Q. Which is a better fiber supplement: psyllium or wheat bran?

A. Jessica Seltz, a dietetic intern at the Frances Stern Nutrition Center, answers: “Fiber, a complex carbohydrate that cannot be broken down during digestion, is found naturally in many plants, including legumes, whole grains, nuts/seeds, fruits, and vegetables. Fiber supplements can help boost fiber intake if diet is not sufficient. (Whether these supplements have the same effect as fibers naturally occurring in foods has yet to be determined.) The two most common fiber supplements, psyllium and wheat bran, contain different kinds of fiber.

“Psyllium, derived from the seeds of an herb called Plantago ovata, is a soluble/viscous fiber. Like other soluble fibers (such as those in oats and beans) psyllium forms a gel with water in the gut, making stool easier to pass. It can be useful in relieving both diarrhea and constipation. Soluble fiber also slows digestion, feeds gut microbes, and helps lower cholesterol levels.

“Wheat bran, the outer layer of a wheat kernel, is an excellent source of insoluble fiber. Insoluble fibers add bulk to stool, which can accelerate movement through the colon and help prevent constipation. Wheat bran is generally well tolerated, although adding too much into the diet at once may cause digestive distress, so it is best to add small amounts at a time.”