LIFESTYLES AND COGNITION

Cognitive decline is a growing medical concern. It includes age-related cognitive decline, mild cognitive impairment, and dementia. Dementia results in considerable dysfunction in life and is associated with an increase in mortality. Since there is no cure at this time, attention is being increasingly directed towards prevention. Lifestyle factors, such as smoking, alcoholism, physical inactivity, poor diet, improper sleep, and loneliness are repeatedly being recognized as modifiable factors that can reduce cognitive decline. This manuscript briefly reviews the lifestyle-cognition relationship.


INTRODUCTION
Cognition, a normal function of the brain, consists of perceiving, processing, and administrating information [1]. This ability progressively declines when we age, and this is considered normal [2]. Mild cognitive impairment (MCI) results when the impairment in cognition is more severe than that which is age-related [3]. People with MCI are, however, able to perform normal activities and can compensate for small changes [4]. MCI prevalence is approximately 10%-20% worldwide [5]. People with MCI have a heightened risk of further cognitive decline and progression to dementia [4]. Dementia results in cognitive decline severe enough to interfere with social and occupational functioning [6]. The global prevalence of dementia is approximately 50 million, and this number is projected to reach 131.5 million by 2050 [7]. It is usually seen in the elderly population and it is estimated that it affects 5-8% of people aged 60+ years [8]. Alzheimer's disease (AD) accounts for 50-70% of dementia cases [9]. AD is a progressive disease [10], and the cognitive decline severely disturbs social and occupational functioning [6]. MCI and dementia greatly reduce the quality of life [11], and are associated with increased mortality [12]. Cognitive impairment is associated with both white matter inflammatory lesions and gray matter pathology, such as cortical lesions and brain atrophy, and this reflects neurodegeneration [13]. There is no cure for MCI or dementia [14]. The focus on healthy lifestyles and their impact on cognition is therefore gaining more attention in the medical circles [15].

