GDF15 and type 2 diabetes mellitus: Metformin is a biguanide drug, most commonly prescribed for the treatment of type 2 diabetes [8, 9], and acts via various molecular pathways to produce tissue-specific effects (e.g. activating AMP-activated protein kinase [AMPK] [10], reducing oxidative stress [11], increasing circulating growth differentiation factor-15 (GDF15) [12] and potentially inhibiting mitochondrial electron transport chain activity [13]).