The old saying you are what you eat applies to animals as well as people. A great
majority of the coat conditions in our pet animals can be remedied by feeding them
properly.  The exception here is bad hair and skin caused by “flea”  allergies but that
is another ball of wax.
  Feeding the wrong type of diet causes many more problems with the animals
besides bad hair and skin. It can and does cause bowel problems and mouth
odors that come from a different source than tarter and plaque build-up. “Bad
Air” in dogs comes from feeding to much people food. It can also be a side
effect of commercial pet food. Generally, commercial pet food that is on the
“inexpensive side”! Geez, didn’t  I say cheap nicely.
  Dogs in particular apparently lack an enzyme in saliva [amylase or ptyalin] which
begins the breakdown of starches. We are all big people here and hopefully do
remember some of what we learned about nutrition so I will not go into what
foods are considered starches.
  The digestibility of food differs more between dogs and cats because of the
length of the digestive systems.  To illustrate:

                Herbivores: [e.g. horses] have a high ratio of intestine length to
                body length because veggies are harder to digest.

                Omnivores: [e.g. dogs/humans] the ratio is smaller because they
                eat a mixture of meat and veggies.

                Carnivores: [e.g. cats] have the smallest length of intestine because
                they are true meat eaters.

 This is the reason you don’t feed dog’s cat food and cat’s dog food. They are
differently structured for different digestive systems.
  Now we get into the area of digestibility.  If the digestibility factor is low then the
animal must eat more in order to get the amount of nutrients it needs during the
course of a day. And the results are generally more for you to pick up in the yard.
Why? Because fecal material is made up of digested and undigested food along
with other bodily wastes.  This is especially true if you are feeding “inexpensive”
food. So how do you judge....simple rule not the most expensive or the cheapest.
Stay away from fancy colors and soft-moist [full of sugars]. Hard and crunchy
is the ticket. Takes longer to eat, better for teeth and gums and will really be
better for the animal. Besides all those colors and shapes are for you. The animals
don’t really care. The exception is cats to whom shapes make a big difference
to palatability.
  A diet that has a high digestibility factor is more readily absorbed by the animal
and therefore less fecal material. Caution: feeding an animal to much of a highly
digestible diet can cause voluminous amounts of fecal material and obesity. A word
to the wise.
  Feeding “people food” messes with the dietary balance of commercial pet foods.
And contributes to bad nutrition, poor hair and skin and basically unhealthy animals.
  So I guess I am back to original statement....you are what you eat!