In the study by Oxford University, over 250 older people with mild cognitive impairment were split randomly into two groups and given either B vitamins or a placebo for two years. Blood levels of several chemicals, including omega-3, were measured before and after the study.

The findings published in the Journal of Alzheimer’s Disease found that people who had high levels of omega-3 in their blood and took vitamin B performed better at thinking and memory tests, compared to those who didn’t take vitamin B or had low omega-3.

Previous results from the same trial indicated that B vitamins were able to reduce brain shrinkage in people with mild cognitive impairment, if they already had good levels of omega-3.

Professor David Smith from Oxford University said: ‚ÄúThe next stage will be to see whether providing a combination of B vitamins and omega-3 supplements can slow the conversion from mild cognitive impairment to Alzheimer’s disease. This would be an important step in the prevention of Alzheimer’s disease. We have high hopes that this trial would work but funding is not easy to obtain for such studies.‚Äù

Dr Doug Brown, director of research and development at Alzheimer’s Society added: ‚ÄúThese results help us to tease apart who could benefit from taking B vitamins, suggesting that they might only improve cognition in people who have high levels of omega-3 oils in their blood.

‚ÄúAs this study shows, the relationship between nutrition and brain health is complex and we need to see increased research efforts to help us understand the role that diet and nutrition can play in reducing a person’s risk of dementia.‚Äù

Evidence for the role of vitamin B in dementia is not clear-cut. B vitamins are responsible for lowering homocysteine, a chemical often found at high levels in the blood of people with dementia, but some clinical trials have shown that taking B vitamins has no effect on brain function, despite their effect on homocysteine.

Next, the group plans to investigate whether a combination of vitamin B and omega-3 supplements can help to slow or prevent the development of dementia in people with mild cognitive impairment.