I'd wager that it's the metformin. Metformin inhibits pathways associated with mTOR (the growth signaling protein). When you inhibit this pathway, it tells the body to start the process of autophagy. It's the same process that occurs during fasting, and it's the same reason that the mTOR inhibitor Rapamycin has been shown to extend lifespan in mice and dogs. This is also why fasting has been shown to increase lifespan. It seems like if you don't constantly tax your body with food (growth signaling), then you can allow the body to repair itself. If you don't give it a break through fasting, then your growth hormones will be continually signaling to activate mTOR within your cells and thus never start the process of autophagy (cell death, and regrowth).