The results suggest that it is not the antioxidant vitamins (vitamins C, E and beta-carotene) but the antioxidant polyphenols that are behind the effects.
"These findings are new and suggest that fruit and vegetable juices may play an important role in delaying the onset of Alzheimer's disease," wrote lead author Qi Dai in The American Journal of Medicine (Vol. 119, pp. 751-759).
"These results may lead to a new avenue of inquiry in the prevention of Alzheimer's," wrote Qi.
Alzheimer's disease is the most common form of dementia and currently affects over 13 million people worldwide. The direct and indirect cost of Alzheimer care is over $100 bn (€ 81 bn) in the US alone. The direct cost of Alzheimer care in the UK was estimated at £15 bn (€ 22 bn).
Although the mechanism of Alzheimer's is not clear, more support is gathering for the build-up of plaque from beta-amyloid deposits. The deposits are associated with an increase in brain cell damage and death from oxidative stress.
It is against the oxidative stress that the polyphenols appear to offer protection.
The researchers followed 1,836 dementia-free Japanese American subjects (average age 72, 54 per cent women) in the Seattle population and collected information on their dietary consumption of fruit and vegetable juices by using a food frequency questionnaire (FFQ). Cognitive function was assessed every two years for up to 10 years.
People drinking juices three or more times per week were reported to be 76 per cent less likely to develop signs of Alzheimer's disease than those who drank less than one serving per week, said the researchers. This was after taking into account dietary intake of vitamins E, C and beta-carotene.
"These [results] identify the possibility that other underlying beneficial elements in fruits and vegetables may contribute to Alzheimer's disease risk reduction," wrote Dai from the Vanderbilt University Medical Center.
"Also, animal studies and cell culture studies confirmed that some polyphenols from juices showed a stronger neuroprotective effect than antioxidant vitamins. So we are now looking at polyphenols," Dai said.
The benefit of the juice consumption also appeared enhanced in subjects who carried the apolipoprotein E epsilon-4 allele, a genetic marker linked to late-onset Alzheimer's disease - the most common form of the disease, which typically occurs after the age of 65.
Interestingly, no relationship between tea drinking and Alzheimer's risk was observed, a result that goes against previous studies that reported significant risk reductions with drinking tea (for example, The American Journal of Clinical Nutrition, Vol. 83, pp. 355-361).
A limitation of the study, and an avenue of research for the future, is the lack of data on specific juice types.
"We don't know if it is a specific type of juice (that reduces risk). That information was not collected in the current study," said Dai. "But we can use plasma to narrow down the kinds of juices."
The next step, therefore, is to test the subjects' blood samples to see if elevated levels of polyphenols are linked to the reduced risk of cognitive decline and Alzheimer's disease, and may point to the types of juice that would be most beneficial.
The researchers sounded a note of caution, however, by stating that future studies are needed to confirm or refute these findings, and that the general public should not "jump the gun" regarding the value of juice as a preventive measure for Alzheimer's disease.
Harriet Millward, deputy chief executive of the Alzheimer's Research Trust told NutraIngredients.com that previous research has reported mixed results with some suggesting that the benefits of fruit and vegetables were short lived.
"But this is a long term study following a relatively large group of people," she said.
"Many scientists believe there is a link between the release of free radicals within the body and early changes to brain cells in people who ultimately go on to develop Alzheimer's disease. Since fruit and vegetable juices are rich in antioxidants which 'mop up' free radicals, this interesting piece of research adds weight to this theory.
"Diet almost certainly plays a part in every person's Alzheimer's risk - and diet is a magnet for research because it could offer a relatively inexpensive way to fight a disease that ruins countless lives and costs the NHS more than cancer, stroke and heart disease put together.
"The Alzheimer's Research Trust funds dietary research looking at everything from oily fish to soya, but much more work is needed before we can say an apple juice a day keeps the doctor away," she said.