97 to 98 percent of the individuals in the group (therefore, for B₁₂ the RDA is 120 percent of the EAR).

Because 10 to 30 percent of people older than 50 years are estimated to have atrophic gastritis with low stomach acid secretion (Andrews et al., 1967; Hurwitz et al, 1997; Johnsen et al., 1991; Krasinski et al., 1986), they may have decreased bioavailability of B₁₂ from food. Therefore, because of the high prevalence of this condition, 50 percent bioavailability of dietary B₁₂ (see Box 9-2) cannot be assumed for this age group, and the EAR would be higher than 2.0 µg. Similarly, 2.4 µg of B₁₂, which is the RDA for younger adults, might not meet the needs of 97 percent of this large age group. There is not sufficient information on which to base a bioavailability correction factor for persons with atrophic gastritis who obtain their B₁₂ from animal foods. However, because the bioavailability of crystalline B₁₂ is not altered in people with atrophic gastritis, the same EAR and RDA would apply if the dietary sources of B₁₂ were foods fortified with B₁₂, supplements, or a combination of both.

The EAR and RDA for B₁₂ for adults ages 51 years and older are the same as for younger adults but with the recommendation that B₁₂-fortified foods (such as fortified ready-to-eat cereals) or B₁₂-containing supplements be used to meet much of the requirement.