Complex sentence: Foods fortified with B 12 are also sources of the vitamin although they cannot be regarded as true food sources of B 12 since the vitamin is added in supplement form, from commercial bacterial production sources, such as cyanocobalamin.
Split into simpler sentences: Foods fortified with B 12 are also sources of the vitamin 12. The vitamin is added in supplement form, from commercial bacterial production sources, such as cyanocobalamin.