Large doses of B-vitamins could slow the cognitive decline in older people that is the precursor to dementias such as Alzheimer's disease, according to a study.

Speaking at the British Science Festival in Bradford on Tuesday, Celeste de Jager, a neuropsychologist at Oxford University, said that taking vitamins B6, B12 and folic acid in medicinal quantities reduced the overall shrinkage of a person's brain by 30% over the course of the two-year study.

Her work, published recently in the International Journal of Geriatric Psychiatry, was carried out on 270 men and women over 70 who had mild cognitive impairment (MCI), a condition that affects one in six elderly people and which can interfere with memory, language and other mental functions. About half of the people with MCI go on to develop Alzheimer's within five years of the initial diagnosis.

Taking B vitamins and folic acid is known to control the levels of an amino acid called homocysteine in the blood. High levels of this chemical can damage blood vessels and are associated with increased risk of dementia.

"High homocysteine is a known risk factor for cognitive decline in the elderly and Alzheimer's disease and also for other kinds of dementia like vascular dementia," said de Jager. "It can be damaging to the endothelial lining of the blood cells. It also binds to receptors in the brain that are on the neurons and it seems to contribute the atrophy that's associated with Alzheimer's."

The elderly are more susceptible to this effect of high homocysteine, she added, because levels rise with older age, possibly due to poorer absorption of B vitamins in diet as people age.

To keep homocysteine levels down, she said, people should eat more meat, fish and green vegetables, and reduce consumption of alcohol, which is known to deplete the body of vitamin B12.

In the study, the volunteers were split into two groups, with one group given placebo pills and the second group given a cocktail of vitamins made from 0.5 milligrams of B12, 0.8mg of folic acid and 20mg of vitamin B6 – all several times more than the standard amount recommended by doctors for a healthy diet. The participants' brains were scanned at the start of the experiment and at two years, and they went through a regular battery of cognitive tests.

Across the whole group, de Jager found that the people taking vitamins had a 30% reduced decline in brain tissue over two years compared with placebo. In people with the highest homocysteine levels in the blood at the start of the experiment, however, the vitamins provided most benefit, reducing brain shrinkage by 50% in these cases.

Rebecca Wood, chief executive of Alzheimer's Research UK, which co-funded the study, said the results were encouraging for the use of B vitamins but larger trials would be needed to work out how protective they might be against mental decline for elderly people. "People should speak to their doctor before embarking on any vitamin plan. Follow-up clinical trials must have a particular emphasis on establishing whether B vitamins could head off conversion from MCI to Alzheimer's."

De Jager is already planning to extend her study to more than 1,000 people. "It will be over two years and the cognitive and clinical outcomes will be the main outcomes this time rather than brain atrophy."

In this next experiment, de Jager will look for the effects of B vitamins on the severity of the cognitive decline in a participant, based on a clinical dementia rating scale. "Most people with MCI have a score of 0.5 on [the scale] and 1 is the first stage of Alzheimer's disease. If we can keep people at the 0.5 or back to zero, that's what we're aiming at. We're looking for a clinically meaningful change."

Meanwhile, she warned that older people should not start consuming lots of vitamin B as a result of her work so far. "Self-dosing needs to be done with some caution because there's some risk of re-activating cancerous cells with folic acid. Also the balance of vitamins needs to be maintained. My approach would be to check that my diet had enough B12 and the other vitamins first."