Think it's because people don't separate energy intake and nutrition as two independent considerations. Re: supersize me. He had an oversupply of energy leading to obesity but also was suffering from malnutrition. Assuming in that specific case his energy needs were well met but possibly his mental performance would have suffered due to malnutrition.
Edit - also the outcomes of too much of either are quite different. Excess energy generally produces obesity, excess nutrition can result in poisoning. Symptoms of either represent themselves in completely different ways physically / mentally.
This is interesting, I wonder if there's anything to it: International variation in IQ – the role of parasites (paper) by Christopher Hassall of U. Carleton.
It strikes me as the sort of thing that could be as big an issue as lead in the environment. Raise the sanity waterline: improve health!