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The alphabet soup of essential vitamins can be confusing, but remind yourself to keep B12 on the menu. Also known as cobalamin, this critical nutrient acts on the central nervous and circulatory systems. A B12 shortage can bring on fatigue, anemia and depression. Synthetic B12 is available, but a healthy natural dose of B12 is available in seafood and protein-rich foods such as beef, liver, lamb, cheese and eggs.