Niacin has been prescribed to older people to prevent confusional states, and severe deficiency causes pellagra, a condition characterised by dementia, diarrhoea, and dermatitis, but its role in Alzheimer's disease has not previously been thoroughly explored, said the researchers based at the Rush University in Chicago.
Much attention has however focused on the relation between dementia and other B vitamins, particularly vitamin B12, B6 and folate.
The new study showed that people consuming more than 22.4mg of niacin daily were 80 per cent less likely to suffer Alzheimer's than those in the lowest intake group.
If confirmed, the findings could have significant implications for public health, write the researchers in the Journal of Neurology, Neurosurgery and Psychiatry (75: 1093-99).
"This is fantastic news as cognitive health is a very important area of concern," said Dan Murray, associate director of nutrition at Lonza, the world's largest producer of niacin.
"Niacin helps general circulation but it is not known as a brainpower vitamin."
Niacin as nicotinic acid is currently marketed in both pharmaceuticals and dietary supplements as a cholesterol-lowering agent. While the new research looked at intake from both food sources -such as lean meat, fish, nuts, dairy products and cereals- as well as supplements, it could open up a new target area for niacin-rich foods and supplements.
There are currently nearly 18 million people with dementia in the world, and the most common cause of this dementia is Alzheimer's disease. By 2025 this figure will rise to 34 million, with 71 per cent of these likely to live in developing countries.
The Rush researchers recruited almost 4000 people aged 65 and older, who had no Alzheimer's disease at the start of the study. They filled out a dietary questionnaire and were checked for any signs of decreasing mental agility (cognitive decline) three and six years later.
At three years, a random sample of 815 people, who had not initially had Alzheimer's disease, were checked for clinical changes and their dietary niacin intake assessed by means of food frequency questionnaires.
Among this smaller group, 131 people were diagnosed with Alzheimer's disease.
After adjusting the results for age, gender, race, educational levels, and the ApoE gene - all important risk factors for the disease - those with the lowest food intake of niacin (an average of 12.6 mg/day) were 80 per cent more likely to be diagnosed with Alzheimer's disease than those with the highest intake (22.4 mg/day).
An analysis of the larger group showed that the rate of cognitive decline among those with the highest niacin intake was almost half (44 per cent) that of those with the lowest intake.
Previous research has indicated that niacin has an important role in DNA synthesis and repair, neural cell signalling, and acts as a potent antioxidant in brain cells, said the researchers.