Diet/study says that if you do X you will lose weight because of Y. You do X and lose weight. The error is thinking that you lost weight because of Y, and not

Diet/study says that if you do X you will lose weight because of Y. You do X and lose weight. The error is thinking that you lost weight because of Y, and not because of a secondary unlisted effect of doing X. The same applies for certain diets/organisms causing conditions/diseases/etc. i.e. the whole cholesterol back and forth. We've learned lots from nutrition and in general informed people live healthier lives. Just because we don't know exactly what's going on inside our bodies doesn't mean that zero improvement has been made. We'll fuck up once in a while and get some bad advice, but in general we've improved lots.