PART 2: The Pervasive Misuderstanding of What the FODMAP Diet Does and Doesn’t Do


The exaggerated confidence given to the FODMAP diet, that it has the ability to favorably alter the intestinal microbiota, starve out organisms, or alter the immune system by reducing histamines, for example, are unfounded in the literature.  The diet is being misrepresented across publications, by practitioners and over social media, and in chat rooms.

In The Pervasive Misunderstanding Of What The Fodmap Diet Does And Doesn’t Do – Part One, I reviewed key terms in FODMAP studies; lactulose breath testing (LBT) and the effects of a low FODMAP (LFD) diet on hydrogen and methane levels; if studies show that the LFD directly treats underlying mechanisms that are at the root of IBS and SIBO; and if the LFD alters immune system markers.

In Part Two, I will discuss the misconceptions around LFD and histamine levels and how this is being incorrectly interpreted across current publications, LFD induced changes in nutrient status, utilization of an LFD in the pediatric population, the drawbacks to implementing a sweeping low FODMAP diet protocol clinically, and how to correctly implement the LFD into your clinical practice.

Does a LFD Reduce Histamine Levels?

Some of you have likely heard the widespread belief that an LFD reduces histamines. This stems from the study, FODMAPs Alter Symptoms And The Metabolome Of Patients With IBS: A Randomised Controlled Trial – McIntosh 37 subjects with IBS were randomized into two groups: LFD vs. HFD (no cross over). [22]

The study’s abstract states, “Histamine, a measure of immune activation, was reduced eightfold in the low FODMAP group (p<0.05).” Let’s take a look at the one study that investigated LFD vs. HFD and histamines.

They measured a LBT, PCR stool testing and 29 different urine metabolites. Of the 29 different urine metabolites, they claim to see differences in three metabolites – one is histamine.

A single point urine test was used to evaluate for histamines. As urinary histamine varies greatly at any one point in time, the standard way to test urinary histamine levels is a 24-hr urine collection. Since the correct test was not utilized to measure histamines, further evaluation of the data and claims is unnecessary, but let’s continue. The standard range of a 24 hour urine histamine measure is 0.006-0.131 mg/ 24 h. The LFD baseline group (prior to the dietary intervention) was 0.006±0.0117, which, if correctly measured, is already a very low urinary histamine level. So, if we are to believe their claims that an LFD reduced histamine levels eight-fold, then the urinary histamine level went from very low, to very, very low.

Within the study, they state, ‘Metabolic profiling of urine showed (histamine within) groups of patients with IBS differed significantly after the diet (p<0.05)” highlighting that this study did not find a statistical significance with their data.

There was no statistical difference when comparing baseline data to post intervention data. The researchers then set baseline data aside and compared the post intervention data only. There was no statistical difference when comparing post data intervention. They state, “In addition, when comparing the post-diet effects, the means of these metabolites were statistically different (p<0.05) between the two diet groups when adjusting for their baseline values.” They have an asterisk on the p-value; and who in research doesn’t love an asterisk on a statistic? The asterisk notation reads, “*Obtained from analysis of covariance and adjusted for the baseline metabolite level, gender, IBS subtype and age.”

The group size was adjusted five different times to reach smaller and smaller sub-groups to finally claim a statistical significance “within groups of patients” (five, because 37 subjects completed the study, yet, they only analyzed urine metabolites in 34 subjects;  tossing out the data of 3 people, but we are not told why).

This is where the assumption that a LFD lowers histamines has propagated – and this is what is cited in the 40 research studies, repeated and recited by practitioners and across social media; Google search returns 104,000 results on ‘histamine + FODMAP’.

A new study that looked at histamine and FODMAP was presented at Digestive Disease Week this year and it may be published soon: “Host Microbe Interactions as Determinants to Response to Fermentable Diets,” which also looked at a single ‘fasting’ urine collection in the morning to assess histamines. They also defined a LFD as 2.6 g/ day +/- 1.4 and a HFD as 8.2 g/ day +/- 5.6, which is not in range with other LFD and HFD studies.

If you are a practitioner and you have been stating that a “LFD lowers histamines,” I urge you to correct this information within your handouts, and online posts.

If you are a researcher with a published study, who has cited FODMAPs Alter Symptoms And The Metabolome Of Patients With IBS: A Randomised Controlled Trial – McIntosh, I urge you to issue a correction. You have a responsibility to the field to make this correction.

Changes in Nutrient Status while on a LFD

There are a few studies that investigated micro and macro-nutrient adequacy on a 4-week LFD and one long term prospective follow up study.  Interestingly, what some of these studies found was that habitual diets of people with IBS did not meet many DRI levels; leading to poor baseline nutrient status and low baseline diet diversity. They showed that although a LFD reduces symptoms, the 4-week LFD didn’t correct these nutritional deficiencies.

