition for Development panel set the TLR cutoff for vitamin A deficiency at 0.1 µmol/g based on changes in biological response of several physiological parameters at or above this cutoff. The criteria used to formulate the DRIs include clinical ophthalmic signs of vitamin A deficiency, circulating plasma retinol concentrations, excretion of vitamin A metabolites in the bile, and long-term storage of vitamin A as protection against vitamin A deficiency during times of low dietary intake. This review examines the biological responses that occur as TLRs are depleted. In consideration of all of the DRI criteria, the review concludes that induced biliary excretion and long-term vitamin A storage do not occur until TLRs are >0.10 µmol/g. If long-term storage is to continue to be part of the DRI criteria, vitamin A deficiency should be set at a minimum cutoff of 0.10 µmol/g and should be set higher during times of enhanced requirements where TLRs can be rapidly depleted, such as during lactation or in areas with high infection burden. In population-based surveys, cutoffs are important when using biomarkers of micronutrient status to define the prevalence of deficiency and sufficiency to inform public health interventions. Considering the increasing use of quantitative biomarkers of vitamin A status that indirectly assess TLRs, i.e. the modified-relative-dose response and retinol-isotope dilution tests, setting a TLR as a vitamin A deficiency cutoff is important for users of these techniques to estimate vitamin A deficiency prevalence. Future researchers and policymakers may suggest that DRIs should be set with regard to optimal health and not merely to prevent a micronutrient deficiency.
维生素 A 是一种脂溶性维生素，参与包括生长、免疫、生殖和视力在内的基本功能。北美人的维生素 A 膳食参考摄入量 (DRI) 建议，最小可接受的总肝维生素 A 储备 (TLR) 为 0.07µmol/g，这并没有明确表示为维生素 A 缺乏的截止值。营养生物标志物发展小组基于该截止值以上的几个生理参数的生物学反应变化，将 TLR 截止值设定为维生素 A 缺乏症的 0.1µmol/g。制定 DRI 的标准包括维生素 A 缺乏的临床眼科迹象、循环血浆视黄醇浓度、胆汁中维生素 A 代谢物的排泄以及长期储存维生素 A，以防止在低膳食摄入期间发生维生素 A 缺乏症。这篇综述检查了 TLR 耗尽时发生的生物学反应。考虑到所有 DRI 标准，综述得出的结论是，只有当 TLR 大于 0.10µmol/g 时，才会发生诱导性胆汁排泄和长期维生素 A 储存。如果长期储存要继续成为 DRI 标准的一部分，那么维生素 A 缺乏症的最低截止值应设定为 0.10µmol/g，并且在 TLR 可能迅速耗尽的增强需求期间（如哺乳期或高感染负担地区）应设定更高的截止值。在基于人群的调查中，当使用微量营养素状态的生物标志物来定义缺乏症和充足症的流行率以告知公共卫生干预措施时，截止值很重要。考虑到越来越多地使用间接评估 TLR 的维生素 A 状态的定量生物标志物，即改良相对剂量反应和视黄醇同位素稀释试验，设定 TLR 作为维生素 A 缺乏症的截止值对于使用这些技术的用户估计维生素 A 缺乏症的流行率很重要。未来的研究人员和政策制定者可能会建议，DRI 的设定应考虑到最佳健康，而不仅仅是为了预防微量营养素缺乏症。
