With no explanation, label text_A→text_B with either "not_related" or "related".
text_A: Vitamin B12 is not produced by humans.
text_B: Vitamin B12 , also called cobalamin , is a water-soluble vitamin that has a key role in the normal functioning of the brain and nervous system , and the formation of red blood cells .. cobalamin. cobalamin. vitamin. vitamin. brain. brain. nervous system. nervous system. It is one of eight B vitamins .. B vitamins. B vitamins. It is involved in the metabolism of every cell of the human body , especially affecting DNA synthesis , fatty acid and amino acid metabolism .. metabolism. metabolism. cell. cell ( biology ). DNA. DNA. fatty acid. fatty acid. amino acid. amino acid. No fungi , plants , or animals -LRB- including humans -RRB- are capable of producing vitamin B12 .. vitamin. vitamin. Only bacteria and archaea have the enzymes needed for its synthesis .. bacteria. bacteria. archaea. archaea. Some substantial sources of B12 include animal products -LRB- shellfish , meat -RRB- , fortified food products , and dietary supplements .. shellfish. shellfish. fortified food. fortified food. B12 is the largest and most structurally complicated vitamin and can be produced industrially only through bacterial fermentation synthesis , typically used to manufacture B12 for fortified foods and supplements .. vitamin. vitamin. bacterial fermentation. bacterial fermentation. Vitamin B12 consists of a class of chemically related compounds -LRB- vitamers -RRB- , all of which show pharmacological activity .. vitamers. vitamers. pharmacological activity. biological activity. It contains the biochemically rare element cobalt -LRB- chemical symbol Co -RRB- positioned in the center of a planar tetra-pyrrole ring called a corrin ring .. cobalt. cobalt. pyrrole. pyrrole. corrin. corrin. The vitamer is produced by bacteria as hydroxocobalamin , but conversion between different forms of the vitamin occurs in the body after consumption .. hydroxocobalamin. hydroxocobalamin. vitamin. vitamin. bacteria. bacteria. vitamer. vitamer. Vitamin B12 was discovered as a result of its relationship to the disease pernicious anemia , an autoimmune disease in which parietal cells of the stomach responsible for secreting intrinsic factor are destroyed ; these cells are also responsible for secreting acid in the stomach .. pernicious anemia. pernicious anemia. autoimmune disease. autoimmune disease. intrinsic factor. intrinsic factor. Because intrinsic factor is crucial for the normal absorption of B12 , its lack in the presence of pernicious anemia causes a vitamin B12 deficiency .. vitamin. vitamin. pernicious anemia. pernicious anemia. intrinsic factor. intrinsic factor. Many other subtler kinds of vitamin B12 deficiency and their biochemical effects have since been elucidated .. vitamin. vitamin. Due to impairment of vitamin B12 absorption during aging , people over age 60 are at risk of deficiency .. vitamin. vitamin
related.