The main culprit is milk’s lactose, which many cats have trouble digesting. The result: diarrhea or stomach upset. Not exactly romantic.
Do cats and dairy ever get along? Can cats drink milk? Here is what cat nutritionists and veterinarians told WebMD.
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.
When a lactose-intolerant cat drinks milk, the undigested lactose passes through the intestinal tract, drawing water with it, according to the Cornell University College of Veterinary Medicine's web site. Bacteria in the colon also ferment the undigested sugars, producing volatile fatty acids.
All that activity might lead to an upset tummy and induce vomiting. But the most common symptom of lactose intolerance in cats is diarrhea, usually within eight to 12 hours, says Susan G. Wynn, DVM, CVA, CVCH, an animal nutritionist in Atlanta and co-author of the Manual of Natural Veterinary Medicine.