One aspect of feeding a balanced diet is whether to give supplements. If vitamins and minerals are healthful, more of them must be better, right? Not necessarily. Too much of anything can cause problems.
For instance, puppies supplemented with calcium can develop deformed bones or have stunted growth. Too much calcium can also interfere with zinc absorption and cause a zinc deficiency. The result is a dog with skin problems, such as thinning or gray hair, spots where the skin is moist, or increased bacterial infections on the skin.
Too many vitamins can also cause problems. Excessive amounts of vitamin D and C are associated with an increased risk of a certain type of urinary stones. Too much vitamin A can suppress the immune system.
Most veterinary nutritionists agree that supplements aren't necessary if a dog eats a complete and balanced diet. Nonetheless, there are some circumstances in which supplements can be helpful. Dogs with skin problems often benefit from essential fatty acid (EFA) supplements, and older dogs, or dogs with health problems, may need certain supplements because they're less able to absorb nutrients. Of course a dog that's eating a homemade diet needs a multivitamin, which is usually recommended in recipes for homemade dog foods. Ask your veterinarian for advice before supplementing your Lab's diet.