What constitutes healthy food?

Most food that is made from scratch at home is more or less healthy. Even pure sugar is mostly less unhealthy than sweet mass produced stuff. Butter is better than the fat added through mass production. Avoiding all those tricky stuff to keep food fresh for months and months might be a good thing, also. And of course - both real life and TS4 bodies are engines that don't need more fuel than they will actually burn.

In TS4 we know that a sim might find lewftovers that were cooked by his great-great-granny, still fresh and good to eat for another 12 hours. A meal that was totally made from scratch. So, real life rules can't always be converted to TS4 rules.

Generelly - both in RL and in TS4, I think one healthy aspect would be "don't eat to get stuffed, but to be less hungry". Stop eating when the hunger bar is full might help to stay fit.

I remember someone saying to me once, "If your grandmother wouldn't immediately recognize something as food, you probably shouldn't eat much of it." I suppose you'd need to ask your great-grandmother these days, since many grandparents grew up in the age of junk food.

Have you noticed that some meals in TS4 appear to come with side dishes? I seem to remember Fried Fish is served with asparagus, and Grilled Chicken with asparagus AND corn on the cob. Or, am I imagining that?

I say if it’s balanced dish. If it’s unbalanced, or too much of one thing, no matter what it is, it’s unhealthy.