What diet is best for IBS, low-carb or low-FODMAP? The answer is clear

On Nutrition

A few months back, there was some buzz about a study that suggested a low-carb diet could be effective for managing symptoms of irritable bowel syndrome (IBS). Because of this buzz and media attention, and because an estimated 10-15% U.S. adults have IBS — which would naturally include many readers of this column — I want to sift through both the good and the bad about this news.

IBS is a disorder of gut-brain interaction, and the primary symptoms are abdominal pain and altered bowel habits — diarrhea, constipation or a mixture of both. While there’s no physical damage to the gut, or large intestine, the gut may be more sensitive to pressure from gas produced as gut microbes ferment fiber from your food. Disrupted signals from the brain may speed up or slow down the contractions of the intestinal wall, contributing to diarrhea or constipation. Many people with IBS also experience gas and bloating, but others don’t, so those symptoms alone don’t indicate IBS.

It’s important to not try to self-diagnose IBS because many health conditions that involve the intestines share similar symptoms, and some of these conditions are quite serious. For example, Crohn’s disease and ulcerative colitis (which fall under the umbrella of inflammatory bowel disease, or IBD) can cause ulcers and bleeding in the intestines, and celiac disease can cause poor absorption of essential nutrients, leading to conditions such as anemia and osteoporosis.

There’s no test for IBS, but the American College of Gastroenterology, in their 2021 clinical guidelines for managing IBS, said that doctors can make the diagnosis based on the presence or absence of certain symptoms. They also recommend having the blood test for celiac disease because this is relatively noninvasive.

When someone does have IBS, many foods can potentially trigger symptoms. While all of those foods are carbohydrates, not all carbohydrates trigger IBS symptoms, which is why the low-FODMAP diet, and not a generic low-carb diet, is the front-line treatment for IBS. FODMAP stands for Fermentable Oligo-, Di-, and Monosaccharides and Polyols. These carbohydrates include the sugars fructose (a monosaccharide) and lactose (a disaccharide) as well as the sugar alcohols sorbitol and mannitol (polyols) and a few groups of fibers (fructans and galacto-oligosaccharides).

The low-FODMAP diet has three phases:

• First: An elimination phase where you avoid foods that are high in these carbohydrates (and are careful not to include too many moderate-FODMAP foods in a single meal or snack)
• Second: A reintroduction phase where you carefully and systematically reintroduce high-FODMAP foods
• Third: A personalization phase where you do more experimentation, ultimately learning which high-FODMAP foods, in what amounts and what frequencies, you can eat without triggering your IBS symptoms

So, yes, you reduce certain carbohydrates temporarily, but you aren’t following a low-carb diet, per se. On the other hand, when you follow a low-carb diet with the hope that it will reduce IBS symptoms, yes, you’ll end up avoiding some high-FODMAP foods, but you also be unnecessarily limiting low-FODMAP carbohydrates. For example, bread, pasta and baked goods made from wheat flour are high-carb and high-FODMAP. Rice and potatoes are high-carb but low-FODMAP. You might also be including low-carb vegetables, such as broccoli and cauliflower, that happen to be high-FODMAP. Also, many dairy foods are lower-carb but high-FODMAP.

The study I referred to, published in The Lancet Gastroenterology and Hepatology, divided its 294 Swedish participants — all of whom had moderate-to-severe IBS symptoms — into three groups. One group was assigned to the low-FODMAP diet, one group to a low-carb diet, and the final group received medication tailored to whether they predominantly had diarrhea or constipation.

The study was promising in that both diets relieved symptoms more than medications — 76% of the low-FODMAP group and 71% of the low-carb group reported a significant decrease in IBS symptoms at the end of the 4-week intervention, compared with 58% of the medication group.

But a few things bothered me as I was reading the study.

• First, while people with IBS who do experience symptom relief with the low-FODMAP diet usually do so within four weeks, it can take up to six weeks.
• Second, while the low-FODMAP group received instructions at the end of the four weeks for how to reintroduce high-FODMAP foods, the low-carb group was told to just carry on, which means they were continuing to eliminate foods that triggered their symptoms and foods that don’t. That’s a situation the low-FODMAP diet is designed to avoid.

While it’s common for someone who experiences real relief from their IBS symptoms by eliminating high-FODMAP foods to be hesitant to reintroduce, or “challenge,” those foods, it’s a critical piece of the overall diet. That’s because most people aren’t bothered by all FODMAPs, and the goal of the diet is to restore as varied a diet as possible while still allowing individuals to manage their symptoms to a degree they are comfortable with. Research has found that staying in the elimination phase for longer than recommended can lead to some nutrient shortfalls.

I do want to note that while the low-FODMAP diet is a go-to treatment for people with IBS, it’s not appropriate in some cases. For example, someone who has current eating disorder, or recent history of one, shouldn’t go on any elimination diet. Medication or gut-directed hypnotherapy would be safer options.

If you suspect you have IBS, get a diagnosis first. If you know you have IBS, are dealing with unmanaged symptoms, and want to try the low-FODMAP diet, you have options. The best option is to work with a registered dietitian who has experience with guiding people through the diet’s three phases. Other options include an app and other resources from Monash University in Melbourne, Australia, which developed the diet (www.monashfodmap.com).