If cats are lactose-intolerant, how did we come to believe that giving cats milk is good?

"Kittens produce this enzyme in smaller amounts as they develop, and by six months of age, most kittens have adult levels of lactase. These adult levels are insufficient to digest milk and their bodies can no longer tolerate it. Basically, like some humans, most cats become lactose intolerant." - Hastings Veterinary Hospital

"Kittens ingest digestible carbohydrates (i.e., lactose or milk sugar) before weaning. Adult cats must rely primarily on gluco-neogenesis from glucogenic amino acids (ketoacids), lactic acid and glycerol for maintenance of blood glucose concentration". - Introduction to feeding normal cats

The answer to the questions is this: kittens eagerly drink their mother's milk - colostrum. Observers would understand that adult cats also drink milk. But in the early days of cat domestication there was less knowledge and they didn't realise that at 6 months of age young cats do not have sufficient lactase (lactase deficiency) to digest lactose and become intolerant of it.

Nowadays, things have changed thanks to the huge amount of education provided by the internet on millions of cat websites!

But there are still people in developing countries and even developed countries where they believe that cats should be fed milk as a treat.

When they do this their cat enjoys drinking the milk because it is fatty. Cats like fatty foods. But it would seem that the adult cat does not understand that they are lactose intolerant.

But they don't need to realise it because they have inherited their wild cat ancestor's character and that wildcat never has the opportunity as an adult to drink milk.

It is only in the human environment as domesticated wildcats that they encounter a bowl of cow's milk containing lactose which they cannot digest but which tastes good normally. Not all domestic cats take to drinking it.

What is lactose?

Lactose, or milk sugar, is a disaccharide sugar composed of galactose and glucose subunits and has the molecular formula C₁₂H₂₂O₁₁.
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P.S. please forgive the occasional typo. These articles are written at breakneck speed using Dragon Dictate. I have to prepare them in around 20 mins.