Vitamin B12 in our diet comes almost exclusively from meat, eggs, dairy products and some seafood. The stomach secretes intrinsic factor which is a protein necessary for vitamin B12 absorption in the small intestine. Since it is a water soluble vitamin, we would think that it would get excreted in our urine when consumed in excess. However, it has a unique feature of being stored in the liver which is important for anyone who’s consuming little vitamin B12 in the diet.
The RDA for vitamin B12 for adult men and women aged 19 years and older is 2.4 micrograms a day. Individuals consuming a vegan diet need to eat vegetable based foods that are fortified with vitamin B12 or take supplements to ensure that they maintain adequate blood levels of this nutrient.
A vitamin B12 deficiency is rare, but is associated with dietary insufficiency or reduced absorption. A deficiency of B12, like folate, is associated with cardiovascular disease due to high levels of homocysteine. Pernicious anemia is a condition when reduced absorption of vitamin B12 happens. It’s caused by inadequate secretion of intrinsic factor by parietal cells of the stomach.
Source: Thompson, Janice, Melinda Manore, and Linda A. Vaughan. The Science of Nutrition. San Francisco, CA: Pearson Benjamin Cummings, 2011. Print.