The most abundant sugar in dairy products is lactose, which is a disaccharide that consists of bonded galactose and glucose fragments and which requires an enzyme called lactase to be able to digest it. The enzyme cleaves the lactose molecule into its two subunits, the simple sugars glucose and galactose, which can then be readily absorbed. When a mammal’s body does not secrete the lactase enzyme from their intestinal villi, they are unable to digest lactose and may suffer from unusual digestive symptoms.
All mammals have high levels of lactase when they are young – they do, after all, drink their mother’s milk. However, this ability to digest lactose generally decreases with age because most mammals do not drink milk in the wild after weaning. For example, there is decreased lactose tolerance in up to 75% of humans (this rises to 90% in some Asian and African countries).
The amount of lactase present in the digestive system of any given dog is thought to vary as widely as it does in humans. Even lactose-intolerant dogs probably usually have small amounts of lactase activity in their gut. When there is insufficient lactase activity to process the amount of ingested lactose, the lactose is not broken down into it’s constituent components and is therefore not absorbed by the dog’s intestine. When lactose is not absorbed by the body, unusual digestive symptoms can arise and the animal is said to suffer from "Lactose Intolerance".
Symptoms of Lactose Intolerance include bloating, abdominal cramps and diarrhea. If your dog suffers from any of these symptoms after consuming dairy products, you should eradicate dairy products from his diet completely. If he suffers no ill-effects, then he is probably tolerant of lactose and there is therefore no inherent danger in feeding dairy products. However, they should always be fed sparingly as large amounts may cause symptoms of lactose-intolerance even where the dog is tolerant. Dairy products, specifically cheese, also have a very high fat content. Yoghurt is often recommended when taking antibiotics because antibiotics indiscriminately kill gut bacteria which temporarily disrupts the digestive functions. Feeding "live" Yoghurt that contains bacteria is thought to help replace the beneficial bacteria in the gut.
If you are keen to make sure that your dog is being fed enough calcium in his diet, but your dog is Lactose Intolerant or you simply do not want to feed him to much dairy, there are other ways to provide him with calcium. Spinach is an excellent source of calcium, and one cup of cooked spinach has a comparable calcium content to 1 cup of reduced fat milk. A cup of spinach can be easily mixed into a bowl of dog food and most dogs will not have any objection to eating it. There are also many calcium supplements available for dogs – ask your vet for specific recommendations for your dog.