Low FODMAP diet not working? Here's why. (2024)

Low FODMAP diet not working? Are you feeling worse on a low FODMAP diet? Or, just not feeling better? Maybe you’re getting bloated on the low FODMAP diet? Here’s a list of all the reasons why a low FODMAP might not be working for you, from a gut health dietitian with over a decade of experience in supporting folks with irritable bowel syndrome.

As a dietitian who focuses on gut health nutrition, I know how incredible a low FODMAP diet can be for people with irritable bowel syndrome…which is why I included detailed information on how to go low FODMAP in my new book Good For Your Gut.

But I also know that low FODMAP doesn’t work for everyone. While going low FODMAP may help as many as 80% of folks with irritable bowel syndrome, that also means that there are some people for whom the diet doesn’t work at all. So in this post, I will cover everything you need to know to investigate why low FODMAP may not be working for you. (And what to do instead!)

• How the low FODMAP diet works
• How long does the low FODMAP diet take to work?
• Maybe you don’t have IBS
• Have you been following the low FODMAP diet correctly?
• Is something else causing symptoms, like stress?
• Troubleshooting on a low FODMAP diet

How the low FODMAP diet works

The low FODMAP diet is a temporary elimination diet that reduces your intakes of the Fermentable Oligo-, Di-, Monosaccharides And Polyols found naturally in plant foods. In English? FODMAPs are carbohydrates that are not fully digested and absorbed, meaning they travel through the gut. And when they do, two main things can happen:

• Some, like lactose, can draw water to themselves, loosening up your bowel movements. Which is why diarrhea is a classic symptom of lactose intolerance.
• FODMAPs are fermented by your gut microbiome, which for most is a very good thing! However, in irritable bowel syndrome, this fermentation can lead to increased symptoms like pain and bloating

Going low FODMAP may also alter the gut microbiome and decrease histamine production in the gut. So for a short period of time we eliminate high FODMAP foods to see if it eases symptoms and then carefully reintroduce to each category of FODMAPs to assess tolerance.

How long does the low FODMAP diet take to work?

The standard advice is that the first elimination phase of the low FODMAP diet should be between 2-6 weeks. In our practice, we do find that 4-8 weeks is more typical. So ideally, within a few weeks, you should start feeling at least a little bit better. If after 4 weeks, you feel absolutely no different than before, it may be time to let the low FODMAP elimination go and seek other solutions. But before you do, ask yourself the following questions.

Low FODMAP diet not working? Here’s why, according to an RD

Maybe you don’t have IBS!

First things first: do you actually have IBS?

Did you get a thorough work up from your doctor? It’s tempting to self-diagnose IBS but doing so can cause you to miss serious issues that could be leading to your symptoms. However, I also have to admit that sometimes, docs aren’t as thorough in diagnosis. Because, in an ideal world, in addition to meeting the diagnostic criteria for IBS, it’s important that your doctor ruled out any other potential cause of the symptoms which can include:

• Small intestinal bacterial overgrowth, or SIBO: SIBO can mimic the symptoms of IBS, but it’s cause is very different: an overgrowth of microbes (bacteria or fungi) in the small intestine, where microbe concentration should generally be low. Slow motility is often a root cause and diet is not as effective in managing SIBO as it is in IBS.
• Bile acid diarrhea: altered bile acid metabolism can lead to diarrhea and a doctor can prescribe bile acid sequesterants to help!
• Moderate to severe constipation: sometimes, clients come to us with a diagnosis of constipation-predominant IBS when in reality, it is traditional constipation that responds favorably to standard constipation nutrition care strategies like a careful increase in fibre.
• Celiac disease: while not all folks with celiac disease have the classic digestive presentation such as diarrhea, it is a common symptom of celiac disease.
• Inflammatory bowel diseases: Crohn’s disease and Ulcerative Colitis are serious diseases that require medical care and often result in multiple bowel movements a day.
• Histamine intolerance: histamine intolerance is tricky to diagnose – and to treat. But if your irritable bowel syndrome is just not improving despite the proper care, talk to your doctor about it.
• Sucrase-isomaltase deficiency: this enzyme deficiency can result in a sugar + starch intolerance that can mimic IBS
• Endometriosis: this disease of the endometrial lining of the uterus can strongly mimic the symptoms of IBS and is often overlooked as a diagnosis

Have you been following the low FODMAP diet correctly?

