Abstract

Vitamin B₁₂ (cobalamin) is an essential cofactor for two metabolic pathways. It is obtained principally from food of animal origin. Cobalamin becomes bioavailable through a series of steps pertaining to its release from dietary protein, intrinsic factor-mediated absorption, haptocorrin or transcobalamin-mediated transport, cellular uptake, and two enzymatic conversions (via methionine synthase and methylmalonyl-CoA-mutase) into cofactor forms: methylcobalamin and adenosylcobalamin. Vitamin B₁₂ deficiency can masquerade as a multitude of illnesses, presenting different perspectives from the point of view of the hematologist, neurologist, gastroenterologist, general physician, or dietician. Increased physician vigilance and heightened patient awareness often account for its early presentation, and testing sometimes occurs during a phase of vitamin B₁₂ insufficiency before the main onset of the disease. The chosen test often depends on its availability rather than on the diagnostic performance and sensitivity to irrelevant factors interfering with vitamin B₁₂ markers. Although serum B₁₂ is still the most commonly used and widely available test, diagnostics by holotranscobalamin, serum methylmalonic acid, and plasma homocysteine measurements have grown in the last several years in routine practice. The lack of a robust absorption test, coupled with compromised sensitivity and specificity of other tests (intrinsic factor and gastric parietal cell antibodies), hinders determination of the cause for depleted B₁₂ status. This can lead to incorrect supplementation regimes and uncertainty regarding later treatment. This review discusses currently available knowledge on vitamin B₁₂, informs the reader about the pitfalls of tests for assessing its deficiency, reviews B₁₂ status in various populations at different disease stages, and provides recommendations for interpretation, treatment, and associated risks. Future directions for diagnostics of B₁₂ status and health interventions are also discussed.

摘要

维生素 B ₁₂（钴胺素）是两种代谢途径必不可少的辅助因子。它主要从动物源性食物中获得。钴胺素通过一系列与从膳食蛋白质中释放、内因子介导的吸收、haptocorrin 或转钴胺素介导的转运、细胞摄取和两种酶促转化（通过甲硫氨酸合酶和甲基丙二酰辅酶 A-变位酶）转化为辅因子形式有关的步骤变得生物可利用：甲基钴胺素和腺苷钴胺素。维生素 B ₁₂缺乏症可以伪装成多种疾病，从血液学家、神经学家、胃肠病学家、全科医生或营养师的角度呈现不同的观点。医生警惕性的提高和患者意识的提高往往是其早期表现的原因，有时在疾病主要发作之前维生素 B ₁₂不足的阶段进行检测。选择的测试通常取决于其可用性，而不是诊断性能和对干扰维生素 B ₁₂标志物的无关因素的敏感性。虽然血清 B ₁₂仍然是最常用和广泛可用的测试，通过全反钴胺素、血清甲基丙二酸和血浆同型半胱氨酸测量的诊断在过去几年中在常规实践中得到了发展。缺乏可靠的吸收测试，加上其他测试（内在因子和胃壁细胞抗体）的敏感性和特异性受损，阻碍了确定 B ₁₂耗尽状态的原因。这可能导致不正确的补充制度和以后治疗的不确定性。这篇综述讨论了目前关于维生素 B _{12 的}可用知识，向读者介绍了评估其缺乏的测试的陷阱，评论 B ₁₂在不同疾病阶段的不同人群中的状态，并为解释、治疗和相关风险提供建议。还讨论了B ₁₂状态诊断和健康干预的未来方向。