Q: My husband and I are vegetarian, and so is our infant son. How can we be sure we're all getting enough vitamin B12?
A: It's actually easy, since the amount of vitamin B 12 our bodies need is tiny. According to the National Institutes of Health, adults require just 2.4 micrograms of B 12 daily. If you're breast-feeding, the requirement inches up to 2.8 micrograms. A benefit of breast-feeding is that your baby gets B12 from you. A bottle-fed baby needs 0.4 microgram through age 6 months, 0.5 microgram from 7 to 12 months, and 0.9 microgram from age 1 to 3 years.
Those are the numbers. I should add that vitamin B12 isn't produced by either plants or animals; it's made by bacteria. Animals have B12-producing bacteria in their digestive tracts, and from there they absorb the vitamin, which ends up in animal food products. So, a cup of nonfat milk contains about 0.9 microgram of B12, and a large egg about 0.6 microgram. However, the B12 in animal products is bound to protein, and our bodies have to pry it loose using stomach acid and digestive enzymes, making absorption unpredictable, especially as we get older and digestion becomes less efficient.
The B12 in supplements and fortified foods such as breakfast cereals and soymilk is more reliably absorbed, which is why the U.S. Department of Health and Human Services recommends supplemental B12 for everyone over 50. This advice is actually of benefit to people at any age, and essential if you're on a vegan diet. Because vitamin B12 is necessary for healthy nerves and healthy blood, it makes sense to boost your dose—say, to 1,500 micrograms a day for at least a month—to be sure to catch up if you haven't been paying attention to B12 for some time. It's not dangerous to exceed the required amount; B12 is not toxic.