Vitamin B12 (cobalamin) is an essential micronutrient and an important cofactor in the one-carbon metabolism. Vitamin B12 has been meticulously studied for decades, and much is known concerning the uptake and actions of this vitamin. However, several aspects still need to be clarified in order for us to understand its role in health and disease. In pursue of pieces for the puzzle, I have addressed three unresolved facets of the cobalamin metabolism in this work: The presence of vitamin B12-binding proteins in fish (Part A); Vitamin B12 during pregnancy and infancy (Part B); and the effects of metformin treatment on vitamin B12 metabolism (Part C).