This article is part of the IBD Index. Last updated on April 20, 2022.
The low-FODMAP diet is somewhat unique among IBD interventions because it promises to intervene in the common but oft-overlooked situation of continuing functional gastrointestinal symptoms even in the absence of inflammation.
I cover the basics and practical aspects of a low-FODMAP diet in The Low-FODMAP Diet for IBD: Everything You Need to Know. In this article, I take a deep dive into all the evidence regarding the effectiveness of a low-FODMAP diet for IBD, both from the perspective of reducing inflammation and managing symptoms. I also address the most common concern about a low-FODMAP diet: its effects on the microbiome.
Table of Contents
Some context: IBS vs. IBD, and “functional” symptoms
Does a low-FODMAP diet reduce inflammation in IBD?
Interlude: not all FODMAPs are created equal
Is a low-FODMAP diet harmful to the microbiome?
Interlude: low-FODMAP does not mean low-prebiotic
Does a low-FODMAP diet reduce symptoms in IBD?
Some Context: IBS vs. IBD, and “Functional” Symptoms
Right off the bat, it’s important to clarify that the low-FODMAP diet is historically a diet for IBS, not IBD. Although the first appearance of the term “FODMAP” was in Peter Gibson’s 2005 paper about susceptibility to Crohn’s disease, it quickly became primarily a means to treat functional gastrointestinal symptoms (like the diarrhea, gas, bloating, etc experienced by IBS patients) that aren’t associated with intestinal inflammation (found in Crohn’s and UC patients).
Now, IBD patients do often present with functional GI symptoms that cannot be attributed to intestinal inflammation. In fact, about one quarter to one third of IBD patients who are in remission still experience significant functional GI symptoms. (Source 1, 2, 3, 4, 5)
Researcher Alexander Ford gives a good high-level summary of IBS vs. IBD here, and shares that even in 2020 when this paper was published, there existed no evidence-based standard of care for IBD patients with ongoing “IBS-like” symptoms; this is likely still true today.
That said, a 2018 clinical practice update (with none other than our friend Peter Gibson as a contributor) suggests a low-FODMAP diet as “best-practice advice” to address functional symptoms after inflammation, structural abnormalities, and alternative pathophysiologic mechanisms have been ruled out. There’s also some very exciting research coming down the pipeline (see the “coming soon” papers here).
All that said, there are two main angles from which the low-FODMAP diet should be assessed for IBD:
1. Effect on disease activity as measured by inflammatory markers and/or histological observation. This may or may not correlate with overall GI symptoms, but is still highly relevant (since the number one goal of IBD treatment is to quell inflammation).
2. Effect on overall GI symptoms (including “functional” symptoms). Essentially, the patient’s day-to-day experience of diarrhea, gas, bloating, constipation, urgency, etc.
I’ll discuss both of these below. I’ll also cover a third angle:
3. Effect on the gut microbiota. The effect of a low-FODMAP diet on the gut microbiota is related to both symptomatic and disease effects in IBD, and is likely a prime explanatory factor mediating these.
Does a Low-FODMAP Diet Reduce Inflammation in IBD?
As I’ve already hinted at, the low-FODMAP diet is not an intervention for IBD inflammation. That’s not what it was designed for, and as yet, there are no obvious direct mechanisms to implicate FODMAPs in the underlying disease pathology of IBD. And with a few minor exceptions, inflammation in IBD does not respond to FODMAP restriction in the clinical trials conducted thus far.
A 2020 review article by Grammatikopoulou et al points out that “most RCTs failed to include objective outcomes like immune activation markers, changes in the gut microbiota or in the gut lumen, and when objective outcomes (such as CRP, fCAL, or T-cell phenotype) were assessed, the lack of significant differences post-LFD adherence was apparent.” However, Bodini et al did observe a decrease in fecal calprotectin after a 6-week LFD trial in IBD patients. (Source)
Section 7 of this 2020 review article from Gibson does a good job summarizing all the research – both animal and human, and not necessarily IBD-specific – relevant to the effect of FODMAPs on intestinal injury and inflammation. As far as human research, one RCT in IBS patients found that a LFD led to reduced levels of urine histamine, which could suggest reduced immune activation. (Source)
A very small study in 6 IBS patients found higher levels of fecal LPS in the IBS patients compared with controls, but after 4 weeks of a LFD their LPS levels had decreased to a level similar to that of the healthy controls. (Source)
Another study, also in IBS patients, found that a LFD led to reduced levels of circulating inflammatory cytokines. (Source) This reduction of inflammatory cytokines was accompanied by a reduction in certain intestinal bacteria believed to be beneficial, and interestingly, a period of supplementation with fructooligosaccharides (a FODMAP) restored levels of those bacteria without increasing levels of cytokines.
