Final answer:
Vegans should supplement with Vitamin B12 (Cobalamin) due to its absence in plant-based diets and its essential role in red blood cell formation, DNA synthesis, and metabolic functions.
Step-by-step explanation:
A vegan should supplement with Vitamin B12 (Cobalamin) as it is not found in plant-based foods. Vitamin B12 is essential for normal maturation and development of erythrocytes (red blood cells), synthesis of DNA, and for certain metabolic pathways such as the conversion of methyl malonyl-CoA to succinyl-CoA and the methylation of homocysteine to methionine. Unlike some other B vitamins, B12 is not typically found in notable amounts in plants, as neither plants nor animals can synthesize it; only certain microorganisms do. Therefore, it is common for vegans to have a risk of Vitamin B12 deficiency, which can lead to serious conditions such as megaloblastic anemia and neurological issues.