Vitamin B12 or cyanocobalamin is a vitamin found exclusively in animal products. Although it is found in small amounts on vegetables that have some dirt on them and algae, there is not enough B12 found in the plant kingdom to maintain health over a lifespan. This is to say that vegans and even vegetarians should supplement with this vitamin consuming at least 3,000 micrograms per week. Vitamin B12 is actually a by-product of bacteria found in soil. Once it is consumed by animals, it is stored in the ileum (a section of the small intestine). Vitamin B12 is a water-soluble vitamin meaning it doesn't store for long periods of time. However, this vitamin is able to store quite well for a water-soluble vitamin. Some have said that you can be without vitamin B12 for up to 5 years without showing any signs of deficiency.
What does vitamin B12 do? Vitamin B12 is involved in the formation of DNA, red blood cells, and nerve health. Vitamin B12 also prevents the formation of a harmful chemical called homocysteine. High levels of homocysteine in the body has been linked to poor heart health and inflammation. Oddly the body creates this harmful chemical naturally, requiring an exogenous source to blunt its creation. Some signs of vitamin B12 deficiency include pernicious anemia, tingling, numbness, mental fog, heart palpitations, vision loss, etc. Vitamin B12 is found in somewhat adequate amounts in meats, eggs, and dairy.
How did we get vitamin B12 in our diet as early humans? If you are familiar with early humans, you know that early humans (in most areas of the world) did not eat an abundance of meat and dairy. These people obtained their vitamin B12, which again is actually a by-product of bacteria, from unwashed vegetables, fruits, and spring water. Now that we thoroughly inoculate our food supply, these bacteria are no longer present in our food.
There are several types of vitamin B12, but the most absorbable forms are methylcobalamin hydroxycobalamin and cyanocobalamin. Also, many alternative dairy products are fortified with vitamin B12 and vitamin D. Consult with your physician to find out which form is best for you to supplement.