Vitamin B-12 doesn’t make headlines the way C and E do, but new evidence suggests it should: Low levels may be a hidden cause of depression and dementia. Even worse, current testing methods may not be up to the task of detecting deficiencies.

Neurologist Florian Thomas at St. Louis University did blood tests on 34 patients. Even though the tests showed the patients all had “normal” B-12 levels, a separate analysis revealed that more than three-quarters also had elevated levels of a natural compound called methylmalonic acid (MMA), a signal that their B-12 levels were actually too low. So the B-12 test may not be telling the whole story.

Thomas says it makes sense for anyone over age 65 to be tested at least for MMA (if not for both MMA and B-12) every other year. Pregnant women who are vegetarians or vegans should do the same.