Abstract

Calorie restriction (CR) and reductions in protein intake in rodents result in increased lifespan and reduced levels of IGF-1. However, the changes in IGF-1 in humans in response to CR and elevated protein intake are confused. We conducted a systematic review and meta-analysis to investigate the effect of Calorie restriction (CR) or increase in protein intake on IGF-1 in humans. The systematic review protocols have been developed based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement guidelines. Two separate systematic searches were undertaken: first for the effect of CR and second on the effect of increase in protein intake on IGF-1. PubMed, SCOPUS and ISI Web of Science databases were searched. In the meta-analysis of the calorie restriction studies, twelve studies met the inclusion criteria (8 clinical trials and 4 observational studies). The meta-analysis of both clinical trials and observational studies revealed no significant effect of CR on IGF-1 (clinical trials: standardized mean difference (SMD) = 0.002 ng/ml, 95% CI -0.14 to 0.14 ng/ml, p = 0.98; observational studies (SMD = -1.14 ng/ml, 95% CI -1.9 to -0.38 ng/ml, p = 0.003). In the meta-analysis of protein intake studies (six studies), a significant increase in circulating IGF-1 levels in response to increases in dietary protein was revealed (SMD = 0.4 ng/ml, 95% CI 0.18-0.61 ng/ml, p < 0.001). In conclusion, in humans, CR was not associated with a significant change in circulating IGF-1. However an increase in protein intake was associated with increased levels of circulating IGF-1. PROTOCOLS REGISTRATION NUMBER: CRD42017073149 for the protein intake meta-analysis and CRD42016046260 for CR meta-analysis.