In the current study, we demonstrate that nutritional deficit is significantly associated with cognitive decline. The prevalence of malnutrition and cognitive impairment was relatively high and increased with age. In particular, malnutrition contributes to global cognition decline and disorientation. Moreover, malnutrition leads to attention problem, calculation problem and language impairment in the elderly who were over 80 years old.
We have used the MNA-SF to evaluate the nutritional status in the present study. Among the 2365 participants, 5.54% were malnourished, 33.45% were at risk of malnutrition. The prevalence was higher in those over 80 years old, nearly half of them (48.63%) were malnutrition or at risk of malnutrition. This data is comparable to those with large samples. A meta-analysis showed that the prevalence of malnutrition was 5.8% and 31.9% of community-living elderly persons were at risk of malnutrition . It indicates that our results are closer to the actual nutritional status of the elderly. Further, we find participants above 80 years of age were at a higher risk of malnutrition, and few previous studies have focused on this age category. We think that aging, as well as consequently frailty progress, is associated with the greater overall prevalence of disability and function limitation, thus directly contribute to the development of malnutrition. Therefore, malnourished is pervasive in the elderly and clinicians should pay more attention to the nutritional status of those over 80 years old.
As far as cognitive status is concerned, in the present study, 63.26% of the participants had normal cognitive status, 36.74% of them had cognitive impairment according to the MMSE score. Recent studies have shown that the prevalence of cognitive impairment was 32.9%-34.4% [23, 24]. Our results are comparable. However, the previous studies showed that the prevalence of cognitive impairment was 11.0%-22.2% as assessed by MMSE in 2003–2011 [25–27], this different prevalence in various periods indicates that there is an increasing trend of cognitive impairment following aging. We also found that the prevalence of cognitive impairment is markedly higher among those over 80 years old (53.65% had cognitive impairment). Previous research indicates that elevated levels of cerebral protein beta-amyloid (Aβ) in apparently healthy persons are found in at least 50% of those over 80 years old . As Aβ accumulation results in cognitive decline, advance ages (over 80 years old) are prone to cognitive impairment. Therefore, the cognitive status of the elderly is not optimistic and requires to be concerned immediately.
In this study, we find nutritional deficient, including malnutrition and at risk of malnutrition, is a risk factor for cognitive impairment. First of all, a correlation was observed between the MNA-SF score and the MMSE score after adjusting for other variables. Secondly, the elderly with cognitive impairment were more likely to be malnutrition while the majority of the elderly with malnutrition had cognitive impairment. In line with the previous study that malnutrition was a significant risk factor for cognitive decline and directly associated with AD pathology [7, 29]. The elderly who had cognitive impairment were more likely to suffer from malnutrition [11, 12]. The interrelationships between malnutrition and cognitive impairment are complex and reciprocal.
As we know, the brain is a complicated organ demand for high oxygen. Several nutrients, as essential constituents of brain tissue, play not only important roles for brain integrity and metabolism, but also prevent cognitive decline by counteracting pathological processes . Nutrients deficiency and an unbalanced diet may contribute to synaptic dysfunction, promote neuronal loss, then lead to cortical thinning and results in cognitive impairment. Epidemiological evidence suggests that cognitive disorder is associated with the deficiency of some specific nutrients[31, 32]. For example, patients with cognitive impairment or dementia have a lower plasma level of several nutrients, including folate, vitamin A, vitamin B12, vitamin C, and vitamin E . It indicates that nutrients deficiency is associated with cognitive decline and the pathological processes of AD. Moreover, the elderly with cognitive impairment may suffer a loss of appetite due to pathological changes in the olfactory system . As the disease progresses, patients may lose their cognitive ability to initiate or continue effective eating strategies , which resulted in decreased nutritional intake and deterioration of nutritional status. To summarize, malnutrition precedes cognitive impairment, and this interplay may result in a vicious circle and thus lead to more pronounced malnutrition.
Our results indicate that malnutrition contributes to disorientation. Nutritional deficits may also lead to a decline in attention, calculation and language in the elderly who was over 80 years old. Considering sub-items of cognition are located in different regions of the brain, we think that different brain regions have different sensitivity to malnutrition. It is reported that the affected brain regions involved in disorientation include mostly the middle temporal and parietal cortices , networks involving the temporal and dorsolateral prefrontal cortex are shown to support language processing [37, 38], posterior cingulate cortex is related to attention function . Thus, temporal cortex, parietal cortex, dorsolateral prefrontal cortex and posterior cingulate cortex are easily affected by malnutrition. A higher Mediterranean diet (MeDi) score and beneficial components of MeDi (fish, vegetables, legumes, and whole grains/cereals) are associated with larger cortical thickness in some specific brain regions, such as temporal, dorsolateral prefrontal, posterior cingulate cortex. Therefore, MeDi might be recommended to prevent cognitive decline associated with malnutrition. In addition, when patients show a sign of disorientation, nutritional status should be screened to find malnutrition earlier.
In this study, we find that lower weight and BMI are independent risk factors for malnutrition. It consists with previous research that the elderly with lower BMI were more likely to suffer from malnutrition [23, 40]. It reminds us that we should monitor the weight of the elderly regularly to detect weight loss and prevent malnutrition. Low education is also a risk factor for malnutrition. Similarly, previous researches have shown illiteracy was found to have a higher prevalence of malnutrition [23, 41]. This may be due to the elderly with higher educational status possibly had higher income and better dietary patterns. Regarding the functional status, we found that the elderly with nutritional deficits performed significantly poorer in the Barthel Index, it indicated that malnourished elderly were more likely to have a functional disability. This is consistent with results from previous studies that malnutrition and frailty were two interrelated factors .