They’re the biggest contributors to improved digestion.

If you struggle with bloating, diarrhea, and constipation, your diet may be partly to blame. Certain foods and medications can cause these digestion problems. Likewise, eating too few fibrous foods can cause constipation. “Most people are not eating the right foods, and they’re not drinking enough water,” says gastroenterologist Dr. Jacqueline Wolf, associate professor of medicine at Harvard Medical School.

The offenders

Dr. Wolf recommends taking an in-depth look at what you’re eating to see if you’re consuming potential offenders. “Too many carbohydrates may make you constipated. Artificial sweeteners such as sorbitol, mannitol, and xylitol are big offenders for diarrhea and bloating,” she explains. Dietary supplements such as calcium and iron can also make you constipated.

Sometimes the culprit may be a physical intolerance. People who are lactose intolerant don’t have the enzyme to break down milk sugar (lactose). Some people are unable to digest casein, a protein in milk. People with celiac disease (a toxic body response to the protein gluten, found in grains such as wheat, barley, and rye) or a gluten sensitivity often struggle with bloating and diarrhea.

Medications may also cause digestion problems. They may be common over-the-counter medicines, such as the pain reliever ibuprofen (Advil) or the heartburn reliever omeprazole (Prilosec), or they may be prescription medicines such as antibiotics or even chemotherapy drugs.

The missing elements

The most common diet shortfalls, points out Dr. Wolf, are water and fiber. Water is necessary to keep stool soft and bulky. Fiber is the nondigestible component of plant food that promotes stool softening and bulking. There are two kinds, and you need them both. Soluble fiber (from foods such as legumes, oats, and apples) dissolves into a gel-like substance, which stimulates the bowels to hold on to water, bulking up the stool. Insoluble fiber (from foods such as whole grains and most vegetables) does not dissolve as it moves through your gastrointestinal tract. This can help push material along, increase stool bulk, and reduce the risk of developing hemorrhoids and diverticulosis.

What you can do

Dr. Wolf recommends talking to your doctor about ways to eliminate or reduce offenders, and adding more water and fiber to your diet to decrease constipation: aim for eight to nine glasses of water and 35 grams of fiber from food per day. A sudden increase in fiber may cause gas, so increasE the amount of fiber in your diet gradually. Dr. Wolf’s other strategies include exercising more, which increases muscle control and stimulates the urge to go to the bathroom; drinking coffee in moderation and if tolerated, which can help stimulate the anal sphincter and increase movement in the lower colon; and using probiotics—colonies of good bacteria—from a supplement or from food such as Greek yogurt.