lab not eating normal

Question
Our female lab (11 and 1/2 yrs. old) all of a sudden stopped eating as much as normal.  We switched her to BilJac food about 2 months ago but she gobbled it up for 5 weeks and now only eats half of it, and comes back later for the rest.  Less energy as well.  Could it be the dog food?

Answer
If in the past, she was a typical Lab, a bottomless pit, I would get her into the vet.   It could be something serious that you don't want to waste time going back to the old food.  

It is also possible the new food is richer,and she doesn't need as much.  Your dog definitely should be narrower at the waist than the hips and chest. You should be able to easily feel the ribs, but not see them. Each dog is different. Standard recommendations are a good place to start, but each dog must have its food and exercise adjusted to its individual needs. Here is a link to a good illustrated guide, http://www.longliveyourdog.com/twoplus/RateYourDog.aspx