Causes of Lactose Intolerance
While milk is not necessary to a dog's or diet, it can be a treat if given from time to time. However, some dogs cannot tolerate milk because they do not produce an enzyme called lactase. Lactase helps with the digestion of lactose, which is found in milk. After weaning, the level of lactase activity falls to about 10 per cent of its peak activity. In some dogs, diarrhea will occur if more lactose (i.e. milk) is consumed than the pet can digest. This is called "lactose intolerance".
A further contributing factor is the fact that cow and goat milk contains 4.5% to 5% lactose, compared to 3.1% in dog’s milk and 4.2% in cat’s milk. The high level of lactose in cow's milk can overpower a dog's ability to digest it. This is why many pups often get diarrhea from drinking cow's milk. This does not mean that milk is unhealthy for dogs. On the contrary, in pets that are able to tolerate it, milk can be an excellent source of protein and calcium.
Pets with milk intolerance can still consume dairy products under certain circumstances. For example, dairy products such as cheese (including cottage cheese) and unpasteurized yogurt usually have the lactose removed or have it partially broken down through bacterial action. As a result, these products are often well tolerated by dogs that would otherwise get diarrhea after drinking milk.
It should be stated that neither boiling milk nor the use of skim milk affects an animal's ability to tolerate milk, since the lactose content remains unchanged in either case. The same applies to pasteurized yogurt and cultured milk (e.g. buttermilk), neither of which is well tolerated by lactase-deficient dogs.