vitamin B12 Any member of a group of cobalamin vitamers that exhibit biological activity against vitamin B12 deficiency. Vitamin B12 deficiency is associated with low red blood cell count and anemia. The vitamers are found in foods such as cereals, meat, fish, and poultry. The vitamers include adenosylcobalamin, hydroxocobalamin, cyanocobalamin, aquacobalamin, nitritocobalamin and methylcobabalamin (also includes their ionized, salt and phosphate derivatives). Any member of a group of cobalamin vitamers that exhibit biological activity against vitamin B<small>₁₂</small> deficiency. Vitamin B<small>₁₂</small> deficiency is associated with low red blood cell count and anemia. The vitamers are found in foods such as cereals, meat, fish, and poultry. The vitamers include adenosylcobalamin, hydroxocobalamin, cyanocobalamin, aquacobalamin, nitritocobalamin and methylcobabalamin (also includes their ionized, salt and phosphate derivatives). Langual documentation note: DIRECTIVE 2002/46/EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 10 June 2002 on the approximation of the laws of the Member States relating to food supplements [http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2002:183:0051:0057:EN:PDF]. B vitamin