Let’s look at three of these studies: 

The Impact of a 4-Week Low-FODMAP and mNICE Diet on Nutrient Intake in a Sample of US Adults with Irritable Bowel Syndrome with Diarrhea, S. Eswaran et. al. investigated changes in nutrient status in response to a 4-week LFD vs. the mNICE diet. [68]

Although subjects followed a 4-week diet, the results of this study are based solely on the analysis of two 3-day food journals which were collected pre-intervention and during week four of the dietary intervention. The authors reported that “some participants complete diary at the end of the day, rather than in real-time diaries may have recall bias and difficulty in estimating quantities.”

The LFD group was encouraged to limit consumption of high-FODMAP foods, yet, the levels (grams) of FODMAPs was not reported in the final analysis, nor are we offered an average FODMAP intake (we are unsure if this study is comparable to other LFD studies, that use 7-9 grams of FODMAP/ day in the low FODMAP arm).

Baseline nutrient measures of both groups showed that their habitual diets failed to meet the DRI for vitamins C, D, and E; and calcium; magnesium; and potassium. The LFD group did not meet the DRI for vitamin K and folate. The authors also noted inadequate dietary vitamin D, which another study found, linking poor vitamin D status with IBS. [60]

In this pilot study, A Low Fermentable Oligo-Di-Mono-Saccharides and Polyols (FODMAP) Diet is a Balanced Therapy for Fibromyalgia with Nutritional and Symptomatic Benefits, A. Marum, et. al., 38 fibromyalgia subjects followed a 4 week LFD. They found no significant changes in calories, macronutrients, nor fiber, calcium, magnesium nor vitamin D. Subjects reported reduced fibromyalgia and IBS symptoms and reduced somatic pain, with 86% reported compliance with the diet. The LFD group consumed 2.6 grams FODMAP +/- 5.4 per day. Baseline measures of nutrient status of the subjects habitual diets was not highlighted by the authors.

This study, Nutrient Intake, Diet Quality, and Diet Diversity in Irritable Bowel Syndrome and the Impact of the Low FODMAP Diet, H. Staudacher, et. al., 130 subjects with IBS were randomized to a 4-week LFD, sham diet or to continue their habitual diet.

They found that the habitual diet quality and diversity was poor. When the LFD group was compared to the habitual diet group and sham diet group, no significant difference was found between calories or macronutrients. The LFD group, averaging 8.6g of FODMAP per day, showed an increase in vitamin B12 intake (dietary increase in eggs and fish) and that calcium and additional nutrient levels measured were found to be no different than the habitual diets.

This study is comparing a 4-week LFD to habitual diets that were already lacking in quality, diversity and micronutrient density for a few minerals – lower in iodine, magnesium, iron and selenium. The data also showed that by week 4, the LFD showed a further reduction in diet quality. Even though the LFD education was performed through one on one consultation with a Registered Dietitian, the authors noted, “This suggests that, even when counseling is provided by a specialist dietitian, the overall ability for individuals to meet the dietary guidelines when following a short-term low FODMAP diet is reduced.”  [62]

There is one long term prospective questionnaire study assessing nutrient levels with LFD, Long-Term Impact of the Low-Fodmap Diet on Gastrointestinal Symptoms, Dietary Intake, Patient Acceptability, and Healthcare Utilization in Irritable Bowel Syndrome, M. O’Keefe, et. al., 103 IBS subjects tracked symptoms and dietary intake while following a low FODMAP elimination and reintroduction diet over 6–18 months. Initial FODMAP education was performed by trained dietitians. They found that 82% continued to follow an Adapted FODMAP Diet ((AFD) personalized adaptation of the diet to self-manage their symptoms in the long term), while 18% had returned to their habitual diet. [57]

At baseline, 20 patients had IBS-C, 39 patients had IBS-D, 21 patients had IBS-M, and 23 patients had IBS-U. To note, stool frequency was marked “normal” (once every 3 days to three times a day) or “abnormal” (less than once every 3 days or more than three times a day).

The AFD group had significant reductions in abdominal pain, bloating, flatulence, incomplete evacuation, and lethargy; with pain, bloating and flatulence decreasing by more than one-third in the long term. The authors noted, “Our study demonstrated benefits of the FODMAP elimination phase, which is only intended for short-term use, to symptoms and QOL in IBS-D patients.”

Total FODMAP intake was significantly lower for the AFD group (20.6±14.9 g/d) compared with the ‘habitual’ group (29.4±22.9 g/d, P=.039. The AFD group ate significantly less fructans and fructose, but there was no difference observed with lactose, galacto-oligosaccharides, sorbitol, or mannitol levels between the groups.

