Reason why domestic cats are lactose intolerant

There comes a time when a mother of kittens decides to start weaning them.  This typically occurs in the fourth or fifth week of a kitten's life but it might be earlier in a large litter or if the mother is unwell or stressed.  The mother instigates the process of weaning as kittens rarely if ever decide to wean themselves.

Original photo by Addy Cameron-Huff. Cat is Finnigan. He looks like a Turkish Van.

The mother starts to spend time away from her kittens and/or blocks access to her milk.  She does this by lying or crouching with her abdomen against the ground.

The kittens get hungry and hunger drives them to search for food.  In the wild the mother brings back prey and tears it up to make it easier for the kittens to eat.

Prey is meat, the flesh of (usually) small mammals.  This forces kittens to develop their ability to eat and digest meat.

These developments include a change to their digestive tract.  The flesh of animals takes longer to digest than milk so kittens' intestines become lined with villi.  These are small finger-shaped projections which increase the amount of nutrients that can be absorbed through the lining of the intestinal wall.

Also, the enzyme that breaks down milk sugar, lactase, is permanently substituted by sucrase which breaks down the sugars in muscle which is meat.

As a result, the adult cat does not have the enzyme lactase in their digestive system to break down milk sugar and therefore it is indigestible or what we call “lactose intolerant". 

Cats are lactose intolerant to varying extents depending on the individual. Humans can also be lactose intolerant. I am. We are not designed to drink cow's milk. It's for cows.

Comment from Christine:

"Many who think they are lactose intolerant find this is only true when they consume pasteurized dairy products. This applies to cats as well. Pasteurized milk appears to remove the enzymes that help digest it."

The classic image from years ago of giving a domestic cat a treat of milk was a misconceived idea. I am sure that there were many cats with diarrhoea in those days although the owners would have been unaware of it as cat litter had not been invented.

Note: early weaning in domestic cats can cause behavioral problems.