Cats Do Not Drink Milk (Except When They Do)

Here is the problem. Adult mammals do not drink milk, ever. They can’t digest it. They lack the enzyme, lactase, required to break down the lactose in the milk and so consuming milk causes great quantities of lactose to be left over in the gut, which are either turned to gas by microbes or just exit the colon, unceremoniously. Some adult humans can drink milk because their ancestors evolved one of several versions of genes for digesting milk as adults. Maybe you are a milk drinker, but you are in the minority, globally speaking.  Most adults simply cannot partake.  Special versions of ordinary genes helped the clans of milk drinkers to survive when clean water, or simply water, and or food were scarce. They survived thanks to milk and their genes spread. But no other animals should be able to drink milk as adults. And this includes your cat.

I have written twice about the story of humans and milk, which is about as fascinating a story of human evolution as we have (see here and here), and both times people have written me to say, “Hey you so-called scientist, my cat drinks milk.” So here is the question, why can Fluffy drink milk? Knowledgeable scientists say she cannot. One possibility is that current knowledge is right and cats do not drink milk, they just get diarrhea and we fail to notice. A second possibility is they actually do digest milk. But if they do, how can this be so? One reader wrote to suggest perhaps cats evolved the ability to digest milk as adults when they moved in with humans and we gave them (or accidentally left out) milk. This seems possible.

Left with my parents for a year, my own cat doubled in size thanks to a steady diet of pudding (I can only point to my Midwestern roots in explaining this particular cat/human daily menu option). So yes, dear readers, I too have seen that cats drink milk.  And so I’m writing humbly to ask for your help. My own research turns up little explanation.  Maybe someone out there knows of a study of drinking in cats. Maybe someone has evidence of one form or another that would suggest another hypothesis. Maybe someone out there wants to do the genetics on cats to see just what they are capable of. In the meantime, all over the world cats (including Snowball, Ollie, Garfield III and Princess among many) are drinking milk and defying our current understanding.

By |2016-11-22T13:47:40+00:00May 1st, 2012|

About the Author:

Rob Dunn
Rob Dunn is a biologist and writer in the Department of Applied Ecology at North Carolina State University. Central to all of his work is the sense that big discoveries lurk not only in faraway tropical forests, but also in our backyards and even bedrooms. The unknown is large and wonderful and Dunn and his collaborators, students, and postdocs love to spend their days in it.

7 Comments

  1. Andrew Durso May 1, 2012 at 12:52 pm - Reply

    According to Hore, P. & M. Messer (1968) Studies on disaccharidase activities of the small intestine of the domestic cat and other carnivorous mammals, Comparative Biochemistry and Physiology. 24(3): 717-725
    cat lactase activity was high at birth, and decreased to relatively low levels in adults, with the sharpest decline in activity between 4 and 7 weeks (i.e., during weaning), a pattern consistent with all other mammals then studied except for humans and guinea pigs (which apparently have consistently low levels throughout life, even prior to weaning). They did isolate at least two beta-galactosidases (the class of enzymes to which lactase belongs) from the cat, so it’s possible that a different form of lactose-digesting enzyme predominates in adulthood. More detail might be found in Kienzle, E. (1993) Carbohydrate metabolism of the cat. 4. Activity of maltase, isomaltase, sucrase and lactase in the gastrointestinal tract in relation to age and diet. Journal of Animal Physiology and Animal Nutrition. 70:89-96., but I can’t access this article and the abstract appears to be in German.

    What I do know is that the ice cream I am eating right now is delicious.

  2. David Schardt May 7, 2012 at 12:24 pm - Reply

    The claim that “most adults simply cannot partake” of drinking milk is a myth. Many randomized, placebo-controlled trials have found that lactose maldigesters can tolerate the amount of lactose found in a cup of milk and many of their symptoms are no worse than after consuming a placebo. See: J Nutr. 2006 Apr;136(4):1107-13. Lactose intolerance symptoms assessed by meta-analysis: a grain of truth that leads to exaggeration. Savaiano DA, Boushey CJ, McCabe GP.

  3. Tom Westheimer August 15, 2012 at 6:39 pm - Reply

    I have heard that pasteurization kills enzymes that aid in digestion? Anyone know about that? I do know “fresh milk” aka raw milk is delicious

  4. Katherine Williams November 14, 2012 at 7:14 pm - Reply

    “Adult mammals do not drink milk, ever. They can’t digest it. They lack the enzyme, lactase, required to break down the lactose in the milk ”

    Most milk consumed in the world (including the United States up until about 1940,) is consumed raw. Raw milk is naturally high in lactase and presents no digestion problems for most people. If natural fresh milk were indigestible by mammals, we would never have domesticated dairy livestock.

  5. Jarvis November 13, 2013 at 10:58 pm - Reply

    “Adult mammals do not drink milk, ever.”

    Not true at all. Go visit a farm with dairy cows. At milking time, all the mammals–even birds–will come running if they aren’t penned in somewhere.

    Katherine is correct. Raw milk from grassfed animals contain bacteria (lactic acid bacteria) that produce lactase and cleave the lactose. The lactic acid bacteria can even survive stomach acid, make it to the gut, and multiply, making more lactase.

    Also, the higher the fat & protein content of milk, the closer it will be to cat’s milk. Apparently cats be raised just fine on raw Channel Island Milk.

  6. JC August 29, 2014 at 8:45 am - Reply

    Over the years I have lived with about 50 different cats. Some would drink milk. One particularly old cat, maybe about 12, he loved it. He would sit at the fridge in the morning waiting for it. I have also lived with cats that don’t like milk, or even meat! No really, the three I live with now (none are mine), none of them will eat meat, not chicken, not beef, not lamb, not even a mouse. I blame the horrible food they were raised on. But they are also the only indoor cats I have lived with. Also the others have been indoor/outdoor.

    But my the one thing I have found is the variety of the cats that LOVED CHEESE. omg. The will all be very vocal about dat cheese. Where is my cheese. I want cheese naow. Give me cheese. Sheesh!

    None of those cats wanted raw milk, just cheese. Cheddar, Colby Jack, Swiss, you name it.

    That milk drinking one from earlier? Never liked cheese.

    The indoor ones, no, no cheese for them either.

    I also had one that loved eating warm sauerkraut and catching cicadas. He was my true love.

  7. Bob August 20, 2016 at 4:01 pm - Reply

    Mammals in the wild don’t consume milk due to the simple fact that it isn’t available to them. For the same reason that wild animals don’t eat domesticated crops or wear clothes. If wild animals had mastered domesticating dairy producing mammals, I’m sure they would be consuming plenty of dairy products.

    I had a dog that absolutely loved unflavored yogurt and kefir.

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