DISCUSSION
Most diseases are only 20-35% heritable [16] and are predominantly dependent on external factors [17]. One such factor is lifestyles. Lifestyles not only impart a significant effect on the genesis and progression of chronic diseases [18], but they also affect mortality [19]. Lifestyle behaviors such as smoking, heavy alcohol consumption, physical inactivity, and poor diet/obesity may be responsible for up to 60% of premature deaths [20]. According to Li et al., adherence to healthy lifestyles could prolong life expectancy at age 50 by 14.0 years in US females and 12.2 years in US males compared with individuals who adopted zero low-risk lifestyle factors [21]. Similar life extension benefits of following a healthy lifestyle have been documented in other countries [22]. Although some cognitive decline is considered 'normal' with aging, MCI and dementia are on the rise, in part due to the surge in the global the aging population [23]. Because there is no established treatment, the search for therapeutic intervention has now increasingly focused on lifestyles/health-related behaviors [15] Unhealthy behaviors have been consistently noted to increase the rate of cognitive decline. On the other hand, healthier lifestyles are associated with better cognitive function [24] and reduce the incidence and progression of MCI and dementia [15].
Smoking: Several studies reported acute exposure to nicotine induces cognitive enhancement [25]. Many psychiatric disorders are associated with cognitive impairments, including deficits in attention, working memory, and response inhibition function [26]. The shortterm enhancing effect of nicotine may explain the high rate of smoking in individuals with psychiatric disorders [27]. Some researchers have also suggested that nicotine may be therapeutic in Alzheimer's disease and several psychiatric disorders [25]. The acute nicotine related effect on cognition may also explain why smokers are unable to quit [28]. The cognition performance enhancement associated with smoking is however short-term [29]. With long term use, smoking harms the cognitive function [30]. It has been causally implicated in the development of neurodegenerative disorders [31] including AD [32] and other dementias [33]. Previous studies indicated that smokers were 1.9-4.3 times more likely to develop AD than non-smokers [34]. The Honolulu-Asia Aging Study noted a dose-response relationship with smoking amount and AD [35]. Smokers exhibit excessive oxidative stress, neuroinflammation, and impaired neuroprotection in the brain [36]. Brain studies reveal a thinner anterior cingulum and prefrontal lobe, and many other structural abnormalities in smokers [37]. Passive smoking increases the risk of cognitive impairment in older adults by 24% and this seems to be especially prominent with non-smokers [38]. Cigarette smoking at an earlier age appears to predict the onset of cognitive impairment at an older age [39]. This risk is directly related to the duration and intensity of smoking and subsides gradually following smoking cessation [40]. Tobacco is a teratogen that impairs fetal brain development and smoking during pregnancy harms the developing fetus [41].
Alcohol: Many studies have demonstrated that low to moderate alcohol intake may prevent cognition decline [42]. However, the relationship between alcohol intake and cognition is U shaped [43]. Excessive alcohol intake and binge drinking are associated with an increased risk of cognitive decline and dementia [44]. Alcohol, when excessive, is neurotoxic to the prefrontal cortex and hippocampal regions of the brain and results in neurodegeneration [45]. The result is functional deficits, especially impairment of executive and motivational functions [46]. Prenatal alcohol exposure may hamper the children's cognitive development [47]. Physical exercise: Physical exercise helps retard cognition decline [48]. Studies have overwhelmingly demonstrated a relationship between physical inactivity and cognitive impairment, including AD [49]. A recent meta-analysis reported significant beneficial effects on cognitive outcomes with exercise in adults aged 50 and older [50]. Another meta-analysis concluded that those with a high level of physical activity showed a 38% less decline in cognitive performance (compared with adults not engaged in physical activity) during 1-12 years of follow-up [51]. A low to moderate level of physical exercise has also demonstrated a 35% less decline in cognitive function [51]. Diet: Nutrition is one of the modifiable risk factors for cognitive decline and AD dementia [52]. Several studies have documented that a poor diet promotes cognitive decline, including AD [53]. A prospective study of 3,718 elderly participants, showed that consumption of greater than two vegetable servings per day decreased cognitive decline over a 6-year follow-up [54]. These individuals appeared about 5 years younger on cognition testing [54]. In cognitively normal populations, the Mediterranean diet and dietary approaches to stop hypertension (DASH) diet, result in lesser age-related cognitive decline [55]. They also reduce the risk for the development of MCI or AD [56]. Consumption of fish is also associated with lesser cognitive decline in older people without AD [57]. Vegetables and fruits contain a plethora of antioxidant vitamins and nutrients, and they help counteract cognitive decline by reducing oxidative stress and inflammation [58]. Vegetables and fruits might also alter the gut microbiota, improving the gut-brain axis, and positively impacting cognitive health [59]. Fish is a good source of long-chain omega-3 fatty acids, and this helps improve cognition [60]. Obesity and its relationship with cognition is mixed [61]. While obesity in midlife appears to be detrimental to cognitive decline, obesity in the old (over 65 years) has been reported to be detrimental, neutral, or even protective [62]. This "obesity paradox" may be explained by the survival of the less obese and healthier individuals as aging occurs [61]. It has also been suggested that this paradox may be insignificant if measurements of central obesity (WC and WtHR) are used, instead of relying solely on BMI [62]. Marijuana abuse: The two most well-studied cannabinoids are Δ9-tetrahydrocannabinol (THC), or its synthetic variants (dronabinol, nabilone), and cannabidiol (CBD). The use of THC has been associated with adverse cognitive effects [63]. Sleep: There is cumulating evidence that sleep quality and duration relate to cognitive processes. Bubu and colleagues, in a meta-analysis, found that sleep difficulty was associated with cognitive impairment in older adults [64]. It is estimated that the risk of developing dementia in patients with sleep disorders was 1.68 times greater [64]. Good quality and sufficient sleep is beneficial in individuals with AD [49]. Socialization: Lack of socialization, with a subjective feeling of loneliness, is associated with decreased cognitive function [65]. Loneliness leads to disturbed selfregulation which also promotes non-compliance with healthy lifestyles [66]. Lonely people are also more likely to be depressed. Depression contributes to cognitive decline [67]. Loneliness adversely affects the HPA axis inflammation and immunity, which influence cognitive decline [68].

CONCLUSION
Cognitive decline is multifactorial, and there is no known cure. Aging is the most common cause of a decrease in cognition. However, MCI and dementia are pathological and are worsened by unhealthy lifestyles. Healthy lifestyles include adherence to a normal body weight (BMI 18.5-24.9 kg/m²), regular physical activity (150 min/week of moderate physical activity), a healthy diet rich in fruits and vegetables, avoidance of alcohol and smoking, getting refreshing sleep (about 8 hours per day) and having a good social life. Most individuals with unhealthy behaviors usually indulge in other unhealthy activities also, and this further worsens cognitive decline.

COMPETING INTERESTS
The author declares no competing interests with this case.

FUNDING SOURCES
None.