PRETTY MUCH EVERYONE SHOULD SUPPLEMENT B12: Vitamin B12 as Protection for the Aging Brain.

European researchers have also shown that giving B12 to people deficient in the vitamin helped protect many of the areas of the brain damaged by Alzheimer’s disease. In a two-year study at the University of Oxford of 270 people older than 70 with mild cognitive impairment and low B12 levels, Dr. Helga Refsum, a professor of nutrition at the University of Oslo, found reduced cerebral atrophy in those treated with high doses of the vitamin.

“A B12 vitamin deficiency as a cause of cognitive issues is more common than we think, especially among the elderly who live alone and don’t eat properly,” Dr. Rajarethinam said.

The academy estimates that between 10 percent and 30 percent of people older than 50 produce too little stomach acid to release B12 from its carrier protein in foods, and as the years advance, the percentage of low-acid producers rises.

But many people do not know they produce inadequate amounts of stomach acid. In fact, evidence from a study of young adults called the Framingham Offspring Study suggests that insufficient absorption of B12 from foods may even be common among adults aged 26 to 49, so the following advice may pertain to them as well.

The academy recommends that adults older than 50 get most of their daily requirement of B12 — 2.4 micrograms for people 14 and older, slightly more for women who are pregnant or nursing — from a synthetic form of the vitamin found in foods fortified with B12 or in a multivitamin supplement. Synthetic B12 is not attached to protein and thus bypasses the need for stomach acid.

Vitamin D is good, too.