Vegetarian diets include foods from plant sources. There are several variations of this diet. Lacto-ovo vegetarians consume milk and eggs. Vegans exclude any animal food sources. A well-planned vegetarian diet that excludes all sources of animal product can provide adequate sources of protein, calcium, iron, and essential fatty acids. However, vitamin B12 is a nutrient that is virtually nonexistent in a plant-based diet. Therefore, when consumption of animal foods is little or none, introduction of B12 either by fortified foods or supplementation is essential.
It is critical to understand specifics of B12 absorption. Using multivitamins can be ineffective and may be detrimental for supplementation of B12 because the vitamin can be degraded in the presence of vitamin C, as well as copper. Absorption from supplements depends on dose and frequency of the intake. For example, it has been noted that a single oral dose of 50 mcg will be absorbed at an amount of just 1.5 mcg; 1,000 mcg at just 13 mcg (Rizzo et al., 2016).
Although there is no recommended supplement amount in pregnancy to meet the daily requirement of 2.6 mcg/day of B12, one oral dose of 100 mcg daily (alternatively, a larger weekly dose of 2,000 mcg in two doses) should be sufficient to meet the needs for healthy pregnant vegetarians. This takes into account the efficiency of absorption and the passive route. Cyanocobalamin is the most economical and most used form of B12 supplementation, hence shown to be safe. Therapeutic administration of oral B12 has proven to be as effective as intramuscular administration. There are no substantial differences between absorption of sublingual and oral forms.
Evaluation of B12 is an important aspect of screening patients on a vegetarian diet. The problem for clinicians is that there is neither a reference method nor gold standard level regarding B12 deficiency (Rizzo et al., 2016). The Institute of Medicine defined 120-180 pmol/L as the range for B12 deficiency. However, the clinician cannot diagnose adequate B12 status based on total B12 levels because deficiencies may exist even at concentrations above the often used 150 cutoff range (Rizzo et al.). Pregnancy and folate deficiency may falsely lower B12 levels. Although the concentration of B12 in the blood may by itself not be sensitive enough to detect early signs of a deficiency, it may be accompanied by other markers to determine status (Langan & Zawistoski, 2011). Lower B12 blood concentration can cause increased mean corpuscular red cell volume and megaloblastic macrocytic anemia, as does a folate deficiency. Elevated levels of both methylmalonic acid serum homocysteine and serum homocysteine are highly sensitive markers for B12 deficiency. However, methylmalonic acid is the more specific of the two in detecting B12 deficiency and hence is the confirmation test of choice for many clinicians.
Risk of B12 deficiency and lack of reserves, even for a short term, must be avoided. In a systematic review of the literature based on the blood concentration of B12 among vegetarians, a deficiency was present ranging from 17% to 39% among pregnant women (Rizzo et al., 2016). Risks related to shortage in children born to vegetarian mothers without adequate reserves have been documented and include B12 deficiency in the neonate, failure to thrive, and neurodevelopment delays and possibly consequences into childhood (Langan & Zawistoski, 2011).
A well-planned vegetarian diet, including vegan, can be nutritionally sound with positive outcomes for mother and baby. However, failure to identify inadequacies can have serious results. Therefore, based on scant availability of B12 in plant sources, and importance of maintaining adequate B12 reserves, appropriate counseling of woman on a vegetarian diet should include discussion of incorporating foods fortified with B12 and consideration of supplementation.
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