The primary sugar in mammalian milk is called lactose. Infant mammals produce an enzyme called lactase. This enzyme is created with the sole purpose of breaking apart the lactose molecule so that the body can use the nutrients. As we pass infancy, in the vast majority of mammals, including many humans, the body cuts production of this enzyme to minimal levels. When this shift occurs, lactose stops being effectively broken down by the mammalian digestive system and causes the gastrointestinal symptoms of lactose intolerance. Some pockets of humanity developed lactase persistence several thousand years ago, and now a little over a quarter of the adult human population is able to process lactose. Although it has not been well studied, it is surmised that some dogs may have a similar persistence.

Lactase is an enzyme that is required for the digestion of the naturally occurring milk sugar lactose. Most dogs lose the ability to break down the lactose in milk shortly after puppyhood.

The symptoms of lactose intolerance are uncomfortable, but not generally life-threatening unless they continue for extended periods of time. Symptoms will start within just thirty minutes to two hours from the ingestion of the dairy product. The lactose concentrations in a female dog’s milk run about 3%, where cow’s milk contains 5%, so even unweaned puppies may not produce enough lactase to digest cow's milk properly, and may show signs of lactose intolerance, though they would be reduced in intensity.

Food intolerance - An intolerance to a food type, like milk, has no immune system involvement and is more likely to cause a gastrointestinal response than allergies do. Changes in the color or consistency of the stools, gas and gurgling sounds from the digestive system are common with a food intolerance. These symptoms can be exaggerated in cases of lactose intolerance, progressing to vomiting and diarrhea. 

Food allergy - An allergy to food is a response by the body’s immune system to defend itself against a threat. An allergic reaction doesn’t happen the first time an individual is exposed to the allergen but rather after repeated exposures. Dogs who continue to be exposed to milk proteins after showing evidence of intolerance may be more likely to develop an allergy later.

The primary diagnosis of lactose intolerance is often made due based on timing and symptomatology. Your veterinarian will probably want to do a physical examination, and will most likely listen to and palpate the abdomen. Most doctors will recommend that you eliminate all dairy from your dog’s diet for several weeks to see if the symptoms disappear. 

Other tests that may assist in a diagnosis are available, but rarely used, in veterinary medicine. A hydrogen breath test will check the amount of hydrogen in the digestive system, and will often remain high for longer after consuming dairy products for intolerant individuals. Lactose tolerance tests verify the amount of sugar in the patient’s blood after consuming lactose. The rise in blood sugar will not be as significant in intolerant canines.

Dairy itself is not required in a dog’s diet, so treatment is generally the removal of dairy from the diet. There are options for the pet parent that wants to find dairy or dairy-similar products:

Lactose-free milk and dairy - There are several varieties of milk and dairy product that are formulated for the lactose intolerant, that may be suitable for your pet.

Soy milk products - Soy is generally a safe food for most canines, but excessive amounts have been shown to increase estrogen-like activity and can lower thyroid levels. 

Almond milk products - This is another alternative that can be stomached by lactase-deficient canines, at least in small amounts. Larger amounts may cause gastrointestinal distress due to the fat content in the milk. 

It is important to ensure that there are no added sweeteners or flavorings that can negatively impact your animal. Read the ingredient label to check for artificial sweeteners and other dangerous additives that may be blended in for sweetness such as raisins, cocoa, or even xylitol. These additives may be deadly to your pet, and should be avoided at all times. 

Human food is not the only source for dairy, and many dog foods also include dairy. Unless your pet is severely intolerant of lactose, a small cheddar dog treat or doggy soft serve cone here and there are unlikely to do any damage, but chronic malabsorption issues like untreated lactose intolerance may lead to weight loss and severe dehydration, particularly if vomiting and diarrhea are occurring on a regular basis. If your dog is showing signs of lactose intolerance without any dairy that you know of, check the ingredient list on your dog food. Cheese flavors or milk fats in their diet may be the culprit.