This article is from the Mastiffs Breed FAQ, by mmcbee@access.mountain.net (Mike McBee) with numerous contributions by others.
A Mastiff will eat anything that is not nailed down! Mastiffs,
being a giant breed, have the capability of chewing on things that
most small breeds can't even get in their mouths.
Oh, you meant food, huh? OK, Mastiffs do best on a food that is in
the medium range for protein (20-25%), mid range for fat (12-18%)
and is well balanced for calcium and phosphorus and high in iodine
(3-5%). Feeding your Mastiff puppy foods high in protein, calories
and fat will push the growth rate and possibly cause joint,
ligament and tendon problems. It is best to grow your puppy at a
slow, steady rate and not try to make him big too soon. Remember,
the Mastiff will grow to what he was genetically programmed to be
no matter how fast or slow that you get him there. It is best to
take your time and grow them out slowly so as to minimize joint
and bone problems and thus have a sound, healthy dog. If the dog
food you intend to use is balanced for nutrition do not add
supplemental calcium to the diet. Too much calcium causes more
problems than too little!
The amount of food is a judgment call, depending on the type of
food you are feeding, the age of the Mastiff, and the body
condition such as too fat, too thin or just right. Feed a good
quality premium food, following the recommendations on the bag and
adjusting the amount according to body condition. Do not let your
Mastiff puppy or young adult get fat and make sure that you can
feel the ribs or at least see the last two ribs when the dog is
moving. Fat dogs have many problems with bones and joints, heart,
liver, kidney, etc. Generally Mastiff puppies eat a lot of food
while growing, until at least the age of two. An adult Mastiff
generally has a slow metabolism and does not eat an exceptional
amount of food, normally about the same as a German Shepherd or
dog of similar or even smaller size.
 
Continue to: