First, what does it mean to be “regular”?

“We consider normal anywhere from going three times per day to three times per week,” says Jacqueline Wolf, a gastroenterologist and associate professor of medicine at Beth Israel Deaconess Medical Center in Boston.

But you could have a bowel movement every day and still be constipated. “Constipation also includes straining, having hard, lumpy stools, or not evacuating completely,” says Wolf.

What’s to blame?

“Certainly a poor diet can have an impact,” says Lucinda Harris, a gastroenterologist and associate professor of medicine at the Mayo Clinic in Scottsdale, Arizona.

“And if you don’t eat, you don’t stimulate your intestines. Eating too little can be an issue in older people.”

If you’re irregular, the first step is to up your fiber intake. But to work, the fiber has to increase stool volume, “and it can’t get fermented by gut bacteria or broken down in transit,” says Nicola McKeown of the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University. (McKeown has received funding from General Mills’s scientific arm and from Metamucil’s manufacturer.)

Fiber comes in two types:

Insoluble. “Coarse, insoluble fiber helps move food through the digestive tract by stimulating the lining of the gut to secrete water and mucus, which softens the stool, making it easier to pass,” explains McKeown. “Coarse wheat bran like Kellogg’s All-Bran is a good example.”

Soluble. Soluble fiber can boost stool bulk by absorbing water. But not all types of soluble fiber get the job done.

The fiber needs to form a gel, allowing it to hold water on its entire journey through the gut.

Enter psyllium, the laxation-​friendly poster child of soluble fibers. It’s in Kellogg’s All-Bran Buds cereal and is the key ingredient in Metamucil.

In contrast, don’t expect much from prebiotic soluble fibers (so-named because they feed gut bacteria). The poster child: inulin—aka chicory root—a processed fiber used in many Fiber One foods, fiber “gummies,” and hundreds of other foods.

In three studies (you can find them here, here, and here) that enrolled a total of 140 people with constipation, only one (funded by an inulin maker) reported more bowel movements in inulin takers than in placebo takers. And inulin made people more gassy.

Those results don’t surprise McKeown

“If the fiber gets fermented by gut bacteria, it can’t hold water, so it can’t add to stool bulk.”

“In fact, few of the isolated fibers that are added to foods actually help with regularity,” she adds. Among the duds: soluble corn fiber and wheat dextrin (the main ingredient in Benefiber).

Instead, stick to whole foods. “Eat a variety of fruits, vegetables, beans, whole grains, nuts, and seeds,” says McKeown. That should give you a good mix of soluble and insoluble fiber.

Tip: Add fiber to your diet gradually

“A high-fiber diet may cause gas and bloating at first in people who were on a low-fiber diet,” says McKeown.

If that isn’t enough, try adding psyllium, coarse wheat bran, or prunes.

“Prunes have both fiber and sorbitol,” says Wolf. Sorbitol, a naturally occurring sugar alcohol, helps retain water in the stool.

“But it can cause gas,” adds Wolf. “And too much sorbitol can cause diarrhea.” So introduce prunes slowly.

(Looking for info about laxatives for regularity? Click here.)

Photos: okkijan2010/ (left plate), nblxer/ (right plate), Kate Sherwood & Jennifer Urban/CSPI (beans).