Doctors now know that stress and lack of fiber are not to blame for irritable bowel syndrome. Highly fermentable sugars and dietary fibers, poorly absorbed in the small intestines, are the culprits — apples, pears, beans, rye and barley, to name a few.

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If you think a single bout of abdominal pain, gas, bloating, diarrhea or constipation can ruin your day, try having it be a daily, or near-daily, occurrence. That’s what most people who have irritable bowel syndrome (IBS) struggle with. When you fear running errands, sitting through a long meeting or going to someone’s home for dinner because you might be gassy or need to run for the bathroom, that’s a profound quality-of-life issue.

For years, a diagnosis of IBS came with advice to give up dairy, eat more fiber or to try to reduce stress. Fortunately, we’ve come a long way. While many individuals with IBS are lactose intolerant, and too much stress can give anyone some gastrointestinal distress thanks to the gut-brain connection, we now know that many types of fiber can actually trigger IBS symptoms. In particular, FODMAPs.

FODMAPs, otherwise known as Fermentable Oligo-, Di-, and Monosaccharides and Polyols, are highly fermentable sugars and dietary fibers that tend to be poorly absorbed in the small intestines of many people with IBS. FODMAPs then travel to the large intestine where they act as “fast food” for gut bacteria, causing excessive gas and bloating. They can also alter the fluid balance in the large intestine, causing diarrhea, constipation, or both.

The irony is that oligosaccharides are a type of prebiotic fiber, which in most of us has the symptom-free benefit of feeding our gut microbiota. This is a perfect example of why one diet does not fit all. Thanks to recent research out of Monash University in Australia, not only do we know about FODMAPs, but we have a way to figure out which foods are causing symptoms. It starts with avoiding high-FODMAP foods for a few weeks, including:

• Milk, yogurt and a few soft cheeses (high in the disaccharide lactose)

• Wheat, rye and barley (high in oligosaccharides)

• Fruits that have a higher fructose-to-glucose ratio (including apples and pears)

• Other foods and condiments high in fructose (a monosaccharide)

• Foods high in sugar alcohols (polyols such as xylitol and sorbitol)

• Beans and lentils (high in oligosaccharides)

• Certain vegetables, including the cruciferous veggies and the onion family (oligosaccharides)

Most people with IBS start to see symptom relief after a few days on a low-FODMAP diet. The next step is to “challenge” each type of FODMAP to learn which foods, in what amounts, cause problems. The ultimate goal is to be able to avoid symptoms while enjoying as varied a diet as possible.

So if you think you have IBS, should you try self-treatment? In a word, no. There are a number of gastrointestinal disorders that have symptoms in common, and some are quite serious. It’s always important to get a proper diagnosis.

IBS affects quality of life, but it doesn’t cause physical damage to the intestines. The same can’t be said for inflammatory bowel disease (IBD), an umbrella term for Crohn’s disease and ulcerative colitis, both autoimmune diseases (when your immune system attacks healthy cells in your body by mistake).

Symptoms of IBS and gluten intolerance may also be similar. Gluten is a protein, but the grains that contain gluten (wheat, rye and barley) also contain oligosaccharides. So some people who think they have celiac disease or non-celiac gluten sensitivity actually have IBS. They have a problem with gluten-containing grains, but with the fiber, not the gluten itself. Another reason why it’s important to seek out a formal diagnosis and get dietary advice from an experienced registered dietitian nutritionist.