Eating poop, called coprophagia, is usually not a sign of illness or nutrient deficiency but is an unfortunate habit. It seems more common in puppies reared in a cage setting as compared to puppies reared in a larger areas. Probably beginning as instinctive behavior to keep the puppy's immediate sleeping area clean but becoming a disgusting situation for the new owner. There are harmless products which can be added to the puppy's food which change the chemical nature of the poop and for many pups this is enough to break the habit. Two useful examples are adding a few shakes of unseasoned meat tenderizer or a product called "Forbid" available through your veterinarian and most pet supply outlets.