Decreased Levels of Insulin-Like Growth Factor-1 Are Associated with Alzheimer's Disease: A Meta-Analysis.

Background: Alterations in levels of peripheral insulin-like growth factor-1 (IGF-1) in Alzheimer’s disease (AD) have been reported in several studies, and results are inconsistent. Objective: We conducted a meta-analysis to investigate the relationship between peripheral and cerebrospinal fluid IGF-1 levels and AD or mild cognitive impairment (MCI). Methods: A systematic search in PubMed, Medline, Web of Science, Embase, and Cochrane Library was conducted and 18 studies were included. Results: Results of random-effects meta-analysis showed that there was no significant difference between AD patients and healthy control (17 studies; standard mean difference [SMD], –0.01; 95%CI, –0.35 to 0.32) and between MCI patients and healthy control (6 studies; SMD, –0.20; 95%CI, –0.52 to 0.13) in peripheral IGF-1 levels. Meta-regression analyses identified age difference might explain the heterogeneity (p = 0.017). However, peripheral IGF-1 levels were significantly decreased in AD subjects (9 studies; SMD, –0.44; 95%CI, –0.81 to –0.07) and MCI subjects exhibited a decreasing trend (4 studies; SMD, –0.31; 95%CI, –0.72 to 0.11) in studies with sample size≥80. Cerebrospinal fluid IGF-1 levels also significantly decreased in AD subjects (3 studies; SMD, –2.40; 95%CI, –4.36 to –0.43). Conclusion: These findings suggest that decreased peripheral and cerebrospinal fluid IGF-1 levels might be a potential marker for the cognitive decline and progression of AD.