I’m a 78-yearold retired medical oncologist. While in residency in New York, I got a gastrointestinal virus that left me with lactose intolerance. After a while, it improved somewhat. But in the past couple of years, using Lactaid tabs when I ate yogurt started not to be enough, and I was plagued by liquid stools once a day to the point where I was having a very hard time. Also, I was having trouble maintaining my weight.

I went to see my GI doc, who put me on Metamucil and seconded my decision to avoid all lactose-containing foods. Things have improved considerably, and I am no longer having weight maintenance problems. I’ve never seen any reports about lactase deficiency causing malabsorption, but I’m pretty convinced that severe lactase deficiency in my case may have caused malabsorption. What do you think? — J.M.C., MD

Answer: Lactose is a natural sugar found mostly in milk and milk products like cheese and yogurt. During adulthood, many people lose the ability to make the enzyme lactase, which breaks down lactose into its component parts. Without that enzyme, lactose cannot be absorbed (malabsorption).

Lactose intolerance is when there are symptoms of the malabsorption, such as the diarrhea you mention. But symptoms also include abdominal pain, bloating and gas. Lactaid is one brand of lactase enzyme. When taken with lactose-containing foods, a lactase replacement allows the body to process those foods normally.

Even though the milk sugar can’t be absorbed in a person with lactase deficiency, weight loss is uncommon. When it occurs, it is often due to people changing their diet to avoid the foods that cause symptoms. Weight loss should prompt an investigation into whether another nutrient is also being malabsorbed. Celiac disease and small intestinal bacterial overgrowth are malabsorption syndromes that can mimic pure lactase deficiency. The fact that your weight stabilized with proper treatment of your lactase deficiency suggests there are no other worrisome causes in your case.