The Recommended Daily Allowance, or RDA, is 2.4 micrograms a day. The average adult diet in the U.S. provides about four to eight micrograms a day. Certain groups are especially at risk for vitamin B12 deficiency. Some people over age 50 make less gastric acid and pepsin in their stomachs and therefore do not absorb enough vitamin B12 from food. This is called atrophic gastritis.
Older people have an overgrowth of bacteria in their stomachs.
The bacteria that are usually killed by stomach acid compete for the vitamin B12 that is available and take it for their own use. The elderly and strict vegetarians should eat fortified cereals and soy foods, or take a supplement, to get enough vitamin B12. If you have a deficiency of vitamin B12, or are at high risk for a deficiency because of your age or diet, the National Academy of Sciences recommends taking a supplement or eating fortified breakfast cereals to make sure you absorb enough vitamin B12. There is no need to boost with extra vitamin B12 for energy. Up to levels of about 100 micrograms, vitamin B12 seems to be safe when taken orally.