One of the Oxford University scientists leading the trial, conducted on 168 volunteers, said it had yielded "very dramatic and striking results".
"It's much more than we could have predicted," David Smith of Oxford's department of pharmacology said on Thursday.
"It is our hope that this simple and safe treatment will delay development of Alzheimer's in many people who suffer from mild memory problems."
But Smith stressed that more research was needed and that people should not rush out and start taking huge doses of vitamin B, as the long-term effects were not yet known.
The volunteers in the study, published in the online journal Public Library of Science ONE, were over the age of 70 and diagnosed with mild cognitive impairment (MCI), a precursor to Alzheimer's and other forms of dementia.
Over a two-year period, half were given a daily tablet containing high doses of the vitamins B folate (B9), B6 and B12. The rest received a placebo pill with no active ingredients.
On average, taking vitamin B slowed the rate of brain atrophy by 30 per cent. In some cases, there were reductions as high as 53 per cent.
'Extremely high doses'
Vitamin B is known to control levels of an amino acid called homocysteine in the blood, and high blood levels of homocysteine are linked to an increased risk of developing Alzheimer's disease.
Helga Refsum, who also worked on the trial, stressed that the vitamins were given in extremely high doses.
"This is a drug, not a vitamin intervention," she said.
The pills, called "TrioBe Plus" contained around 300 times the recommended daily intake of B12, four times daily advised folate levels and 15 times the recommended amount of B6.
Experts commenting on the findings said they were important and called for larger, longer full-scale clinical trials to see if the safety and effectiveness of vitamin B in the prevention of neurodegenerative conditions could be confirmed.
Paul Matthews, a professor of clinical neurology at Imperial College London, said that although the vitamins used are generally safe and inexpensive, the study "should not drive an immediate change in clinical practice".
"Instead, it sets out important questions for further study and gives new confidence that effective treatments modifying the course of some dementias may be in sight," he said.
An estimated 37 million people worldwide live with dementia, with Alzheimer's disease causing the majority of cases, according to the World Health Organisation.
Mild cognitive impairment (MCI) affects around 16 per cent of people aged over 70 worldwide and is characterised by slight problems with memory loss, language or other mental functions.
MCI does not usually interfere with daily life, but half of people with the condition develop dementia within five years.
Source: Al Jazeera and agencies