One of the most common pet myths is that milk is good for cats! Not only is it bad for your cat's waistline — one saucer of milk is equivalent to four burgers (based on a 4.5kg cat and an average size woman) — but giving your cat milk can also cause upset tummies, cramps and even severe diarrhoea.
Diarrhoea can be fatal to kittens because it dehydrates their little bodies very quickly. After weaning, milk is not necessary in a cat’s diet. Their ability to digest lactose (the sugar in milk) reduces. Cats can even become intolerant to milk, resulting in the diarrhoea mentioned above. Owners should give cats limited milk — or none at all.
Water is actually all cats will ever need. Fresh water is a must for cats! If you do wish to give your cats milk, then you can buy lactose-free milk (lactose is the sugar found in milk) from your pet food store, vets or supermarket, but it’s not essential. Indeed, some cats, especially Siamese, will not drink milk at all.
Most cats are lactose intolerant because they lack the enzyme lactase, which aids in digesting lactose. As undigested lactose passes through the cat's digestive system it draws water from the intestines. Lactose cannot pass easily through the intestinal wall into the bloodstream, so it remains in the intestines.
Next, gut bacteria invade and fermentation sets in causing large amounts of gas to form. The result is pain and diarrhoea for your cat. Very young kittens produce the enzyme lactase when suckling from their mother but the ability to produce the enzyme diminishes after weaning.
Soft tissues such as meats, essential in a cat's diet, and offal are very low in calcium and if they are fed as the sole food source, calcium deficiency will occur. You might think that milk is the only source of calcium and the answer to this problem, but good proprietary diets will contain adequate supplies of the major and trace minerals, which includes calcium.