Hi Jennifer-
To answer your first question, those numbers are the recommendation from the NRC (National Research Council) which in conjuntion with AAFCO, sets the minimums for the nutrient profiles in all dog foods. As far as I know there is no legal obligation to NRC for companies unlike AAFCO, however they influence AAFCO greatly and are a good rule of thumb. These are nutrient guidelines you would want to consider in preparing homemade and raw diets as well.
For number 2, my best advice is what you already know. Keep them lean, all throughout life. I find that I can still feed my older large breed (8 year old 75lb Lab) high calorie foods, but still keep him at a good weight, so long as I consider his activity level and any extras that he gets. I also do prefer leaner meats like chicken, turkey and fish. The other thing that certainly helps their metabolism is them being intact. It’s going to be much different with neutered/spayed dogs, as their energy requirements change. It’s said now, with some of the new research, that senior dogs require more of a high quality protein because of their inability to metabolize protein as efficently as their adult counterparts. Some still argue that high protein damages the kidneys etc. Others argue that processing excess protein is a passive process of the kidneys and has no effect on a healthy dog.
