A vitamin found in fish, nuts, dairy products, tea and coffee may offer protection against the development of Alzheimer's disease, research has shown.

Scientists also found that niacin -- or vitamin B3 -- in the diet could help prevent a decline in mental agility. The team, writing in the Journal of Neurosurgery and Psychiatry, said their findings could have important implications for the future prevention of Alzheimer's.

The researchers, from the Rush Institute for Healthy Aging in Chicago, questioned almost 4,000 people over the age of 65 who had no history of Alzheimer's, which is characterized by severe dementia and confusion.

Participants were asked about their diet and checked for signs of decreasing mental agility -- cognitive decline -- three and six years later.

After three years, samples of 815 people were checked for clinical changes, and their dietary niacin intake was assessed; 131 of them were diagnosed with Alzheimer's.

The researchers, led by Martha Morris, adjusted the results for important risk factors for Alzheimer's -- age, gender, race, educational levels and a gene known as the ApoE. They concluded 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.

The researchers also looked at the mental agility of the larger group after six years. They found that cognitive decline was reduced by 44% among those with the highest niacin intake compared with those with the lowest intake.

The researchers noted that niacin had been prescribed to older people in order to prevent confused states. Severe deficiency of the vitamin causes pellagra -- which is characterized by dementia, diarrhea and dermatitis -- but its role in Alzheimer's has not been thoroughly explored, they said.

"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."