Variation in the plasma membrane monoamine transporter (PMAT) (encoded by SLC29A4) and organic cation transporter 1 (OCT1) (encoded by SLC22A1) and gastrointestinal intolerance to metformin in type 2 diabetes: an IMI DIRECT study - Dawed, A Y; Zhou, K; van Leeuwen, N; IMI DIRECT Consortium (Diabetes Care, 2019)