Is cows' milk bad for cats?

Question

My mother used to tell me that I shouldn't give our kitten cow milk, even if she seemed to enjoy it. Since I loved my kitten, and was quite disobedient at that time, I did it fairly often, and never observed any unwelcome consequences.

I did a little bit of research and found several topic-related links.

Generally speaking, no, cats shouldn't drink milk. Most cats are lactose (which refers to the sugars in milk) intolerant, and drinking milk may result in an upset tummy or cause diarrhoea.

And another quote.

Once cats mature, many become lactose intolerant. This makes cow's milk a poor choice to use as a treat for your cat. Milk can cause your pet to have stomach ailments and indigestion since they are not used to it. In some cases milk can actually make your cat very ill and cause vomiting and/or diarrhea.

However, these articles seem, scientifically speaking, poorly documented. Some of them recommend a special brand of milk designed for cats, which makes me suspect they might have commercial interests in warning people against generic cow milk.

So, what's the truth in it? Is there a solid, biological reason why cats shouldn't drink cows' milk?

Answer 1

TLDR It depends. Evidence suggests that many cats at least, can drink milk in some forms, particularly unpasteurized milk.

There was a famous series of studies done on, among other things, cats and milk consumption, by Dr. Francis Pottinger, in the 1930s and 1940s.

As the study has been around so long, the results have been analyzed time and again, and you can find much on google.

Wikipedia summarizes the findings of the study quite well:

In ... the "Milk Study," the cats were fed 2/3 milk and 1/3 meat. All groups were fed raw meat with different groups getting raw, pasteurized, evaporated, sweetened condensed or raw metabolized vitamin D milk. The cats on raw milk were the healthiest while the rest exhibited varying degrees of health problems.

From this we can see that some forms of milk are indeed bad for cats.

I am searching for the original text of the study to cite here, which I would expect to be public domain by now, and so far have not found it.

There are a number of attributed differences between pasteurized and unpasteurized milk, which may affect the health of a cat, but the simplest to recognize is that of lactase (note the distinction from lactose: Lactose is a sugar, lactase is the enzyme which helps digest lactose). This is the simplest to recognize, due to the nearly immediate symptoms associated with its absence, as well as the well understood mechanism by which it works.

The main culprit is milk’s lactose, which many cats have trouble digesting. The result: diarrhea or stomach upset. Not exactly romantic. source

Lactase is an enzyme naturally present in cow's milk, which is essential for the complete digestion of lactose (milk sugar). The process of milk pasteurization destroys this enzyme, thus leaving the consumer of the milk (a cat in this case) to his own devices to digest the lactose, and many cats (and other animals) don't have the gut flora necessary to digest it, thus leading to digestive problems.

The above part of my answer appears to be incorrect (thanks to @Scott for raising the question); although that doesn't change the results of the above quoted study; only the mechanism by which these results can be explained. I will try to edit this question as I find additional information.

Answer 2

It's bad for most cats.

To digest lactose, mammals require the enzyme lactase. As mammals grow to adults, lactase production decreases (as you would expect after weaning).

Cow's milk contains much more lactose than cat's milk.

Undigested lactose leads to diarrhea. Severe diarrhea can be fatal.

---

Cats and Dairy Fact 1: Lactose Intolerance Is the Norm

Just like people, cats can be lactose intolerant. And although we tend to think that’s a problem, it’s actually completely normal, says Linda P. Case, MS, adjunct assistant professor at the University of Illinois College of Veterinary Medicine and author of The Cat: Its Behavior, Nutrition, and Health.

“The only time animals are exposed to lactose is when they’re babies — in their mother’s milk," Case says.

To digest lactose, a milk sugar, the human and feline digestive systems must contain the enzyme lactase. We have plenty of this enzyme in our systems at birth, and it helps us thrive on our mother’s milk.

But as we grow up, it’s normal for people and cats to begin producing less lactase. Less lactase means less ability to digest lactose. The result may eventually be lactose intolerance.

...

Though kittens have lactase in their system, there’s just not enough of it to tackle the lactose overload found in cow’s milk.

From http://pets.webmd.com/cats/guide/cats-and-dairy-get-the-facts

---

the lactose content of cow's milk is nearly three times that found in bitch's milk, For this reason, puppies that are fed straight cow's milk will develop sever diarrhea

Canine and Feline Nutrition: A Resource for Companion Animal Professionals By Linda P. Case, Leighann Daristotle, Michael G. Hayek, Melody Foess Raasch

Excerpts show this book quotes scientific studies in its references section but I don't have access to it.