PPY and rheumatoid arthritis: Interestingly, we observed that active GLP-1 (p < 0.01), GIP-1 (p < 0.0001), Insulin (p < 0.05), PP (p < 0.05) and c-Peptide (p < 0.01) were all significantly increased in the serum of RA patients, when compared to HC (Fig. 1D).