Low-FODMAPs Diet Not Working? An RD Shares 7 Possible Reasons Why + What To Do

Registered Nutritionist & Dietitian By Nour Zibdeh, M.S., RDN
Registered Nutritionist & Dietitian
Nour Zibdeh is a functional and integrative dietitian and nutritionist, author, and speaker. She received a B.S. in Human Nutrition from Virginia Tech and an M.S. in Health Sciences from James Madison University.
Low-FODMAPs Diet Not Working? An RD Shares 7 Possible Reasons Why + What To Do

Graphic by mbg creative / Istock

FODMAPs, which stands for Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols, are types of carbohydrates that do not get digested or are poorly absorbed in the small intestine.

FODMAPs tend to be short carbohydrate chains and are easily fermentable by bacteria in the small intestine. Fermentation increases gas production and draws water into the colon, which leads to distention, bloating, stomach pain, gas, and discomfort, ultimately resulting in constipation or diarrhea.

The low-FODMAP diet restricts these carbohydrates to ease stomach and bowel symptoms.

While the diet typically helps 50% to 80% of people who follow it, there are still others who don't get these positive results. If you're among these people, it can be frustrating and confusing. Here are some reasons that may explain why the diet didn't work for you and additional things you can consider trying. 

The problem: You don't follow the diet correctly.

The first and most important thing to check is that you have actually followed the diet correctly. The low-FODMAP diet eliminates common foods like garlic, onion, shallots, wheat, celery, and others, so it can be easy to accidentally break the diet at restaurants or when your food is prepared by someone else.

You may have grabbed gluten-free crackers or salad dressing that also have dehydrated or powdered onion or garlic. Or maybe you had a healthy energy bar that is sweetened with pear or apple concentrate. Even a small amount of these ingredients can cause a problem.

Even if you are being careful about hidden sources of FODMAPs, it is important to remember that FODMAPs can have a bucket-like effect, which means, when you eat carbohydrates with FODMAPs you slowly add to the bucket and gradually get closer to a threshold after which symptoms start to appear. The threshold varies from one person to another. So, if you eat half a cup of roasted broccoli with two Brussels sprouts and sprinkle 2 tablespoons of pine nuts on top, you could trigger symptoms even though all are within the allowed portion limit for each food.

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The solution:

For the diet to be successful, you must follow the initial elimination plan fully for at least 2 weeks. If you slip often and eat foods that are not allowed intentionally or by accident, you're probably not going to get the anticipated results. The studies that have looked at its efficacy usually have qualified dietitians instructing participants on the diet, so you may want to reach out to a professional to make sure you're following it correctly. 

The problem: You're eating too many allowed starches.

On the low-FODMAPs diet, you can eat rice, white potato, corn, quinoa, and other starches. While these carbohydrates are not considered FODMAPS, they can still be difficult to digest. Digestion involves enzymes produced by the stomach, pancreas, and intestinal cells, with the support of the liver and gallbladder. People who don't produce enough enzymes, possibly due to aging, the use of proton-pump inhibitors, current or past gut infections, gut inflammation, or for any other reason, will not be able to digest starches with long carbon chains like these. These long-chain carbohydrates eventually get fermented by gut bacteria and will have the same effect as foods that contain FODMAPs.

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The solution:

If you are following a low-FODMAPs diet, you probably eat these carbohydrates, sometimes even in excess to make up for the ones that have been eliminated. Try removing all grains and starches, including white potato, quinoa, corn, buckwheat, oats, and others for two weeks and see if that makes a difference.

The problem: You're eating too much sugar.

Sugars and carbohydrates, whether they are low or high in FODMAPs, feed yeast. It's possible that your symptoms are related to yeast overgrowth. Yeast is part of your gut flora and is kept within a normal healthy range thanks to good healthy gut bacteria. Antibiotics that wipe beneficial bacteria can lead to yeast overgrowth. Estrogen promotes the growth of yeast, and the use of hormonal contraceptives can affect yeast balance in the gut. If you have a history of yeast infections, it's possible that you still have a yeast imbalance in your gut.

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The solution:

Try removing all sugars and carbohydrates for two weeks, and see how that makes you feel. This includes sugars allowed on the low-FODMAPs and natural sugars including maple syrup, cane sugar, grapes, bananas, and other fruit.

The problem: You can't tolerate foods that are allowed on the low-FODMAPs plan.

Nuts, seeds, leafy greens, oils, raw vegetables, meats, in addition to all sugars and grains can trigger bloating, pain, discomfort, diarrhea, or constipation. Although these foods don't have high levels of the fermentable carbohydrates and are allowed on the low-FODMAPs diet, they can still cause a reaction.

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The solution:

If the diet isn't giving you satisfactory results, know that there are some details and additional things that you may have missed. A dietitian who has experience helping people with IBS can probably help you pinpoint the specific issues your own digestive system is struggling with. 

Nuts, leafy greens, and raw vegetables may be difficult to digest due to their hard cell walls. Instead, try soaking nuts, sticking to nut and seed butters only, or removing them altogether. Avoid raw salads and cook all vegetables, especially leafy greens.

Cook meats in liquid, for example by simmering or making a stew, as these will be easier to digest compared to dry grilled pieces.

High fat intake can slow down gastric emptying and give you the feeling of indigestion and bloating. If your liver and gallbladder don't make and release enough bile salts, you may not absorb fats and oils properly, which will lead to diarrhea or loose stool.

