http://www.nytimes.com/2011/11/29/health/vitamin-b12-deficiency-can-cause-symptoms-that-mimic-aging.html?src=me&ref=generalThose at Risk
Natural plant sources are meager at best in B12, and the vitamin is poorly absorbed from them. Many strict vegetarians and all vegans, as well as infants they breast-feed, must consume supplements or fortified breakfast cereals to get adequate amounts.
Certain organisms, like the bacterium Spirulina and some algae, contain a pseudo-B12 that the body cannot use but may result in a false reading of a normal B12 level on a blood test. Despite claims to the contrary, laver, a seaweed, and barley grass are not reliable sources of B12.
In animal foods, B12 is combined with protein and must be released by stomach acid and an enzyme to be absorbed. Thus, chronic users of acid-suppressing drugs like Prilosec, Prevacid and Nexium, as well as ulcer medications like Pepcid and Tagamet, are at risk of developing a B12 deficiency and often require a daily B12 supplement.
Stomach acid levels decline with age. As many as 30 percent of older people may lack sufficient stomach acid to absorb adequate amounts of B12 from natural sources. Therefore, regular consumption of fortified foods or supplementation with 25 to 100 micrograms of B12 daily is recommended for people over 50.