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.