As indicated in Chapter 9, it is possible to estimate the length of time body stores of vitamin B₁₂ held in reserve will maintain adequate nutriture when dietary intake is nonexistent as a result of adopting a vegan or similar diet very low in B₁₂ or when decreased absorption of vitamin B₁₂ occurs as a result of atrophic gastritis or pernicious anemia.

This estimate is based on being able to ascertain what the expected turnover rate will be (0.1, 0.15, and 0.2 percent have been found experimentally to occur). It has been estimated that in individuals with normal absorption and reabsorption rates for vitamin B₁₂, the daily turnover is 0.1 percent per day. For individuals with pernicious anemia, who cannot absorb or reabsorb vitamin B₁₂, the turnover rate is about 0.2 percent of the body pool or stores per day. As individuals develop various degrees of atrophic gastritis, it is possible that a turnover rate of 0.15 percent is appropriate.

To estimate how long body stores of vitamin B₁₂ can be depended on to maintain health, it is also necessary to know the lowest pool size of vitamin B₁₂ consistent with health, which could be considered to be the threshold before which signs of inadequate B₁₂ would begin to occur. As is indicated in Chapter 9, this has been estimated to be approximately 300 μg (Bozian et al., 1963).

By calculating the ratio of the total body stores of vitamin B₁₂ to this assumed threshold (or to a lower threshold if expected because of differences in individual requirements), it is possible to estimate how long body stores will meet the needs of the individual.

Table N-1 provides the ratio of the expected stores of an individual (in milligrams) to the threshold level of stores at which signs of vitamin B₁₂ deficiency may appear. Usually the threshold value is not known, but studies suggest that it may be approximately 300 μg of vitamin B₁₂ for adults (Bozian et al., 1963).

TABLE N-1

Table of Ratios Used to Estimate the Extent of Protection from Vitamin B₁₂ Stores.

Tables N-2 and N-3 give the expected length in days (Table N-2) or years (Table N-3) for a given turnover rate and ratio from Table N-1 that body stores of vitamin B₁₂ will sustain health in the individual. For example, from Table N-1 the ratio for an initial store of 3 mg of vitamin B₁₂ and a threshold of 300 μg of vitamin B₁₂ is 10. If the turnover rate is 0.1, the store would be expected to last 2,303 days, or 6.3 years.

TABLE N-2

Time in Days until Vitamin B₁₂ Threshold Is Reached.

TABLE N-3

Table of Time in Years until Vitamin B₁₂ Threshold Below Which Deficiency Signs May Occur Is Reached.