These days, most health warnings about vitamins focus on the danger of overdose. For vitamin B12, though, it's more likely that people are getting too little. In fact, B12 deficiency is the most common nutritional deficiency in the developing world and possibly in the United States as well, reports the August issue of the Harvard Health Letter.

Sometimes the only symptoms of a B12 deficiency are subtle cognitive and neurological changes. More serious shortages can result in dementia or anemia. Breast-fed infants of mothers with a B12 deficiency are at risk for severe developmental abnormalities and irreversible neurological damage.

Vegans (people who don’t eat any meat, dairy, or eggs) are most at risk for developing a B12 deficiency because, aside from fortified breakfast cereals, the only reliable dietary sources of vitamin B12 are animal-derived products. But even vegetarians who eat eggs and dairy products consume, on average, less than half the adult Recommended Dietary Allowance of 2.4 mcg of B12, notes the Health Letter.

Some older people are also at high risk for developing B12 deficiency. Up to 30% of people ages 50 and over suffer from atrophic gastritis, a thinning of the stomach lining. This condition reduces the amount of B12 absorbed by the small intestine. Certain other digestive conditions and surgeries also can interfere with B12 absorption.

The Harvard Health Letter recommends that vegetarians and older people with atrophic gastritis take a multivitamin, eat fortified breakfast cereal, or both. A deficiency of vitamin B12 may affect balance, memory, and perhaps mood. If you have these problems and you’re in an at-risk category, ask your doctor about getting a B12 blood test.

Also in this issue:

• Type D personality and cardiovascular risk

• Shingles and whooping cough vaccinations for adults

• Chronic constipation

• How best to treat and avoid cancer, heart disease, and other chronic diseases