N° 16 | October 2007

Fruit and vegetable consumption and age-related cognitive decline

Based on evidence from numerous epidemiologic and animal studies, food components can modify the rate of cognitive decline that occurs with aging. Because foods are a complex mixture of nutritive and nonnutritive constituents, many professionals believe these relationships should be not be examined only at the level of nutrients, but as foods or food groups or as a function of food patterns. Thus far, clinical trials designed to test the efficacy of nutrient supplements on health outcomes have not produced expected or optimal outcomes. Perhaps, it is the complement of food constituents (some yet to be identified) as found in foods consumed together at the table that contribute to cognitive health, either positively or negatively.

Animal experiments, in particular those feeding whole food extracts such as blueberry, spinach extracts or apple juice concentrates to rats show that neuronal function can be preserved with aging(1, 2). Rats fed vitamin E, strawberry and spinach extracts from adulthood to older age did not experience the losses in cognitive performance shown in rats fed standard chow. Blueberry supplemented diets increased neurogenesis in the dentate gyrus of the hippocampus of rats, a region critical for shortterm memory.

Prospective cohort designs provide the best opportunity to examine the relationship between dietary behavior and changes in cognitive performance. Associations between cognitive change and fruit and vegetable intake have been studied in two large wellcharacterized cohorts. One includes a sample from the Nurses’ Health Study (NHS); 13,038 stroke-free participants completed multiple food frequency questionnaires and were interviewed by telephone using six standardized cognitive tests twice over a 2 year period(3). Such relationships were also explored in the Chicago Health and Aging Project or CHAP. Specifically, 3718 elderly residents over a median follow-up of 5.5 years(4) were interviewed in their homes using standardized cognitive tests at least twice on everyone approximately 3 years apart and for 1946 persons, three times over a median followup of 6.3 years. The global cognitive score was based on four cognitive tests, all of which were also used in the NHS study.

In both the NHS and CHAP cohorts, dietary information was garnered from participants who completed similar semi-quantitative food frequency questionnaires that have been shown to provide valid and reproducible nutrient estimates(5-7). In NHS, the questionnaire includes 15 fruits (juices too) and 30 vegetables (excluding French fries and potato chips). Total fruit consumption was based on 14 items listing 21 different fruits on the CHAP questionnaire, while 19 items representing 28 different vegetables, excluding potatoes are present. Food intakes in NHS analyses were based on averaged intakes across four to five questionnaires, from 1984 to the first cognitive test. For the CHAP analyses, the food group data were derived from a single questionnaire administered at baseline. Both included older persons. Whereas the NHS sample was exclusively female, the CHAP sample was slightly more than one third male (38%) and 62% black.

In both, the reported rate of change in cognitive scores was a decline of 0.04 standardized units/year! NHS women consumed a median of 3.1 servings of vegetables daily, and 2.4 servings of fruits. In CHAP, the averages were slightly lower: 2.3 servings per day of vegetables, and for fruit, 2.2. In both, however, a slower rate of cognitive decline was observed with higher vegetable intakes, and, in particular, higher intakes of green leafy vegetables (NHS, median, 0.8; CHAP, 0.36 daily servings). In NHS, the rate of decline among persons in the top fifth of vegetable intake was equivalent to being 1.5 years younger in age. In the CHAP cohort, the decline rate was seen in the top two-fifths of the sample (more than 2 vegetable servings per day) and was equivalent to 5 years of younger age. In NHS, higher intakes of legumes were also associated with reduced rates of decline; this association was not observed in CHAP. In both, fruit consumption was unrelated to cognitive changes.

It is always possible that the reported associations may be due to residual or unmeasured confounding. Yet, it is assuring that in two different population samples of older adults, the findings are remarkably consistent. Moreover, in the CHAP analyses, the inverse association between green leafy vegetable consumption and cognitive change was attenuated when models included vitamin E. These vegetables are often the richest in vitamin E content and also usually consumed with added fats (salad dressing, margarine, etc) that are also only vitamin E rich but enhance the absorption of vitamin E and other fat soluble nutrients. In CHAP, intakes of vitamin E and total vitamin C in food were related to slower cognitive decline over 3 years(8). Other constituents of vegetables, in particular, green leafy vegetables, have been associated with rates of cognitive decline. These include folate, and the class of antioxidant –rich polyphenolics known as flavonoids. Cognitive performance was the secondary endpoint in the randomized clinical trial, FACIT. In that 3 year trial, daily supplements of 800 mcg folate (against placebo) improved memory and other cognitive domains that decline with age(9). Two groups(10, 11) found that higher dietary flavonoid intake was related to a lower risk of AD in the Rotterdam cohort and the French cohort, PAQUID (Personness Agées Quid), respectively. In a 10 year follow-up of PAQUID cohort(12), cognitive evolution was again better with higher dietary flavonoid intake.

Further research is needed to confirm the vegetable-cognitive association. We suggest refinements in present dietary instruments used in epidemiologic studies ---first, queries regarding the mode of culinary preparation used for vegetables and to a lesser extent, fruits might be included. Many polyphenols exist primarily in the outer section or skins of fruits and vegetables. Thus, boiling or peeling can remove much of the polyphenolic content. Second, meal based questionnaires may be used. These changes could provide more sensitive and discriminatory tools to evaluate the contributions fruits and vegetables play in cognitive aging.

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