Background: Irisin is a newly discovered hormone that is proposed to be a promising therapeutic target in obesity and type 2 diabetes. It has received remarkable attention recently, while inconsistent results have been shown regarding its association with chronic exercise training in adults.
Objective: The aim of this study was to evaluate the effects of chronic exercise training on circulating (plasma/serum) irisin in adults by meta-analyzing randomized controlled trials (RCTs) and non-randomized studies (NRSs) separately.
Methods: We conducted a search of the PubMed, Web of Science and Cochrane Library databases from January 2012 to September 2014 for studies published in English. Studies were included if they had an assessment of chronic (≥8 weeks) exercise training effects on circulating irisin in the general or clinical population (mean age ≥18 years) with reported outcomes of circulating irisin, and involved more than five participants. Pooled effect sizes (Cohen's d) with 95 % confidence intervals (CIs) for changes in circulating irisin were calculated by a random-effects model. Subgroup analyses were performed to evaluate the association between exercise modes and changes in circulating irisin.
Results: Of the 181 articles screened, 12 studies in eight articles were included, of which three were RCTs and nine were NRSs. In the three RCTs (a total of 173 healthy, untrained participants), chronic exercise training was associated with a moderate and significant overall effect in decreasing circulating irisin compared with the control (d = -0.46; 95 % CI -0.76 to -0.15). Chronic resistance exercise training showed a moderate and significant effect in decreasing circulating irisin compared with the control (d = -0.41; 95 % CI -0.75 to -0.06), while endurance exercise training only had a trend (d = -0.64; 95 % CI -1.32 to 0.04). In the nine NRSs (a total of 113 intervention participants), chronic exercise training was associated with a trivial and non-significant overall effect in decreasing circulating irisin compared with baseline (d = -0.04; 95 % CI -0.30 to 0.23).
Conclusions: Chronic exercise training leads to significantly decreased circulating irisin levels in the RCTs, while evidence remains inconclusive in the NRSs. Well-designed RCTs that measure dietary intake and report changes of body fat percentage or insulin sensitivity/resistance index following chronic exercise training are required to confirm these findings.