I can think of more than one cause for this unexpected result. How about this: Unhealthy fat people with big atheroma plaques enroll in study. The study gives

I can think of more than one cause for this unexpected result. How about this: Unhealthy fat people with big atheroma plaques enroll in study. The study gives them some motivation to improve their health (to look good on results). They start a diet => they get included in the "restricted diet" in the study, they also start exercising, atheroma plaque wall ruptures due to stress => heart attack => death.