There were no significant differences between groups at long-term follow-up for energy and nutrient intakes, except for folate and vitamin A which were both higher in the AFD group compared with the ‘habitual’ group in comparison to the UK recommendations.

The AFD group reported that the diet was more expensive and that they had difficulty eating at restaurants, with family and while traveling, and they had a greater dependency on supplements. [57]

LFD in the Pediatric Population

There have been a few studies investigating the efficacy of an LFD in the pediatric population. [63-66] Although these studies have shown that an LFD reduces abdominal pain, bloating and diarrhea symptoms in children, can the statement be made that an LFD is safe and beneficial for children?

The study, Low FODMAP diet in children and adolescents with functional bowel disorder (FGD): A clinical case note review, S. Brown, et. al., reviewed 29 cases where a LFD was utilized over a three month period, subjects and families met with a dietitian three times and were instructed to follow an LFD elimination and then reintroduction phase. [67]

We need to recognize that this is a retrospective clinical case review, in which they contacted 29 children to fill out the questionnaires 2- 29 months later, “A 16‐point IBS satisfaction survey was utilized in the current study to measure symptoms (bloating, pain, alternating stools, etc.); however, this was completed retrospectively and, in some instances, more than 2 years after the child received his or her initial dietetic consult.”

Twenty‐three (79%) participants reported an improvement of symptoms, complete resolution of GI symptoms was observed in 92% of those with bloating, 87% of those with diarrhea, and 77% of those with abdominal pain. However, nine (31%) children were not satisfied with the improvement of their overall symptoms, and 17 (59%) were not interested in changing their diet further to improve symptoms.

During the reintroduction phase, several participants experienced symptoms to more than one carbohydrate as follows: fructans was the most common intolerance (18, 67%), followed by lactose (16, 56%), polyols (2, 7%), fructose (2, 7%), and GOSs (2, 7%). Six (24%) children specifically identified that apples (fructose and sorbitol) triggered symptoms.

17% of children in this study experienced stress and anxiety. Children with FGD are known to have higher risks of anxiety and stress. The younger subjects (10 year old subjects) had more success with this study (likely because the parents had more control over their diet). So, placing them on a restricted diet for three months (plus the time that this takes to reintroduce FODMAPs) may be too drastic of an approach.

Instead, why don’t we learn from studies like this and reverse the approach, by focusing on single FODMAPs that are more likely to trigger GI symptoms; trial single eliminations to fructans and lactose, then fructose and assess symptoms.

The authors state, “LFD is safe and beneficial for children.” This is a broad reaching claim that is unsubstantiated by the data presented in this study. A more concise statement would be, “In a group of 23 children with FGD, a short term low FODMAP elimination diet, followed by a reintroduction phase reduced bloating, diarrhea, and abdominal pain symptoms in 77%, 87% and 92% respectively, yet increased anxiety in 17%.”

What are the Drawbacks?

The FODMAP diet is an elimination-style three-phase plan to investigate which Fermentable Oligosaccharides, Monosaccharides, and Polyolys (FODMAP) are triggering symptoms. Phase 1: 4week FODMAP elimination; Phase 2: reintroduction (challenge); and then Phase 3: maintenance.

There are many drawbacks to implementing a FODMAP elimination diet.

