Plain language summary
Alzheimer’s disease (AD) is a chronic neurodegenerative disease that is characterized by progressive memory loss and cognitive deficits. Mild cognitive impairment (MCI) is a prodromal form of AD that is characterised by neurocognitive dysfunction. However, pathological changes associated with AD begin to occur in the brain at least 10 years before the onset of overt symptoms and clinical manifestations, making the discovery of early diagnostic methods and timely interventions important for AD management. The aim of this study was to determine the relationship between circulating methionine (Met) components and AD/MCI. This study is a systematic review and meta-analysis of thirty studies (10 case– control studies, 10 cohort studies, and 10 cross-sectional studies). Results show that: - individuals with AD compared with controls had significantly increased levels of homocysteine. - for people with MCI, there was no significant difference in blood homocysteine levels in the control group. - there were no differences in blood vitamin B12 levels between patients with AD or MCI and controls. Authors conclude that circulating Met components affect patients with AD compared to cognitively normal individuals, with patients exhibiting higher blood homocysteine levels. Additionally, high Met and S-adenosylmethionine levels are risk factors for AD, which supports the association between Met cycle components and AD/MCI.
Abstract
BACKGROUND Circulating methionine components have been reported to be associated with Alzheimer's disease (AD) and mild cognitive impairment (MCI), although outcomes are not always consistent. MATERIALS AND METHODS Database searching was conducted using PubMed, Embase, Cochrane Library, and Web of Science from inception to 26 December 2021. In this study, two reviewers independently identified eligible articles and extracted the data. We used Joanna Briggs Institute (JBI) Critical Appraisal tools to assess the overall quality of the included studies. STATA software was employed to perform meta-analysis evaluating the standardized mean difference (SMD) with its 95% confidence intervals (CIs) using random-effects models. Evidence quality was assessed using the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) criteria. RESULTS Totally, 30 observational studies were eligible for inclusion. Compared with cognitively normal controls, patients with AD had increased homocysteine (Hcy) levels in the blood [standardized mean difference (SMD) = 0.59, 95% confidence interval [CI]: 0.36-0.82, P = 0.000], plasma (SMD = 0.39, 95% CI: 0.23-0.55, P = 0.000), and serum (SMD = 1.56, 95% CI: 0.59-2.95, P = 0.002). Patients with MCI were not significantly different from controls (SMD = 0.26, 95% CI: -0.07-0.58, P = 0.127). Patients with AD or MCI did not significantly differ from controls of blood vitamin B12 levels, AD (SMD = -0.05, 95% CI: -0.19-0.08, P = 0.440), or MCI (SMD = 0.01, 95% CI: -0.16-0.17, P = 0.94). Some cohort studies have suggested that higher Hcy, methionine, and S-adenosylmethionine levels may accelerate cognitive decline in patients with MCI or AD, and vitamin B12 deficiency is a risk factor for the disease; however, the results of other studies were inconsistent. According to the GRADE system, all these outcomes scored very low to low quality, and no high-quality evidence was found. CONCLUSION Only Hcy levels in the plasma and serum were found to be inversely related to the risk of AD. However, due to the low quality of supporting these results, high-quality studies are needed to verify these findings. SYSTEMATIC REVIEW REGISTRATION http://www.crd.york.ac.uk/PROSPERO/, identifier CRD42022308961.
Methodological quality
Jadad score
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Not applicable
Allocation concealment
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Not applicable
