Fruits, Vegetables and Cognitive Decline in Older Persons

In developing countries, ageing of the population leads to an increase of cognitive impairment, the most dramatic form being dementia. Dementia is characterized by the alteration of superior cognitive functions such as memory, attention or executive functions. The most prevalent cause of dementia is Alzheimer's disease. By now, no treatment is available to cure Alzheimer's disease and prevention becomes an important way to reduce the prevalence of the disease. The search for risk factors is essential for a better understanding of the mechanisms leading to Alzheimer's disease. The oxidative stress hypothesis has been raised as a putative mechanism. Nutrition plays a role in the ageing process, but there is still a lack of knowledge about nutrition-related risk factors in cognitive impairment. We shall review several epidemiological studies that showed an association between fruit and vegetable intake and cognition, or between anti-oxidant nutrients and dementia. Epidemiological studies have shown that high fruit and vegetable intake may lower the risk of developing Alzheimer's disease. Several cross-sectional studies reported a direct relationship between antioxidant intake and better cognitive functions. By contrast with the cross-sectional studies, the prospective studies published so far give a more complex picture. Some found that antioxidants intake is associated with a better cognitive evolution, whereas other found that dietary and not supplement-derived antioxidant intake is associated with a lower risk for Alzheimer's disease. Finally, clinical trials using vitamin E supplementation showed no benefit on the rate of cognitive decline in Alzheimer's disease or in subjects with a mild cognitive impairment. In conclusion, human observational epidemiology studies have, in general, been consistent with the hypothesis that there is an inverse relationship between antioxidant levels/intake, cognitive function, and ultimately the risk to develop Alzheimer's disease. However, randomized clinical trials with antioxidants do not fulfil the promises of those studies. This shows that antioxidant vitamins are not the only nutrients to be considered and fruit and vegetable intake cannot be reduced to their antioxidant contents.