  • If someone is a responder, and symptoms are greatly diminished, it can be difficult to get them to reintroduce and challenge FODMAPs and to expand their diet.
  • Many assume that the FODMAP diet is a direct treatment for IBS and SIBO, and, that by following the diet they will be free of symptoms and be cured. In reality, this is only managing some symptoms. Symptom and quality of life improvements are helpful, and the root issue still needs to be investigated, targeted and treated.
  • If, in response to symptoms, a person has already begun to restrict foods, a shift to the elimination phase of the FODMAP plan may not provide as much relief. This may lead to more restriction as the patient continues to work towards major symptom control.
  • If a person is low weight, hypoglycemic, anxious, malnourished, exhibit tendencies towards disordered eating, and/ or they already have other dietary restrictions in place, further restriction of their diet may do more harm than good.
  • Many people with SIBO feel that they MUST go on a carbohydrate restricted diet to directly treat SIBO, with the goal of ‘starving out the organisms.’ This has not been shown in the literature, nor does this play out in the clinical setting. Patients coming into my practice have been following a restrictive FODMAP diet from 4 months and up to 7 years and they still have SIBO.
  • People are becoming ‘stuck’ on this diet, with fear that every symptom that they experience is a worsening of a SIBO ‘infection’ that is growing in their intestines. This fosters anxiety and food fear and this in turn will negatively affect the entire body. As previously explained, an altered carbohydrate diet can temporarily alter symptoms, but it cannot starve out the organisms. SIBO is not an infection; it is a secondary condition, an overgrowth, brought about by a variety of factors. The goal when treating SIBO is to identify and treat the root set of factors that set SIBO up in the first place.
  • If a patient has methane dominant SIBO, the LFD is contraindicated. No study has shown that the LFD, nor HFD, change methane levels. Pulling fiber will likely exacerbate constipation, which is often linked with methane dominance. [15, 22, 29, 31, 53]
  • We see across studies that there is a high attrition rate; up to 70% in some studies. [47] The diet is difficult to follow by many for any length of time.
  • People with IBS and SIBO are already dealing with chronic symptoms that leave them feeling socially isolated. [54] A restricted diet will further add to their feeling of social isolation, by making eating at restaurants and sharing a meal with family members, near impossible. This also makes shopping and cooking for a family a huge barrier to healing.
  • Unless a person is working directly with a health care provider, they are often not educated on the fact that the LFD is an elimination diet. They are staying on this well beyond the 4 week elimination phase, with limited data on the longer term efficacy and safety of the LFD with respect to nutritional adequacy, precipitation of disordered eating behaviors, effects on fecal microbiota and metabolomic markers, and subsequent translation to clinical effects. [55]
  • Based on two reviews, the authors recommend a LFD for short-term use only in managing IBS symptoms, after which, high FODMAP foods should be reintroduced. As such, reintroduction of FODMAPs after a 6-wk LFD likewise be encouraged among IBD patients due to a lack of evidence regarding the long-term effects and consequences of the diet. [34,56]

Conclusion/ How to Correctly Implement the LFD in Clinical Practice

Since we know that there are striking variances among the pathophysiology of IBS subtypes and SIBO, and we know that a variety of factors influence an individual’s perceived response, and differences in factors that affect fermentation can change on any given day, [58] then the dietary approach should always be evaluated on an individual basis.

A 4-6 week low FODMAP elimination diet, followed by a challenge and maintenance phase, may be best implemented as adjunct support for managing symptoms in IBS-D and SIBO patients with diarrhea, yet every patient with IBS and SIBO with diarrhea does not need to attempt this diet.

If IBS patients present with poor dietary status, we can start with the basics, by improving eating patterns, providing education on achieving a nutrient dense diet, and supporting digestion and absorption. Although assessing nutrient adequacy of a 4-week LFD is necessary, to assess the safety and offer insights for clinical use, i.e. prescribing a well absorbed multivitamin along with a 4-week LDF is a good idea, many people are not using this as a short term elimination diet.

Practitioners may want to consider first targeting individual FODMAP foods containing fructans, fructose, and then lactose, versus pulling all FODMAPs from the start. Consider reversing the full low FODAMAP elimination diet for those patients with whom a foundational approach of managing stress, cleaning up the diet and basic approaches to supporting digestion, motility, and bowel habits have failed to reduce symptoms to a manageable level. Consider pulling single FODMAPs, which have been shown to be more likely to cause symptoms in patients with IBS, fructans, lactose and, lesser so, fructose.

Since yoga and gut-directed hypnotherapy, as seen in The Efficacy of Gut‐Directed Hypnotherapy is Similar to that of the Low‐FODMAP Diet for the Treatment of IBS, have been as effective as the LFD at reducing symptoms in IBS patients, practitioners should incorporate these interventions. [3] [59]

When implementing the LFD in your practice, personalize the approach by providing adequate one-on-one counseling on the implementation of this diet and by speaking directly to the patient’s needs, their clinical presentation, and their lifestyle.

Be sure to emphasize that this is a 4-6 week short term elimination diet and the goal is to challenge and expand the diet, or further harm may be done to the gut microbiota.

In functional medicine, we pride ourselves at getting to the root of the issue. A LFD may provide adjunct support and symptom relief within a subset of your patients. Yet, since these only address symptoms, please consider that this is placing a high level of time and energy into a sweeping dietary change that is not a direct treatment for IBS and SIBO and in the end, it may do more harm than good.

Angela Pifer, MSN, LN, CLN, FMN known as ‘SIBO Guru,’ has been in private practice for the past 15 years, blending functional medicine and integrative functional nutrition into evidence based work at her clinic in Seattle, Washington. She is an academic research practitioner, contributing to the field of functional gut disorders and  SIBO and through education and online tools, – a FODMAP recipe site and education that helps people expand their diet, and products, – the first commercially available low FODMAP bone broth,  that help people navigate and heal from SIBO. She carries a Master’s degree in Nutritional Science from Bastyr University and is a State Licensed Certified Nutritionist; training in Functional Medicine through the Institute of Functional Medicine.
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