Evidence-Based Prevention of Alzheimer's Disease: Systematic Review and Meta-Analysis of 243 Observational Prospective Studies and 153 Randomized Controlled Trials

Background: Evidence on preventing Alzheimer's disease (AD) is challenging to interpret due to varying study designs with heterogeneous endpoints and credibility. We completed a systematic review and meta-analysis of current evidence with prospective designs to propose clinical recommendations on AD prevention. Methods: Electronic databases and relevant websites were searched from inception to March 1, 2019. Both observational prospective studies (OPSs) and randomized controlled trials (RCTs) were included. The multivariable-adjusted effect estimates were pooled by random-effects models, with credibility assessment according to its risk of bias, inconsistency, and imprecision. Levels of evidence and classes of recommendations were summarized. Findings: A total of 44,676 reports were identified, and 243 OPSs and 153 RCTs were eligible for analysis after exclusion based on pre-decided criteria, among which 104 modifiable factors and 11 interventions were included for the meta-analyses. In the OPSs, 26 risk factors and 8 protective factors were found to significantly modify AD risk by an effect size of over 25%, amongst which eight factors were rated at a moderate-to-high level of evidence. Among the interventions tested in RCTs, physical exercise and the homocysteine lowering treatment were the most promising interventions to reduce AD risk. Finally, 21 recommendations are proposed based on the consolidated evidence, with 'Class I' recommendations targeting 19 factors: ten are with 'Level A' strong evidence (cognitive activity, hyperhomocysteinemia, high BMI in late-life, depression, stress, diabetes, head trauma, hypertension in midlife, orthostatic hypotension, and education) and nine with 'Level B' weaker evidence (obesity in midlife, weight loss in late-life, physical exercise, smoking, sleep, CVD, frailty, atrial fibrillation, and Vitamin C). In contrast, two interventions are not recommended: estrogen replacement therapy (Level A2) and acetylcholinesterase inhibitors (Level B). Interpretation: Our study mapped the current evidence profile and assessed the findings in order to guide future research directions for AD prevention. Evidence-based recommendations are proposed, offering clinicians and stakeholders current guidance for the prevention of AD. Funding Statement: The authors stated: "There was no direct funding source for this study." Declaration of Interest: JTY serves as an associate editor-in-chief for Annals of Translational Medicine, is senior editor for Journal of Alzheimer's Disease. SA has received grants from Europe, Ipsen, and France Alzheimer, served as a consultant for Ipsen, Pierre Fabre, Lilly, Nestle, Sanof, and Servier, and received non-financial support from Biogen, Nutrition Sante, Pfzer, and Icon, and other support from the AMPA Association. SG has received clinical trial support from Lilly and Roche in DIAN-TU, TauRx Therapeutics (TauRx), and Lundbeck; has been a data safety monitoring board (DSMB) member of ADCS, ATRI, API, and Eisai; and has been a scientific adviser to Affiris, BoehringerIngelheim, Lilly, Roche, Servier, Sanofi, Schwabe, Takeda, and TauRx. PSA has received grants from the US Alzheimer’s Association, Janssen, Lilly, the US National Institute on Aging, and Toyama; and consulting fees from Abbott, Abbvie, Amgen, Anavex, AstraZeneca, Biogen Idec, Biotie, Bristol-Myers Squibb, Cardeus, Cohbar, Eisai, Elan, Eli Lilly, Genentech, Ichor, iPerian, Janssen, Lundbeck, Medivation, Merck, NeuroPhage, Novartis, Pfizer, Probiodrug, Roche, Somaxon, and Toyama, outside the submitted work. BV reports grants from Pierre Fabre, Avid, Exonhit, AbbVie, Lilly, Lundbeck, MSD, Otsuka, Regenron, Sanof, Roche, AstraZeneca, LPG Systems, Nestle, and Alzheon, and personal fees from Lilly, Lundbeck, MSD, Otsuka, Roche, Sanof, Biogen, Nestle, Transition Therapeutics, and Takeda. All other authors declare no competing interests. Ethics Approval Statement: The authors followed the recommendations by the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) 2009 guidelines.