MrPedantic: 1. If I would eat better food, I would be healthier.
2. If he would eat better food, he would be healthier.
#1 sounds odd to me not for grammatical reasons, but because to speculate upon your own willingness to do something as if you had no say in the matter seems odd.
I agree, Mr P that it is hardly the norm but why couldn't a person say this in a moment of despair? Say they had just had some serious life threatening incident and they say to the doctor or a friend;
If I WOULD eat better food, I would be healthier, but that just ain't gonna happen.
Or as a reply to someone who had said;
A: If you would eat better food, you would be healthier.
B: You're right of course; if I wooouulld eat ...
What about this one:
3. If I would eat better food, I would be healthier? What on earth are you talking about! I eat perfectly healthy food!
It's fine to my mind.