Evidence shows that metabolic syndrome (MS) is associated with a greater risk of developing Parkinson's disease (PD) because of the increase in oxidative stress levels along with other factors such as neuroinflammation and mitochondrial dysfunction. However, because some studies have reported that MS is associated with a lower risk of PD, the relationship between MS and PD should be investigated. This study aimed to investigate the effect of MS on PD. Two authors searched five electronic databases, namely, MEDLINE, PubMed, Scopus, PsycINFO, Web of Science, and Science Direct, for relevant articles between September and October 2020. After screening the title and abstract of all articles, 34 articles were selected for full-text review. Finally, 11 articles meeting the eligibility criteria were included in the study. The quality of articles was critically evaluated using the Joanna Briggs Institute. Overall, we evaluated data from 23,586,349 individuals (including healthy individuals, with MS and PD) aged 30 years or more. In cohort studies, the follow-up period varied between 2 and 30 years. MS contributed considerably to the increase in the incidence of PD. In addition, obesity, a component of MS, alone can increase the probability of developing neurodegenerative diseases. However, despite few studies on MS and PD, changes in cognitive function and more rapid progression of PD disease has been documented in patients with MS using methods commonly used in research.

Various functions in the central nervous system, such as growth, development, and cognition can be influenced by vitamins and minerals, which are capable of helping to maintain brain health and function throughout life. Cognition is understood as the aspects related to knowledge, learning, and understanding, as well as the ability to develop these functions. A possible association between low levels of vit D and deficit in the performance of cognitive functions in healthy humans or with some pathological condition is discussed. Because of this, the present systematic review analyzed only randomized clinical trials carried out in healthy non-athlete adults about intellectual and/or mental processes involving cognitive functions to identify whether these individuals with different levels of vit D are capable of interfering with the performance of the cognitive function. To do so, we adopted the PRISMA method criteria and registered it in the PROSPERO database. The search was performed in PubMed (MEDLINE), PsycINFO, Science Direct, Scopus, and Web of Science databases, 2,167 records were identified. The 5 most frequent cognitive domains in the selected studies were: processing speed, attention, verbal learning/memory, executive function, and general cognitive functions. We found that there are positive changes in the following domains: verbal memory and verbal working memory, learning memory, attention, executive function, and also cognitive function in general. We highlight the following suggestions for improvements that vitamin D supplementation may promote in the cognitive domains of healthy adults: a) low doses between 400 and 600 IU/d seem to be more effective when compared to doses between 2,400 and 5,000 IU/d and b) food fortification and enrichment with vit D, need further studies, as they seem to be more or as effective as synthetic supplementation. We evident that there is a need for trials that evaluate the control of vit D levels for healthy adult individuals is important, as they have the potential to minimize health problems, especially those involved in the reduction of cognitive abilities. Thus, the development of more clinical trials to obtain satisfactory answers on this topic needs to be encouraged.Systematic review registrationhttps://www.crd.york.ac.uk/PROSPERO/, identifier: CRD42021262413.

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