Lower muscle protein synthesis in humans with obesity concurrent with lower expression of muscle IGF1 splice variants

This study tested the hypothesis that expression of insulin-like growth factor 1 (IGF-1) protein and mRNA splice variants is lower in skeletal muscle of humans with obesity who have a lower mixed-muscle protein fractional synthesis rate (MMP-FSR) when compared with individuals without obesity.

The study included nine participants with obesity (OB, mean [SD],  BMI = 35 [3] kg/m² , MMP-FSR = 0.06%/h [0.02%/h]) and nine participants without obesity (W-OB, BMI = 24 [3] kg/m² , MMP-FSR = 0.08%/h [0.02%/h]; for both BMI and MMP-FSR p < 0.05). MMP-FSR and mitochondrial protein FSR were measured following an overnight fast.

Along with lower MMP-FSR, OB participants displayed lower mitochondrial protein FSR (p = 0.03) compared with W-OB participants. Expression of IGF-1 (p = 0.04) and IGF-1 receptor (p < 0.01) proteins was lower in muscle of OB participants. In addition, OB participants had lower (p < 0.05) mRNA expression of IGF1 variants Eb and Ec. This study demonstrates that lower protein synthesis in muscle of humans with obesity occurs concurrently with lower expression of muscle IGF-1 and IGF-1 receptor proteins, as well as lower mRNA expression of the IGF1 splice variants.

These findings indicate that lower protein synthesis observed in muscle of humans with obesity may result from diminished muscle IGF1 gene expression.