I wanted to add that this site says that recent reseach is suggesting a link between being spayed or neutered and weight gain.
http://www.andoveranimal.com/alter.html
At the time the dog we had was middle aged. Yet still very active. After getting her fixed her endergy level went way down. We took outside for daily excerise and limited her food intake yet she still gained weight. As for her personality well for whatever reason she was NOT the same dog. I can't explain it she just wasn't. So like I said everyone is entitled to their opinion.