Numerous studies have suggested an overall association between pre-diabetes and future stroke, but several confounding factors (such as age, obesity, and hypertension) make it impossible to know the exact size of this association.

Pre-diabetes and “not pre-diabetes” cannot be randomly assigned, so the best that we can hope for is an observational design that controls for all important cardiovascular risk factors. But researchers disagree about which factors need to be controlled for. For example, one study controlled for seven factors,1 whereas another controlled for 11 (only three of which were also controlled for by the first study).2 Unsurprisingly, the two studies yielded markedly different estimates of the association between impaired glucose tolerance and future stroke (14% increased risk …