MONDAY, Sept. 26 (HealthDay News) -- Too little vitamin B-12 may be associated with smaller brain size and more problems with thinking skills as people age, new research suggests.

And the number of people who suffer from B-12 deficiencies may be greater than thought because current methods for measuring levels of the vitamin may not be accurate, said Christine C. Tangney, lead author of the study published in the Sept. 27 issue of Neurology. The study was funded by the U.S. National Institute on Aging.

The researchers assessed the study participants' vitamin levels not only from B-12 levels themselves, but from blood metabolites that are considered markers of B-12 activity (or lack of it) in the tissues.

But the findings aren't nearly enough to start recommending people take B-12 supplements to jumpstart their brains, cautioned Dr. Marc L. Gordon, chief of neurology of Zucker Hillside Hospital in Glen Oaks, N.Y. Gordon was not involved with the study.

"It's not clear exactly if you have a measurement like this whether it's causal or that lowering the marker will drive a change in the risk," he said.

And unless you're a strict vegan, most people do get enough B-12, which is critical for brain health, from their diet -- mainly from animal-derived products, added Gordon, who is also an Alzheimer's researcher at The Feinstein Institute for Medical Research in Manhasset, N.Y.

B-12 is critical for brain health but can become an issue as people get older because the body becomes less able to absorb it. Also, certain drugs can affect absorption. These include proton pump inhibitors, widely used to reduce stomach acid, and the hugely popular diabetes drug metformin (Glucophage).

The authors of the new study looked not only at B-12 levels but at five different blood markers for the vitamin that indicate "where B-12 is active in the tissues," said Tangney, who is associate professor in the department of clinical nutrition at Rush University Medical Center in Chicago.

These markers may actually be better indicators of how much B-12 is absorbed in the body than B-12 itself, she added.

In this study of 121 black and white seniors participating in the Chicago Health and Aging Project, volunteers had their blood drawn and tested for B-12 and related metabolites; they also took 17 tests to measure their memory and mental acuity (cognitive skills).

About 4.5 years later, the researchers measured the participants' brain volumes using MRI scans, and checked for other signs of brain damage. High levels of four of the five markers were linked with smaller brain volume and/or lower scores on cognitive tests, compared with people who had lower levels of the markers.

"This suggests that measuring B-12 levels in itself is not enough to tell if a person is deficient or not," Tangney said. "We need to be careful and think about other indicators."

If a person's B-12 levels are borderline normal, it might be reasonable to check other measures, said Gordon.

Tangney said the study results suggest that B-12 deficiencies contribute to brain atrophy (shrinkage), which in turn can contribute to cognitive problems. However, she also warned against making dietary changes or drawing too-firm conclusions from these findings, noting that they were based on data from only a small number of people.

More information

The U.S. National Institutes of Health's Office of Dietary Supplements has more on vitamin B-12.