Is my lab healthy?

Question
I have a 9 month old silver lab.  He seems awfully thin.  I feed him more than 4 cups a day of Nutro Maxx Dog food. Should I feed him more.

He is an American build so he is tall and lean, but i can feel the top of his spine and it kind of curves?  Is that normal?

Answer
Most of my 9 month olds did fine on 3 cups of Pro Plan.  Lucky needed more like 6 cups.  He also had larger softer stools.  Compare your dog to this illustrated guide, http://www.puppychow.com/products/popup_body_condition.aspx  You might have the vet confirm your judgment the next visit.  Also look at his stool for sign of worms.  They are less of a problem now with so many dogs on flea/heartworm remedies that kill other common worms too.  

It is best to keep a Lab lean, and make an early switch to adult chow.  Most Labs would eat more than is good for them and look for more.  See  http://www.photolocker.net/images/Labman/zekeandtheproplan.jpg