Vitamin C is an important antioxidant that scavenges free radicals, can help reduce inflammation, and is essential for collagen synthesis. Fortunately, dogs synthesize vitamin C on their own (in the liver). Supplementation may be useful for dogs with liver disease, but the decision to do so should be based on factors your veterinarian will consider.
A decline in plasma ascorbate in sled dogs during the racing season was largely prevented by ascorbic acid dietary supplements. Supplementing with vitamin C also may help racing dogs by facilitating oxidation of fatty acids by mitochondria in working muscles. Let's not kid ourselves though. Family pets and even the majority of canine athletes aren't in the same league as sled dogs. So, do our family dogs benefit from supplementation?
Vitamin C supplements come with some downsides. Some are more serious than others.
1. Stomach upset, diarrhea (easily solved by feeding less of the supplement and and/or trying Ester C which is easier on the stomach of some dogs)
2. In cases of cancer where antioxidants can become prooxidants
3. Acidifies urine which can create a good environment for the formation of bladder stones
4. Vitamin C is excreted in urine, and some dogs also excrete calcium this way. When the two combine, we have oxalic acid which is the perfect set-up for calcium oxalate stones
5. Breed matters! Dogs with a genetic predisposition to forming calcium oxalate stones are being pushed toward them when fed vitamin C supplements.
6. Disease matters! As an example, Cushing's syndrome affects calcium metabolism and makes dogs, regardless of breed, more likely to produce calcium oxalate stones even without being fed vitamin C supplements.
7. Some medications (long term use of prednisone, for example) can lead to stone formation. Vitamin C supplements increase the risk.
Foods that provide vitamin C don't pose the risks above. Your dog can get all the benefits just by eating fresh. Good choices: kiwi, strawberries, broccoli, red and yellow bell peppers