Vegetarianism and vitamin B-12 (cobalamin) deficiency

Vegetarian diets can be classified as either lactovegetarian, ovovegetarian, lactoovovegetarian, or vegan if they include, respectively, dairy products, eggs, both dairy products and eggs, or no animal products at all. Vegan diets have a very low cobalamin content, but a study by Herrmann et al (1) in this issue of the Journal forces us to reevaluate the shortcomings of the other forms of vegetarianism. Herrmann et al show that vegans and, to a lesser degree, lactoovovegetarians and lactovegetarians have biochemical evidence of cobalamin deficiency based on increased blood total homocysteine and methylmalonic acid (24) and low holotranscobalamin II concentrations; the test for the latter is still under investigation for addition to the diagnostic algorithm for vitamin B-12 deficiency (5). The adverse health consequences in 2 closely related groups, voluntary vegetarians who base their dietary preferences on religious or philosophical grounds and persons whose near-vegetarianism is imposed by poverty, are worth reexploring. Worldwide, vegetarians number in the hundreds of millions, so public health initiatives that seek to improve the health of this population will have a global effect.

Whereas vegetarianism is present in all geographic areas, only in the past 50 y was it recognized that vegetarians have consistently lower vitamin B-12 concentrations than do nonvegetarians and that vegetarians are at greater risk of vitamin B-12 deficiency than are nonvegetarians. Because vitamin B-12 is produced in nature only by vitamin B-12–producing microorganisms, humans must receive vitamin B-12 solely from the diet (3). Although there are abundant vitamin B-12–producing bacteria that colonize the large bowel, that organ is too distal to allow normal vitamin B-12 absorption. Herbivores obtain vitamin B-12 primarily from plants contaminated with nitrogen-fixing, vitamin B-12–producing bacteria that grow in roots and nodes of legumes and from plants contaminated with feces. Carnivorous lower animals receive their vitamin B-12 by eating insects and other animals and via coprophagy. Nonvegetarians obtain most of their vitamin B-12 through eating meat, whereas lactoovovegetarians obtain most of their vitamin B-12 from milk, dairy products, and eggs. Plants contaminated with vitamin B-12–producing bacteria through fertilization with manure may also be a source of vitamin B-12, so, in theory, "organically grown" leafy vegetables may have higher vitamin B-12 concentrations than do leafy vegetables exposed to chemical fertilizers.