The remaining findings he discusses (which I won’t summarize here) begin to implicate FODMAPs in more serious intestinal barrier dysfunction, injury, and inflammation, but this data is from animal studies that have tenuous relevance to humans at best. Not only are the doses of FODMAPs used in these animals quite high, but the diets are so deconstructed they can scarcely be called food. And as I’ll explain below, the effects of FODMAPs can be quite dependent on context, with whole-food FODMAPs having notably different effects from isolated supplemental FODMAPs.
That said, Gibson has not abandoned his hypothesis that excessive intake of FODMAPs could contribute to the development of IBD (see Section 8 of the previously mentioned review article), so it’s certainly still an open question in the research community. But at this time, there isn’t much support for the use of a LFD as an intervention for IBD inflammation.
Interlude: Not all FODMAPs are Created Equal
In his 2005 “personal view,” Gibson presents a rat study showing that supplementation with rapidly-fermented fibers caused significant intestinal injury. He mentions this study again in his 2020 review paper, but presents no new evidence to that effect. Based on this and other tangentially-related animal evidence, he suggests that FODMAPs could potentially cause intestinal injury by virtue of their rapid fermentation in the distal small intestine and proximal colon.
First of all, although FODMAPs are indeed rapidly-fermented by definition, this rat study did not use a FODMAP in the experiment – it used potato starch (a source of resistant starch). But more importantly, the harmful effects were reduced or abolished by the concomitant supplementation of wheat bran, a slow-fermenting fiber. It appears that consuming slow-fermenting fibers along with rapidly-fermented fibers shifts fermentation towards the distal end of the colon, which our intestines seem to prefer over excessive fermentation in the proximal colon and ileum.
This topic could be explored in far more depth, but I take this as yet more evidence that there’s something to be said for eating foods in their whole form, since most FODMAP-containing foods include many different types of fiber, including the slowly-fermenting kind.
Is a Low-FODMAP Diet Harmful to the Microbiome?
The same Gibson 2020 review paper I extensively referenced in the last section also does a good job summarizing what we know about the effect of a LFD on the intestinal microbiota, which is…not much.
Based on the evidence so far, we can surmise with some certainty that a LFD leads to decreased abundance of microbes in feces (without associated decrease in richness/diversity), and decreased abundance of the phylum Actinobacter and genus Bifidobacterium, but all other effects on the microbiota seem to be inconsistent and unpredictable, owing in large part due to the huge number of variables that aren’t strictly controlled for across studies: specific composition of the LFD, FODMAP content of subjects’ habitual diet, differential effects of different types of FODMAPs in the diet, etc. (Additional source for review of the evidence)
And any useful interpretation of these results is further complicated by the fact that despite claims and appearances, we don’t actually know what constitutes a “healthy microbiome.” (This fact alone shows how much we still have to learn in the field of microbiome science, but for now it should at least release us from the cognitive dissonance when confronted with symptom reductions that correlate with “unfavorable” microbiome shifts, for instance.)
As far as the effects on SCFA, one would expect that a LFD would lead to reduced SCFA production, since bacteria would have less fermentable substrate. That may well be true in many cases, but most of our human data is from SCFA measurements taken from feces, and since SCFA are readily absorbed from the intestine, the SCFA that show up in feces were probably produced by bacterial fermentation of long-chain fibers (non-starch polysaccharides and resistant starch) rather than FODMAPs.
This inference is supported by the fact that studies that matched fiber content between LFD and control diets (usually by adding resistant starch to the LFD) showed no difference in fecal SCFA, while those that did show a difference did not match fiber content.
Much of the discussion on the effect of a LFD on gut microbiota gets into real can-of-worms territory by calling up what I see as one of the Big Central Questions of Intestinal Health and Disease (or BCQIHD…think it’ll catch on?), namely: starve the bad or feed the good?
This question, referred to in at least one article as “Gibson’s Conundrum,” is at the mechanistic intersection of therapeutic diets such as the Specific Carbohydrate Diet, low-FODMAP diet, and even carnivore or ketogenic diets, and therapeutic approaches emphasizing increased consumption of prebiotic fibers (whether through diet or supplements) like the bean protocol.