The problem: You have food allergies or sensitivities.

The low-FODMAPs diet doesn't account for individual food sensitivities or allergies. Allergies are IgE immune reactions where someone might experience acute symptoms like hives, swelling, and restricted air pathways right after eating the foods.

Food sensitivities, on the other hand, are completely different immune reactions that often trigger subtle, delayed, and less severe symptoms. When immune cells can't recognize certain foods as "safe" anymore and tag them as foreign, they start to attack by releasing inflammatory compounds like cytokines, leukotrienes, histamine, and others. This immune-inflammatory response to food can contribute to symptoms like bloating, water retention, gas, stomach pain, and diarrhea. Food sensitivities often come with other symptoms like brain fog, muscle and joint pain, fatigue, headache, and mood swings.

If you have followed the low-FODMAPs diet strictly but didn't feel significant improvement in your symptoms, it's possible you have an immune response to foods that are allowed. You may be reacting to salmon, chicken, blueberries, cucumber, parsley or any other "safe" low-FODMAP food. Or your immune system may be reacting to the casein in dairy, not just the lactose. If the diet made you feel worse, you may have started eating a food that you have a sensitivity to more often because it's allowed on the diet, and that ended up backfiring.

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The solution:

If cutting out or reducing grains, sugars, dairy, nuts, raw vegetables, and other potential problematic foods still doesn't help, consider food sensitivities next. Consult with a dietitian who can help you identify and test your own individual food sensitivities.

The problem: You have histamine intolerance.

Histamine has important functions in the body such as promoting muscle contractions in the gut and stimulating the production of hydrochloric acid in the stomach. It also has an immune role as it's released by mast cells when exposed to foreign substances. Histamine is also found in certain foods.

A healthy body can keep histamine levels in check. There are two enzymes that help break down histamine, and one of them is mostly produced in the gut. For a variety of reasons, including certain medications, gut infections, and dysbiosis, these enzymes don’t work properly, and your body loses the ability to get rid of excess histamine. The result is a condition known as histamine intolerance.

Histamine intolerance symptoms include redness or flushing in the skin, eczema, rashes, hives, sneezing, congestion, postnasal drip, low blood pressure, rapid heart rate, nausea, vertigo, dizziness, headaches, migraines, muscle and joint pain, insomnia, and other mysterious symptoms.

The low-FODMAPs diet can reduce histamine levels in the body. While this is a point in favor of the diet, if you have histamine intolerance, you're probably still taking in too much histamine from food without knowing it. Aged hard cheeses, vinegar, tomato, eggplant, spinach, bone broth, and canned tuna are high in histamine, despite being low in FODMAPs. And eggs, strawberries, oranges, banana, pineapple, some nuts, tomatoes, eggplant, spinach, and shellfish can stimulate your body to make more histamine. Leftover foods, even a day or two later, can accumulate enough histamine to trigger symptoms. On top of that, having food sensitivities means that your body is releasing histamine, along with other immune compounds, as a reaction to foods, regardless of their histamine or FODMAPs levels.

The solution:

If you have stomach pain, bloating, diarrhea, and loose stool in addition to these other symptoms, you may need to adjust your diet further. Remove the foods that contain or release histamine mentioned above for two weeks, and eat only same-day-cooked meals. A diet that is both low in histamine and FODMAPs is challenging, so you might want to consult with a dietitian who can help you address both. Consider consulting with a medical professional who can help diagnose and manage histamine intolerance. 

The problem: You have SIBO.

SIBO stands for small intestinal bacteria overgrowth. It's characterized by the excess of pathogenic bacteria or even normal healthy bacteria in the small intestine. Too much bacteria in the wrong place in the digestive tract leads to excessive fermentation, resulting in distention, bloating, stomach pain, gas, burping, diarrhea, and constipation.

It's estimated that 4% to 78% of people with IBS have SIBO. It's more common in people with IBS who are female, older, have diarrhea-predominant IBS, experience bloating and gas, and have used proton-pump inhibitors or narcotics.

The low-FODMAPs diet tends to help people who have SIBO. However, they may need additional dietary restrictions, such as removing all starches, grains, legumes, and sugars. People with SIBO also have a hard time with the reintroduction phase. Because the presence of bacteria in the small intestine isn't addressed with the elimination diet alone, adding back foods that are easily fermented will bring on the symptoms again.

The solution:

If you suspect that you may have SIBO, consult with a health practitioner who can help you diagnose the condition. Eradicating the bacteria, whether through a specific type of antibiotic or through herbal supplement protocol, is necessary to be able to reintroduce some of the foods that were eliminated on the plan.

A last word on FODMAPs.

No matter where you fall in the statistics and how successful the low-FODMAPs diet is for you, keep in mind that it's not a lifestyle diet. It's meant to be used short-term to allow your gut to rest and digest and to give you the opportunity to narrow down your triggers so you avoid the fewest types of foods possible.

The diet can affect the composition of the gut microbiome, which can lead to other health implications later. One study found that following the low-FODMAPs diet for 21 days reduced total bacteria abundance in the gut. Another study found that four weeks on the low-FODMAPs diet reduced bifidobacteria, a beneficial type of bacteria that prevents infections and produces B vitamins and short-chain fatty acids (SCFA). Eating nutrient-rich, high-fiber foods like apples, pears, cauliflower, celery, onion, garlic, artichoke, and others is important for maintaining a healthy gut and preventing many chronic diseases. 

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