You can't run a 40-year trial to test whether a daily pill keeps you healthy at 80. So they did the next best thing: took the Diabetes Prevention Program from the 1990s and linked its participants to their Medicare records 21 years later. buff.ly/roQTbQ4
Still, it's hard to ignore. The habits you build seem to change your health for decades. A pill, even a good one, doesn't carry that kind of memory.
To be fair to metformin, it may not be inert. There's evidence it blunts some of the gains from exercise (VO2 max, for one). Its effect here might be good and bad cancelling out, rather than nothing at all.