They try to. Unfortunately you can’t account for all of them, because we don’t know all of them. Once it’s published that X is healthy or Y is unhealthy then p

They try to. Unfortunately you can’t account for all of them, because we don’t know all of them. Once it’s published that X is healthy or Y is unhealthy then people change their lifestyle in accordance. You can’t tell if some other thing that people do because they heard it is healthy is the real cause.