Low FODMAP not working? Generally speaking, in order to truly know if low FODMAP is working for you, you have to follow it 100%. Eating 1-2 high FODMAP meals a week while low FODMAP can absolutely alter the effectiveness of the elimination. Which is why going low FODMAP may not be right for everyone, even if it is an evidence-based dietary approach for IBS.

Even if you have a good low FODMAP foods list, there are some potential hiccups, especially if you are eating out at restaurants, where it can be difficult to avoid hidden FODMAPs such as garlic powder.

Also, it’s worth noting that foods on the low FODMAP food list aren’t all FODMAP free. Portion control matters. For example, while ½ cup of canned, rinsed lentils are low FODMAP, if you ate ¾ cup of canned lentils – or ate ½ cup of home cooked lentils, you would be eating a high FODMAP portion.

What’s more, is eating too many low FODMAP foods at a single serving can cause you to go over your FODMAP threshold. Especially for fruit. If you make a low FODMAP smoothie with 4 servings of low FODMAP fruit, that smoothie may actually be high FODMAP.

This is why it is super helpful to do low FODMAP with one-on-one support from a gut health dietitian. However, I know that it isn’t always possible, which is why I write these blogs, create low FODMAP recipes, and why I wrote Good For Your Gut.

Is something else causing symptoms, like stress?

Irritable Bowel Syndrome is a disorder of gut-brain communication, which is why it’s important to know that while nutrition is important for healing your gut and managing symptoms, it’s not everything. Here’s a great example:

A university student is enjoying success 4 weeks into a low FODMAP diet, but all of a sudden, symptoms take a nosedive. When questioned about changes in their life, it is revealed that the student is going through exams, and is super stressed. The fix? Remaining on the low FODMAP diet until exams had passed. Symptoms resolved and then the client felt confident to begin reintroductions.

Your gut is seriously affected by stress, so in our practice, we offer clients strategies for supporting the gut brain connection such as time in nature, meditation, yoga, breathwork and even gut-directed hypnotherapy. In my opinion, it’s often the missing ingredient in IBS care.

Troubleshooting symptoms on a low FODMAP diet

Low FODMAP diet not working? Did new symptoms pop up when you went low FODMAP? Or, did symptoms get worse? Let’s look at some common issues and why they might occur.

• Constipation: maybe you’re not getting enough fibre. Going low FODMAP means eliminating some gluten-containing grains like whole wheat and greatly reducing intake of high fibre legumes, which can decrease your fibre intake and lead to constipation. Try to increase your intake of FODMAP-friendly high fiber foods like certain nuts, seeds and other low FODMAP plant foods.
• Bloating: bloating is super complex – and anything from inactivity to drinking fizzy drinks can be sneaky causes of bloating. But lack of fibre can lead to the slowing down of the gut, exacerbating bloating, especially if movement slows enough to make you constipated.
• Diarrhea: In IBS, intestinal transit can be too quick, which can contribute to diarrhea. So if you are eating less fibre, particularly soluble fiber that can help bind stools, you may find that diarrhea persists or gets worse (although rare!). Soluble fibre-rich low FODMAP foods like rolled oats may help. You may also be eating more starches and sugars from hyper-processed gluten free, low FODMAP packaged foods than usual, which may overwhelm the absorption capacity of a sensitive, hyper-active gut. If you suspect this, try eating more fruits, vegetables, nuts and seeds and see how you feel.

Was this post helpful? Please let me know your questions about troubleshooting low FODMAP diets in the comments below so we can grow this post into a comprehensive resource together in order to help more people!