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.