Dear Dr. Roach: My wife believed she had lactose intolerance. After some tests, it was found that she was sensitive to casein rather than lactose. Are there other things in cow's milk besides lactose and casein that people are sensitive to?

— K.A.

Dear K.A.: There are three main components of cow's milk that can lead people to having a bad reaction.

Lactose is a type of compound sugar called a "disaccharide," which includes two separate single sugars ("monosaccharides") — glucose and galactose — that are connected by a chemical bond. This chemical bond is broken by a specific enzyme called lactase. Deficiency is rare in children but common in adults, especially in people with ancestry from Asia and Africa, as well as Native Americans.

Without the ability to break lactose into its com

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