If as we are told healthy eating can see off such diseases as cancer, heart disease etc, then why can't it see off fatness if that in itself is supposed to be a vehicle for these things?
The effect of the failure of healthy eating to make people slim has meant some fat people who follow healthy eating regimes believe they're healthy because of that. This is in keeping with the healthist idea that diet and activity are the primary means to regulate health.
Many slim healthists instinctively recoil from the idea that healthiness doesn't necessarily equal slim, even if you eat and exercise according to shared dictates. Rather mean and pompous of them ,but the whole situation raises an interesting point. Either one can be fat and healthy and therefore healthy eating is health promoting.
Or one can only be slim and healthy, in which case, weight is the key factor in health, not what you eat.