As part of your dog's natural process, he will lose a certain amount of protein every day through loss and turnover of hair, skin, nails and bodily secretions.
While it's true that an older dog's system isn't as efficient in metabolizing protein as younger dogs, that doesn't mean it should be excluded from your dog's diet. In fact, for this reason your older dog may need more protein in his diet.
There are really two main reasons why your dog requires protein. First to provide the essential amino acids that his own tissue can't make. Secondly, to provide nitrogen for making non-essential amino acids and other nitrogen-containing compounds.
As part of your dog's natural process, he will lose a certain amount of protein every day through loss and turnover of hair, skin, nails and bodily secretions. What's more, about 1% of body protein is broken down and re-synthesized every day. However, if the body isn't taking in enough protein, this process can break down and result in a protein deficiency.
So how much protein does your senior dog require? To a certain extent it depends on his breed, environmental factors, and variations in the dietary source of protein. If your senior dog has special needs, consult with your veterinarian.