It's not n=13 it's 13 with genetic defects.

Also not adjusting for confounders is statistically ridiculous. As well as n=13 studies.

i'll call n the total number of patients. if n * 9 / 1000 = 13, you'll find out that n is around 1400, which is a pretty high number in discovery studies. Maybe not for diabetes though, i remember hosting anonymized data from a lot more patients two years ago, i felt this was THE subject all the medical founding was put into (that and air quality and its effect studies).