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error] and 60.3 ± 7.9, respectively, p < 0.02), and urinary thiocyanate excretion (an index of the exogenous cyanide load) was inversely associated with serum B12. Similarly, in a study of pregnant women, the distribution of values of serum B12 was slightly lower for smokers than for nonsmokers. However, in a cross-sectional study, differences in B12 concentrations of smokers and nonsmokers were not significant in multivariate analyses. The effect of smoking on the B12 requirement thus appears to be negligible.

Gender

In a cross-sectional study of 77 young men and 82 young women (Fernandes-Costa et al., 1985), the women were found to have significantly higher serum B12 values and unsaturated cobalamin binding capacity than did the men (p < 0.001 and 0.05, respectively). Subjects were excluded if they were taking vitamin supplements, oral contraceptive agents, or other medications other than patent analgesics. Mean serum B12 values were 477 and 604 pmol/L (647 and 819 pg/mL) for men and women, respectively—well above the cutoff of adequacy. Other investigators have reported similar findings (Low-Beer et al., 1968; Metz et al., 1971). Studies that have found no difference in mean B12 values were smaller and less well-controlled for other factors that could influence B12 values (Rosner and Schreiber, 1972; Scott et al., 1974). Taken together, these studies do not provide sufficient evidence on which to quantitate a difference in B12 requirements by gender.

Nutrient-Nutrient Interactions

Folate with B12

Although adequate or high folate intake may mitigate the effects of a B12 deficiency on normal blood formation, there is no evidence that folate intake or status changes the requirement for B12.

Vitamin C with B12

Low serum B12 values reported in persons receiving megadoses of vitamin C are likely to be artifacts of the effect of ascorbate on the radioisotope assay for B12 (Herbert et al., 1978) —and thus not a true nutrient-nutrient interaction.



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