This study examined the synergistic effect of PA and F&V intake on cognitive decline in older Taiwanese. Independently, high PA and high intake of F&V were associated with decreased likelihood of the risk of cognitive decline. When PA and F&V intake were combined, the simultaneous of high PA and high F&V intake significantly reduce the risk of cognitive decline as high as 63% in older Taiwanese.
Presently, it is known that PA and F&V intake are widely recommended health behavior. F&V intake have been independently proved with reduced risk of diabetes, stroke, heart disease and cognitive functional decline [44–47]. As well, although much work has been done to explore the relationship between PA and cognitive function, this relationship has been mainly explored in older adults [48, 49]. To date, very little is known about the combined effect of PA and F&V intake and cognitive function, in older Asian adults.
The present study adds significant and creative information as follows. First, the results support important findings of the synergistic effects of PA and F&V intake on decreasing cognitive decline in Taiwan, and showed that PA and F&V intake play a vital role in improving the older Taiwanese's cognitive functions. PA and F&V intake are predictors of cognitive decline and negatively related to the cognitive risk of the elderly. We argue that PA and the intake of F&V simultaneously are beneficial to decreasing cognitive decline. Practicing these habits at the same time brings more significant health benefits than practicing them alone. In the present study, F&V intake was not independently related to cognitive decline. Compared with high PA and high intake of F&V separately, the risk of cognitive decline decreased 63% when high PA and high intake of F&V were combined. Unlike western countries with highly PA habits and facilities, Asians used to have fewer PA habits due to lack of insufficient facilities .
Second, the study reported an important longitudinal result of the national population-based cohort study of cognitive functions in older Taiwanese. To our knowledge, this is the pioneer study that explore the 16-year longitudinal relationship between PA and F&V intake of older adults in Taiwan. Third, the generalized estimating equations (GEE) models used herein to analyze the longitudinal data during the 16-year follow-up period. GEE for this study are a convenient and general approach to the analysis of several kinds of correlated data. The main advantage of GEE resides in the unbiased estimation of population-averaged regression coefficients despite possible misspecification of the correlation structure [42, 43].
Although the mechanism of the positive effect of PA and F&V intake on cognitive decline is unclear, there are several possible mechanisms for the cognitive decline. The first is our research results show that high PA can effectively reduce the risk of subsequent cognitive decline. This finding is consistent with many previous studies. Daily PA has been found to prevent cognitive decline in the elderly and help delay the cognitive decline that they already have; the severity of the elderly, PA is one of the risk factors for cognitive decline and associated with better cognitive performance [51–53]. Different kinds of sports lead to mental stimulation properties, such as the need for eye-hand coordination and visual and spatial memory, which further enhance their impact on cognitive function . In 2018, 26 researchers representing nine countries and a variety of academic disciplines met in Snekkersten, Denmark, to reach evidence-based consensus about PA and older adults. They presented the consensus on the effects of PA on older adults' fitness, health, and cognitive functioning, functional capacity, engagement, motivation, psychological well-being and social inclusion.Studies have also suggested that the reason why exercise has the effect of preventing dementia may be related to the stimulation of neurotrophic factor (brain-derived neurotrophic factor, BDNF) secreted by the brain, which can prevent the hippocampus from shrinking and maintain cognitive function. In daily life, we perceive external stimuli through our eyes, ears, tongue, nose, and skin. These perceptions can be roughly divided into five types: sight, hearing, taste, touch, and smell, which are also called the "five senses." PA can promote the activity of our five senses. At the same time, proper PA is also very helpful to prevent dementia and restore cognitive function [55, 56]. This is the same result that this study found that PA is a protective factor and high PA helps reduce the risk of cognitive decline.
Second, the analysis in this study found that high F&V intake is an important factor in significantly reducing the risk of cognitive decline. The results of recently published integrated analysis studies consistently found that increased intake of F&V, is associated with a decrease in the risk of cognitive decline. This finding is in line with recent studies, indicating that diet plays a related role in people's cognitive decline. Foods rich in antioxidants, such as fruits, vegetables, and nuts, can improve the prevention or delay of the occurrence of cognitive decline, higher intake of is related to reducing or preventing cognitive decline [47, 57]. There are many known protective mechanisms, such as polyphenolic compounds in many plant foods, and bioactive compounds in various fruits, vegetables, legumes, nuts and whole grains, including antioxidants, vitamins, and polyphenols. Other phytochemicals and unsaturated fatty acids, by reducing oxidation, can enhance synaptic plasticity and neuron survival [58, 59], alleviate cognitive decline, help cognitive health, and improve specific cognitive areas (especially the frontal lobe executive function). Polyphenols in fruits and vegetables can regulate tau hyperphosphorylation and β-amyloid aggregation in animal models of Alzheimer's disease [60, 61]. Taken together, for the elderly, the different nutrients contained in fruits and vegetables can reduce the risk of cognitive decline. Third, this study also has a multiplier effect on cognitive decline by "combining high F&V with high PA", which can reduce the risk of dementia by 67%; further evidence is strengthened to support the impact of the combination of PA and F&V intake on cognitive decline .
Why PA and F&V have synergy? According to Walker and Avant's health literacy concept analysis , elderly people with health literacy pay more attention to health and are more diligent in acquiring, understanding and applying health information or knowledge. Factors, personal factors, and situational factors may change, but such elderly people understand that they are in a state of degraded physical, mental, and social functions. Their health literacy will encourage them to develop the habit of physical activity simultaneously and focusing on ingesting fruits and vegetables to achieve self their purpose of health management, health care and prevention of cognitive decline.The elderly in Asia and Taiwan, because young urbanites are busy at work, they generally have bad habits such as eating imbalance and lack of exercise, and the risk of chronic diseases in their later years will also increase . Because the body is getting older and paying attention to health, both PA and F&V intake are healthy behaviors that require continuity and discipline, and gradually develop exercise and diet habits in life, so a healthy attitude towards PA and F&V intake is more important. According to research by Harooni et al., regular healthy living habits and healthy living discipline are very important for the elderly. Both PA and F&V intake require regular and disciplined maintenance in order to have good health effects, healthy and successful aging, and prevent cognitive decline is the best proof.
PA and nutrition have complementary effects, and there are risks if they are separated. As far as cognition was concerned, according to the results of a survey in Taiwan , the most common physical activities were: walking (69.6%), gymnastics (14.9%) and hand-shaving (8.5%). It can be seen that PA of the elderly stay in low-intensity and low-resistance had less health-promoting effects, and most of them were engaged in simpler physical activities in parks or schools near their homes and intensity of PA was insufficient. Nutrient supplements with F&V are needed to reduce the risk of cognitive decline. Conversely, although a great amounts of F&V intake can increase the nutritional content, if without cooperate with PA, the effect of preventing cognitive decline will be reduced to 40% and 23%.
This study has limitations. Cognitive decline was not diagnosed by physicians but self-reported by the elderly. Although it has acceptable accuracy, it inevitably has some shortcomings. The cognitive status of this study was based on the topics of the Chinese version of the SPMSQ scale, which was widely used in domestic studies, and its reliability and validity had also been confirmed. This study used a relatively long period of time between 1999 and 2015, which lasted for 16 years. As dietary and physical activity may change with age, health status, social environment and other factors. The research subjects were elderly people in Taiwan, so the results may not be applicable to younger Taiwanese. Furthermore, data was collected for frequency of intake without quantity (such as servings or serving sizes), so food intake cannot be quantified.