Tackling that question is very much outside the scope of this article, and a satisfying synthesis of the dual hypotheses depicted below continues to elude even cutting-edge researchers, so I’d certainly fare no better. Suffice it to say that a low-FODMAP diet most certainly affects the intestinal microbiome, in ways that may have bearing on IBD pathology, but as yet we don’t have a consistent picture of what those effects are or whether they’re helpful or harmful or some combination of the two.
Interlude: Low-FODMAP Does Not Mean Low-Prebiotic
Gibson introduces this issue clearly and succinctly: “FODMAPs are defined by their molecular size, absorptive characteristics in the small intestine, and fermentability. Prebiotics are defined by their effects on microbiota and health, and are substrates that are selectively utilized by host microorganisms conferring a health benefit.” (Source)
He goes on to explain that many dietary carbohydrates that have prebiotic actions are FODMAPs, including FOS, GOS, polyols, and possibly fructose. I would also add “possibly lactose” to that list. (Source 1, 2, 3)
But although many FODMAPs are prebiotics, not all prebiotics are FODMAPs. Resistant starch is probably the best example of a fiber that is commonly considered a prebiotic, but is not a FODMAP. In fact, in LFD trials where fiber content is matched between the experimental LFD and the control diet, resistant starch is typically added to the LFD to make up for the reduction in fiber content from high-FODMAP sources. (Sunfiber is another low-FODMAP prebiotic option.)
So setting aside the question of whether and how and to what extent prebiotics are necessary or beneficial for managing IBD (or for maintaining the health of the microbiome in general)…I want to at least make the point that the categories “FODMAP” and “prebiotic” are not one and the same, and that there are low-FODMAP options for selectively encouraging the growth of “beneficial” bacteria for those who desire to do so.
Does a Low-FODMAP Diet Reduce Symptoms in IBD?
And finally, we reach the main event. In contrast to its effects on intestinal inflammation and the microbiome, the effects of a LFD on functional GI symptoms (both in people with IBD and IBS) are fairly well established, so this is where a low-FODMAP diet really comes into its own: in IBD patients who have ongoing functional symptoms that are not explained by inflammation.
The controlled trials by Cox et al (2020), Bodini et al (2019), and Pedersen et al (2017) provide evidence that initiating a LFD can improve symptoms and quality of life for patients with quiescent IBD, while results from the trials by Cox et al (2017) and Halmos et al (2016) provide evidence for the flip side of the same coin – that increasing FODMAP intake in IBD patients leads to worsened symptoms. (See the review of the literature section in my other low-FODMAP article for additional details on all these studies, plus other relevant papers.)
The mechanisms by which FODMAPs contribute to functional GI symptoms are also fairly well understood, lending credence to the causal relationship. As explained here, what makes a FODMAP a FODMAP is both 1) its molecular structure/size as a short-chain carbohydrate, and 2) its tendency to be poorly absorbed and readily fermentable by bacteria.
Based on these characteristics and confirmed by ileostomy study, breath testing, and other techniques, researchers have confirmed that FODMAPs can lead to GI symptoms via 1) osmotic effects, resulting in more water being drawn into the intestine, and 2) increased gas production from bacterial fermentation. (Source)
Although this is a bit out of scope for this article, it’s important to note here that an inevitable causal linkage need not be drawn from FODMAPs > increased gas > GI symptoms. In reality, the relationship is much more complex, with factors such as composition of gut bacteria, visceral hypersensitivity, and gas disposal mechanisms also at play. After all, plenty of healthy people can consume quantities of FODMAPs without issue.
The Bottom Line
- Some very preliminary evidence indicates that a low-FODMAP diet could help reduce inflammation, but it is far from being a validated strategy for that purpose.
- On the other hand, we have fairly good evidence that low-FODMAP diets are effective for reducing lingering functional gastrointestinal symptoms in IBD patients whose disease is in remission.
- Although a common concern with low-FODMAP diets is potential adverse effects on the microbiome from removing fermentable substrates, its actual effects on the microbiome are not clear and predictable on the basis of current research.
- Not all prebiotics are FODMAPs, so it’s possible to support the growth of beneficial bacteria through resistant starch or products like Sunfiber even on a low-FODMAP diet.