Although it is clear that protein-bound B₁₂ is less well absorbed than crystalline B₁₂, the effect varies greatly with the specific protein and may be modified by gastric factors (see “Food-Bound B₁₂ Malabsorption”). Data on absorption from different types of diets (e.g., high in dairy products or beef) are not sufficient to use as a basis for adjusting the estimated requirement for B₁₂.

No evidence was found that a high-fiber diet increases the amount of B₁₂ that should be consumed. A single study (Doi et al., 1983) was found that examined the effect of dietary fiber (specifically, konjac mannan, or glucomannan) on the absorption of B₁₂. A 3.9-g dose of the fiber with a meal did not change the rate of B₁₂ absorption in either normal subjects or those with diabetes mellitus.

Underutilization of B₁₂ has been reported in individuals with genetic defects that involve deletions or defects of MMA-CoA mutase, transcobalamin II, or enzymes in the pathway of cobalamin adenosylation (Kano et al., 1985; Rosenberg and Fenton, 1989).

An Adequate Intake (AI) is set for the recommended intake for infants. The AI reflects the observed average vitamin B₁₂ intake of infants fed principally with human milk.

Reported values for the concentration of the vitamin in human milk vary widely, partly because of differences in methods of analysis and partly because of differences in maternal B₁₂ status and current intake. Despite high intraindividual diurnal variability within a group of lactating women, no consistent effect on B₁₂ concentration of time of day, breast, or time within a feed has been demonstrated. Thus, casual samples of human milk can be used to represent concentrations for the group (Trugo and Sardinha, 1994). However, the wide intraindividual variability may lead to inaccuracies in reported mean values if the number of individuals sampled is small. Median values are substantially lower than average values (Casterline et al., 1997; Donangelo et al., 1989). Acceptable meth-