Overgrowth of bacteria: Some people develop vitamin B 12 deficiency as a result of conditions that slow the movement of food through the intestines (diabetes, scleroderma, strictures, diverticula), allowing intestinal bacteria to multiply and overgrow in the upper part of the small intestine
In the study by Gayatri et al
Anal Biochem 98(1):154159 Massa CM, Liu Z, Taylor S, Pettit AP, Stakheyeva MN, Korotkova E, Popova V, Atochina-Vasserman EN, Gow AJ (2021) Biological mechanisms of S-nitrosothiol formation and degradation: how is specificity of S-nitrosylation achieved
2017;109(8) (Epub 2017/04/05)
Pigmentation pigmentosa melanopsin wasn't discovered when