The brains of the elderly and younger people with autism and schizophrenia may share a common link: Both have low levels of vitamin B12, researchers say.

The facts that blood levels of B12 do not always mirror brain levels of the vitamin, and that brain levels decrease more over the years than blood levels, may imply that various types of neurological diseases — such as old-age dementia and the disorders of autism and schizophrenia — could be related to poor uptake of vitamin B12 from the blood into the brain, the scientists said.

The findings, reported last month in the journal PLOS ONE, support an emerging theory that the human brain uses vitamin B12 in a tightly regulated manner to control gene expression and to spur neurological development at key points during life, from the brain's high-growth periods during fetal development and early childhood, through the refining of neural networks in adolescence, and then into middle and old age.

Vitamin B12, also called cobalamin, plays a crucial role in blood formation and the normal functioning of the nervous system. The vitamin is found in foods derived from animal sources, although some plant-based foods can be fortified with B12. [6 Foods That Are Good For Your Brain]

In the new study, scientists led by Richard Deth, a professor of pharmacology at Nova Southeastern University in Fort Lauderdale, Florida, examined the brains of more than 60 deceased individuals, ranging in age from a fetus in a late stage of gestation to 80 years. The study included 12 people who had autism and nine with schizophrenia.

This is the first study to compare the levels of vitamin B12 in the brain across the human lifetime, Deth told Live Science. The vitamin B12 levels in the brain were 10 times lower in the oldest people compared with the youngest, reflecting a gradual, natural, and consistent decline over the years.

For the elderly, this decline might not be a bad thing. Lower levels at advanced ages may offer some degree of brain protection by slowing cellular reactions and the production of DNA-damaging chemicals called free radicals, Deth said. In previous work with his colleague Yiting Zhang of Northeastern University in Boston, Deth found that the body's creation of biologically active forms of vitamin B12 produces free radicals as a waste product.

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But levels of B12 that are too low can be detrimental. "At some point, an extreme decrease in metabolism…is not compatible with cell survival," Deth said. Similarly, lower vitamin B12 levels can have negative consequences for people of younger ages, as the brain is still developing. Deth's group found that the levels of vitamin B12 in the brains of young people with autism and in middle-age people with schizophrenia were about one-third of the levels found in similarly aged people who did not have these neurological conditions.

The people in the study with autism, who were all under age 10, had levels similar to those found in a 57-year-old. It's not clear what these low levels imply, but the uptake of too little B12 might hinder the brain's ability to establish important neural connections between regions, Deth said.

Those with schizophrenia, all between ages 36 and 49, had levels similar to those found in a 72-year-old. Although their brains were mature by this age, the below-normal level may have manifested itself during adolescence, when the seeds of schizophrenia are thought to take root. But even in middle age, the lower levels may contribute to a loss of previously normal function, Deth said.

Daniel Smith, a neurologist and vice president of innovative technology at Autism Speaks, an autism advocacy group based in New York that sponsors autism research, who was not involved in this research, said the study was interesting and worth pursuing further. However, he noted that the study remains speculative in its hypothesis that vitamin B12 deficiencies at a cellular level lead to changes associated with the autism spectrum of brain traits.

Numerous studies have searched for an association between vitamin deficiencies and neurological disorders. There has been no definitive study, however, indicating that autism and schizophrenia can be caused by a deficiency or treated through vitamin supplementation.

In fact, a study published last year in the Journal of the Academy of Nutrition and Dietetic found that few children with autism benefit from vitamin supplements and may be at risk for overdosing.

Follow Christopher Wanjek @wanjek for daily tweets on health and science with a humorous edge. Wanjek is the author of "Food at Work" and "Bad Medicine." His column, Bad Medicine, appears regularly on Live Science.

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