University of Lorraine, INSERM UMR_S 1256, Nutrition, Genetics, and Environmental Risk Exposure (NGERE), Faculty of Medicine of Nancy, Nancy, France; Department of Biochemistry, Molecular Biology, Nutrition, and Metabolism, University Hospital of Nancy, Nancy, France; Department of Hepato-Gastroenterology, University Hospital of Nancy, Nancy, France.
University of Lorraine, INSERM UMR_S 1256, Nutrition, Genetics, and Environmental Risk Exposure (NGERE), Faculty of Medicine of Nancy, Nancy, France; Department of Biochemistry, Molecular Biology, Nutrition, and Metabolism, University Hospital of Nancy, Nancy, France.
Vitam Horm. 2022;119:241-274. doi: 10.1016/bs.vh.2022.01.016. Epub 2022 Mar 1.
Vitamin B12 is assimilated and transported by complex mechanisms that involve three transport proteins, intrinsic factor (IF), haptocorrin (HC) and transcobalamin (TC) and their respective membrane receptors. Vitamin deficiency is mainly due to inadequate dietary intake in vegans, and B12 malabsorption is related to digestive diseases. This review explores the physiology of vitamin B12 absorption and the mechanisms and diseases that produce malabsorption. In the stomach, B12 is released from food carrier proteins and binds to HC. The degradation of HC by pancreatic proteases and the pH change trigger the transfer of B12 to IF in the duodenum. Cubilin and amnionless are the two components of the receptor that mediates the uptake of B12 in the distal ileum. Part of liver B12 is excreted in bile, and undergoes an enterohepatic circulation. The main causes of B12 malabsorption include inherited disorders (Intrinsic factor deficiency, Imerslund-Gräsbeck disease, Addison's pernicious anemia, obesity, bariatric surgery and gastrectomies. Other causes include pancreatic insufficiency, obstructive Jaundice, tropical sprue and celiac disease, bacterial overgrowth, parasitic infestations, Zollinger-Ellison syndrome, inflammatory bowel diseases, chronic radiation enteritis of the distal ileum and short bowel. The assessment of B12 deficit is recommended in the follow-up of subjects with bariatric surgery. The genetic causes of B12 malabsorption are probably underestimated in adult cases with B12 deficit. Despite its high prevalence in the general population and in the elderly, B12 malabsorption cannot be anymore assessed by the Schilling test, pointing out the urgent need for an equivalent reliable test.
维生素 B12 通过涉及三种转运蛋白（内因子 (IF)、触珠蛋白 (HC) 和转钴胺素 (TC) 及其各自的膜受体）的复杂机制被吸收和转运。维生素缺乏主要是由于素食者饮食摄入不足，B12 吸收不良与消化疾病有关。本文探讨了维生素 B12 吸收的生理学以及导致吸收不良的机制和疾病。在胃中，B12 从食物载体蛋白中释放出来并与 HC 结合。HC 被胰腺蛋白酶降解以及 pH 值的变化触发了 B12 在十二指肠向 IF 的转移。Cubilin 和 Amnionless 是介导 B12 在回肠远端摄取的受体的两个组成部分。部分肝脏 B12 随胆汁排出，并经历肠肝循环。B12 吸收不良的主要原因包括遗传性疾病（内因子缺乏、肠病性肢端皮炎、艾迪生病性贫血、肥胖、减重手术和胃切除术。其他原因包括胰腺功能不全、阻塞性黄疸、热带口炎性腹泻和乳糜泻、细菌过度生长、寄生虫感染、卓-艾综合征、炎症性肠病、回肠末端慢性放射性肠炎和短肠综合征。建议对减重手术的患者进行 B12 缺乏的评估。成人 B12 缺乏症患者中，B12 吸收不良的遗传原因可能被低估。尽管 B12 吸收不良在普通人群和老年人中普遍存在，但不能再通过 Schilling 试验进行评估，这突显了迫切需要等效可靠的试验。
