The prevalence of vitamin B12 deficiency increases in the elderly, mainly because atrophic gastritis decreases the production of the acid and digestive enzymes needed to cleave protein-bound vitamin B12 from the natural chemical form of vitamin B12 found in meat, poultry, fish, and dairy foods. Depending on the biochemical criterion that is used, 5% to more than 20% of older adults have marginal or frank vitamin B12 deficiency. Adults over 50 years of age should meet the Recommended Dietary Allowance by consuming vitamin B12 in the crystalline form, which does not require gastric acid or enzymes for initial digestion. A recent clinical trial suggests that an oral dose of 500 microg/d of crystalline vitamin B12 is needed to reverse biochemical signs of vitamin B12 deficiency in older adults.