When I started eating mostly vegetarian a few months ago (by accident, I just didn't eat meat for a couple of days) I noticed my energy level shooting up, and I had the good (energetic) kind of hunger as dinnertime approached.

When I briefly tried low-carb breakfast and lunch last week, I got the same effect.

Is there an established theory that could explain this? The only thing I can think of is a crackpot-sounding book I once read that said you shouldn't mix carbs with protein since they're hard to digest together.