Dear Dr. Roach: In a recent column that touched on taking laxatives daily, you mentioned a sugar called lactulose that you recommend for constipation. I’m wondering if other sugars have the same effect. My 70-year-old wife drinks several cans daily of diet lemon-lime soda that contains sucralose. She also tends to have fairly loose, even runny bowel movements. Could the amount of sucralose that she’s ingesting be responsible for this? — T.T.

Answer: Your logic is completely sound. Lactulose is a synthetic sugar that was designed to be nonabsorbable. It stays in the bowel and brings water along with it, making it a useful treatment for constipation. It’s also used to treat people with bad liver disease such as cirrhosis by helping trap ammonia in the gut, since a damaged liver can’t metabolize ammonia. This helps treat and prevent a condition called hepatic encephalopathy. Of course, the major side effect will be loose bowel movements (sometimes diarrhea).

Sucralose is a synthetic sweetener that also isn’t well-absorbed. Like lactulose, it mostly remains in the gut. (Only 15% is absorbed and metabolized.) It is very sweet, 500 or so times sweeter than sugar, so very little is needed. The effect of trapping water inside the gut isn’t as prominent as with lactulose; however, it is absolutely true that some people will be very sensitive and develop diarrhea. This is not only due to it trapping water, but also due to changes in the types of bacteria in the intestines (called the microbiome).

Decreasing the amount she drinks would be a useful diagnostic test. If her symptoms get better immediately, it’s likely the water-trapping effect. If it gets better after a few days to a few weeks, it’s likely the microbiome effect. If it doesn’t get better at all, then it probably wasn’t due to sucralose.

Other compounds used as sweeteners, especially sugar alcohols like sorbitol, xylitol, maltitol and erythritol, can cause similar symptoms in some people.

Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or send mail to 628 Virginia Dr., Orlando, FL 32803.