A vitamin found in a range of common foods could protect against Alzheimer's Disease, researchers have claimed.
A team from the Chicago Institute for Healthy Aging found niacin - vitamin B3 - was also linked to a reduced risk of age-related mental decline.
It is found in dairy products, poultry, fish, lean meats, nuts, and eggs.
The team said their findings, published in the Journal of Neurosurgery and Psychiatry, could help prevent Alzheimer's developing.
The US researchers looked at the diets of almost 4,000 people aged 65 and over between 1993 and 2002.
None had any history of Alzheimer's disease. The researchers then monitored for any signs of decreasing mental agility.
After three years, a sample of 815 people were checked for clinical changes
and their dietary intake was assessed.
Among this group, 131 were diagnosed with Alzheimer's disease.
The researchers, led by Dr Martha Morris took into account other risk factors such as age, gender, race, educational levels and a gene known as the ApoE.
They found that those with the lowest food intake of niacin - around
12.6mg a day - were 80% more likely to be diagnosed with Alzheimer's than those
with the highest intake - around 22.4mg a day.
When they examined the mental agility of the larger group after six years, the researchers found cognitive decline was "significantly reduced" by 44% among
those with the highest niacin intake compared with those with the lowest
The researchers, led by Dr Martha Morris, said severe niacin deficiency causes pellagra - characterised by dementia, diarrhoea and dermatitis - but its role in Alzheimer's has not been
Writing in the journal, they said: "There has been little previous examination of dietary niacin and Alzheimer's disease.
"In this study, we observed a protective association of niacin against the
development of Alzheimer's disease and cognitive decline within normal levels of
dietary intake, which could have substantial public health implications for
disease prevention if confirmed by further research."
Harriet Millward, deputy chief executive of the Alzheimer's Research Trust, said: "This research will be of interest worldwide since there hasn't been a serious study of a possible link between niacin and Alzheimer's previously.
"The results will be less relevant in the UK since on average dietary levels of niacin are higher in the UK than they are in the US where this study took place.
"Research into protective factors is not only valuable in helping us to identify ways of reducing people's risk of developing Alzheimer's, but can also help increase our understanding of the mechanism of the disease."
Catherine Collins, of the British Dietetic Association, said a typical diet in the UK included significantly higher levels of vitamin B3 than even those than the highest seen in the US study.
She added: "There are a lot of other risk factors in relation to Alzheimer's, but people should feel assured that if they eat a balanced diet, they are doing all they can to protect themselves against Alzheimer's in relation to niacin levels."