B12

Initially I was going to just write about B vitamins (since there are 8 of them), but there was too much on B12, so we'll save B6 for a later date. Why is B12 so important? A lot of the writing on B12 is about defiency and it's effects on the brain.
B12 is also called cobalamin. It is the largest and most complicated vitamin.
As a water-soluble vitamin, it accumulates and is stored in the liver and other tissues. It works with folic acid to synthesis DNA and red blood cells, and is vital for the nerve cell (myelin sheath) health. The richest sources of B12 are: liver (lambs especially) and kidney; eggs, cheese, and some kinds of fish.
Low levels of B12 are associated with: asthma, depression, AIDs, multiple sclerosis, tinnitus, diabetic neuropathy, and low serum counts. Worst of all B12 deficiency is associated with the development of neurological dysfunction-Alzheimer's. A study on elderly, showed that those with lower B12 levels had a brain shrinkage, which explains the link to Alzheimer's because dementia has been linked with brain shrinkage. Also, studies have shown that B12 levels tend to be lower in women with breast cancer. Not only does it have such an effect on the brain but also the heart. B12 deficiency increases homocysteine, which is a known risk factor for heart disease and stroke.
So who is at risk most for B12 deficiency. Studies have shown that elderly and vegetarians tend to be suseptible to lower levels of the vitamin. B12 deficiency is such a big deal to the elderly because of the link to Alzheimer's. Some believe this increase is due to a decline in the intrinsic factor needed to absorb the vitamin from the small intestine, that most elderly face. The link to vegetarians is easy, because they're not consuming the products in which B12 are found, they would have to supplement in order to get the daily recommended value.
Bottom-line on B12: eat a varied diet rich in fish, liver, and eggs. Getting the vitamin in your diet should be substantial to providing B12, but if you are not ingesting it through food, consider a B complex vitamin (or most multi-vitamins contain B12), and help prevent problems in the future (honestly who wants dementia!)