The discovery that people in the early stages of failing memory can retain more of their mental faculties for longer if they take the tablets regularly could lead to treatments for the condition. Some participants in the Oxford University trial saw their neurological decline reduced by as much as half after using B vitamins.
That breakthrough has raised hopes that the vitamins, which are sold in chemists and health food stores, could at least slow down, if not prevent, the shrinkage that affects many older people's brains.
Vitamin B tablets are popular among vegans, who do not receive it because they shun the foods in which it is found – fish, meat and milk – and among sufferers of pernicious anaemia.
"It is our hope that this simple and safe treatment will delay the development of Alzheimer's in many people who suffer from mild memory problems," said David Smith, a professor emeritus in Oxford University's pharmacology department and co-leader of the study. About 1.5m people over 70 in the UK who suffer from mild cognitive impairment (MCI) – who have a 50/50 chance of going on to develop full-blown dementia within five years – could benefit from the discovery, Smith added. But while the results were "immensely promising", it was not yet certain, he stressed, if B vitamins could slow or prevent the development of Alzheimer's.
Healthy middle-aged people hoping to avoid dementia and older people exhibiting early signs of memory loss might now be tempted to start routinely taking the vitamins, he said. But they should not do without first talking to their doctor, as the tablets could help stimulate the growth of early-stage cancer, he warned.
Chris Kennard, chair of the neurosciences and mental health board at the Medical Research Council, said the findings "bring us a step closer to unravelling the complex neurobiology of ageing and cognitive decline and hold the key to the development of future treatments for conditions like Alzheimer's disease."
Rebecca Wood, chief executive of the Alzheimers Research Trust, said: "These are very important results, with B vitamins now showing a prospect of protecting some people from Alzheimer's in old age. The strong findings must inspire an expanded trial to follow people expected to develop Alzheimer's, and we must hope for further success."
Some 820,000 people in the UK have dementia, predominantly Alzheimer's, and their numbers are expected to soar as the population ages.
Smith and his colleagues at the Oxford Project to Investigate Memory and Ageing gave one group of people with MCI daily tablets comprising folic acid, vitamin B6 and vitamin B12, and another group a placebo. The vitamins were chosen because they control the amounts of an amino acid called homocysteine in the blood. High levels of homocysteine have been linked to a greater risk of Alzheimer's.
After two years participants' brains were examined using MRI scanners and their mental faculties assessed using tests of cognition. They found that those who had been receiving the supplements had experienced on average 30% less brain atrophy than those receiving the dummy pills. The former saw their brains shrink by 0.76% a year, while the placebo group saw theirs reduce by 1.08%. Those who started the trial with the highest levels of homocysteine experienced the greatest benefit – 50% less brain shrinkage.