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N
Estimation of the Period Covered by Vitamin B12 Stores

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.



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DRI Dietary Reference Intakes: For Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline N Estimation of the Period Covered by Vitamin B12 Stores 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.

OCR for page 527
DRI Dietary Reference Intakes: For Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline TABLE N-1 Table of Ratiosa Used to Estimate the Extent of Protection from Vitamin B12 Stores   Initial Store, mg Vitamin B12 Threshold, µg 1 2 3 4 5 6 7 8 9 10 350 3 6 9 11 14 17 20 23 26 29 300 3 7 10 13 17 20 23 27 30 33 250 4 8 12 16 20 24 28 32 36 40 200 5 10 15 20 25 30 35 40 45 50 150 7 13 20 27 33 40 47 53 60 67 100 10 20 30 40 50 60 70 80 90 100 a This represents the initial vitamin B 13 stores divided by the level of stores at which signs of vitamin B12 deficiency may become evident. 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). 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. REFERENCE Bozian RC, Ferguson JL, Heyssel RM, Meneely GR, Darby WJ. 1963. Evidence concerning the human requirement for vitamin B12. Use of the whole body counter for determination of absorption of vitamin B12. Am J Clin Nutr 12:117– 129.

OCR for page 527
DRI Dietary Reference Intakes: For Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline TABLE N-2 Time in Days until Vitamin B12 Threshold Is Reached   Turnover Rate Ratio of Initial Stores to Threshold, from Table N-1 0.1 0.15 0.2 70 4,248 2,832 2,124 60 4,094 2,730 2,047 50 3,912 2,608 1,956 40 3,689 2,459 1,844 30 3,401 2,267 1,701 20 2,996 1,997 1,498 17 2,833 1,889 1,417 16 2,773 1,848 1,386 15 2,708 1,805 1,354 14 2,639 1,759 1,320 13 2,565 1,710 1,282 12 2,485 1,657 1,242 11 2,398 1,599 1,199 10 2,303 1,535 1,151 9 2,197 1,465 1,099 8 2,079 1,386 1,040 7 1,946 1,297 973 6 1,792 1,195 896 5 1,609 1,073 805 4 1,386 924 693 3 1,099 732 549

OCR for page 527
DRI Dietary Reference Intakes: For Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline TABLE N-3 Table of Time in Years until Vitamin B12 Threshold Below Which Deficiency Signs May Occur Is Reached   Turnover Rate Ratio of Initial Stores to Threshold, from Table N-1 0.1 0.15 0.2 70 11.6 7.8 5.8 60 11.2 7.5 5.6 50 10.7 7.1 5.4 40 10.1 6.7 5.0 30 9.3 6.2 4.7 20 8.2 5.5 4.1 17 7.8 5.2 3.9 16 7.6 5.1 3.8 15 7.4 4.9 3.7 14 7.2 4.8 3.6 13 7.0 4.7 3.5 12 6.8 4.5 3.4 11 6.6 4.4 3.3 10 6.3 4.2 3.2 9 6.0 4.0 3.0 8 5.7 3.8 2.8 7 5.3 3.6 2.7 6 4.9 3.3 2.5 5 4.4 2.9 2.2 4 3.8 2.5 1.9 3 3.0 2.0 1.5