Shellfish, liver and mackerel contain the highest levels of vitamin B12 per 100 gram serving. Clams, oysters and mussels have the highest levels among shellfish. Salmon, herring and tuna also have significant levels. Like the other B vitamins, B12 is involved in energy metabolism and related biological processes.
Vitamin B12 is crucial for cardiovascular health, DNA production and neurological health. The production of red blood cells and prevention of the accumulation of homocysteine depend on sufficient levels of B12. Excessive levels of homocysteine are associated with coronary artery disease, stroke and peripheral vascular disease. Anemia results from an untreated B12 deficiency and causes easy bruising, excessive bleeding and fatigue. Milder symptoms of insufficient levels include rapid heartbeat and breathing, pale skin tone, upset stomach and other digestive problems.
Pernicious anemia, Crohn's or celiac disease, alcoholism and autoimmune disorders diminish the body's ability to utilize the nutritional content of a normal diet. In these cases, health care providers often prescribe vitamin B12 injections or supplements.
Strict vegan diets do not contain the appropriate levels of vitamin B12, and supplements are recommended to prevent deficiency. Vegetarian diets are less likely to result in a deficiency because of the B12 obtained from milk, eggs and cheese. Diets that include meat and fish are unlikely to result in a B12 deficiency unless an underlying medical condition is present.