Kalon, I have found the same thing... and it mimics life, IMO, just like the game is supposed to. In the beginning of adulthood, we may not eat as well as we should and we're running like chickens trying find that 'niche' in life, etc.
With a little more experience and more stability through money, people tend to be a bit more responsible and eat better.
OK, maybe I'm reaching, but your theory is right as far as I can see, I have it working on 3 families and the one that doesn't eat well is tired and has low energy all the time.
Tis a shame people rushed to complain and make it such a big issue when it's a pretty easy fix that comes with a little more time in game playing.