Vitamin B₁₂ All naturally occurring vitamin B₁₂ is produced by microorganisms. The dietary source of the vitamin for human beings is animal products, such as meat, fish, poultry, and eggs. A deficiency in vitamin B₁₂ may result in megaloblastic anemia, the release of fewer but larger-than-average red blood cells into circulation. Symptoms include fatigue, shortness of breath, and numbness in the extremities.