"We anticipate that the day may come when foods like apples, apple juice and other apple products are recommended along with the most popular Alzheimer's medications," said the leader of the study, Professor Thomas Shea, from the University of Massachusetts Lowell (UML).
Although the mechanism of Alzheimer's is not clear, more support is gathering for the build-up of plaque from amyloid deposits. The deposits are associated with an increase in brain cell damage and death from oxidative stress.
The UML team previously showed the benefits of feeding apple juice in mice bred to be prone to Alzheimer's disease, as well as benefiting naturally aged mice - an effect attributed to the antioxidant content of apples and apple juice (Journal of Alzheimer's Disease, Dec. 2005, Vol. 8, pp. 283-287).
The results from the new study however appear to show that the benefits may not be exclusively related to the antioxidant properties of the fruit since the data indicate that apple juice consumption could increase the production of the essential neurotransmitter acetylcholine in the brain, resulting in improved memory. Neurotransmitters such as acetylcholine are chemicals released from nerve cells that transmit messages to other nerve cells.
Levels of acetylcholine decrease naturally during ageing, but a substantially bigger drop in acetylcholine levels are associated with Alzheimer's disease.
Indeed, the role of acetylcholine in the brain forms the basis of many medications for Alzheimer's that seek to increase brain levels of the neurotransmitter to retard mental decline in Alzheimer's patients.
The research, sponsored by the US Apple Association and the Apple Products Research and Education Council, divided normal, Alzheimer's-like, or aged mice into three groups and fed them for one month on a diet supplemented with vitamin E (50 IU/kg) or folic acid (4 mg/kg), or a diet lacking these nutrients and containing a pro-oxidant (iron). An additional group was fed the vitamin E, folic acid deficient diet but received a supplement of concentrated apple juice (0.5 per cent in water).
Among those fed the apple juice-supplemented diet, the mice showed an increased production of acetylcholine in their brains.
For the normal mice, the apple juice group had acetylcholine levels about 25 per cent higher than the mice on the deficient diet. The mice with Alzheimer's-like symptoms supplemented with apple juice had acetylcholine levels about 33 per cent higher than the mice on the deficient diet. For the aged mice, the apple juice group had acetylcholine levels about 130 per cent higher than the mice on the deficient diet.
Also, after multiple assessments of memory and learning using traditional Y maze tests, researchers found that the mice who consumed the apple juice-supplemented diets performed significantly better on the maze tests.
"The decline in cognitive performance, and its prevention by supplementation with apple juice concentrate, is likely due to acetylcholine depletion, rather than oxidative damage," wrote lead author Amy Chan in the Journal of Alzheimer's Disease (Vol. 9, pp. 287-291)
The results obtained were from animals consuming moderate amounts of apple juice, said the researchers. In humans this would be comparable to drinking approximately two eight-ounce glasses of apple juice or eating two to three apples a day.
"The findings of the present study demonstrate the usefulness of consumption of antioxidant-rich foods such as apple products as part of a therapeutic approach to maintain acetylcholine levels," concluded Chan.
The next step is to test if such results can be reproduced in human studies, and Shea said that such a clinical trial is planned for the future.
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 $100bn (€ 81bn) in the US alone. The direct cost of Alzheimer care in the UK was estimated at £15bn (€ 22bn).