The low-FODMAP diet is a very popular diet among people who suffer from gastrointestinal illness, in particular those who have Irritable Bowel Syndrome (IBS) – an extremely common condition in today’s society. If you go see a doctor, gastroenterologist, or dietitian and tell him you regularly experience bloating, flatulence, and/or other IBS-related gastrointestinal problems, then chances are you’ll be advised to take up a diet that’s low in Fermentable Oligo-, Di- and Mono-saccharides And Polyols (FODMAPs).
One of the primary reasons the low-FODMAP diet has become so popular, both among patients and clinicians, is that it has a solid body of clinical research integrated into its foundations. Few diets have been subjected to as much scientific scrutiny as the low-FODMAP diet. Several clinical trials have looked into the therapeutic potential of this special diet, which was developed for the management and treatment of gastrointestinal illness. The results of these studies indicate that the low-FODMAP diet effectively reduces gastrointestinal symptoms such as diarrhea and bloating in patients with IBS and may be useful in the treatment of organic gastrointestinal disorders such as Inflammatory Bowel Disease (IBD) (1, 2, 3, 4).
A lot of people look at the results of these studies and jump to the conclusion that patients who suffer from gastrointestinal illnesses such as IBS should be prescribed the low-FODMAP diet, which they should stay on for a long or indefinite period of time. In today’s article I’ll explain why it’s probably a mistake to make this assumption.
Let’s take a step back and get an overview of the problem
As I’ve pointed out many times here on the site (e.g., here, here), it’s a huge mistake to base our understanding of nutrition and health solely on the results of clinical trials, meta-analyses, and other similar studies. These types of studies are undoubtedly powerful; however, they only provide us with some of the pieces we need to put together the complex puzzle of medicine. Before we take out our shovels and dig into the depths of PubMed, we have to make sure we are equipped with the knowledge and tools we need to make sense of the wealth of information that we’ll likely come across on our journey. Most importantly, we need to understand the evolutionary and biological mechanisms that play a role in the condition or issue we’re on a quest to explore.
The foods that we human eat are composed of a long range of nutrients. These nutrients can broadly be categorized according to where they are digested in the human body. Fats, proteins, simple sugars, and starch (excluding resistant starch) are primarily digested in the upper part of our gastrointestinal tract (the mouth, stomach, and small intestine), whereas oligosaccharides and non-starch polysaccharides are primarily digested in the large bowel. There are some exceptions and caveats to this rule though. For example, in lactose intolerant individuals, lactose may pass undigested through the small bowel and become subjected to microbial fermentation in the colon. Moreover, the lines can sometimes get a little blurred, with some nutrients being exposed to digestive enzymes both in the lower and upper part of the gastrointestinal tract. However, in general, it’s safe to say that the aforementioned principle holds true.
The observant reader has undoubtedly understood why this is important to know in the context of the therapeutic potential of the low-FODMAP diet.
The low-FODMAP diet is a weird diet. It doesn’t exclude non-starch polysaccharides, although they pass undigested through the upper gut, but it does exclude fructose and lactose, which are absorbed just fine in the small intestine of many people. Personally, I don’t like the approach the designers of the low-FODMAP diet used to classify carbohydrates.
With that said, as an overarching rule, the nutrients that are excluded on the low-FODMAP diet belong to the latter category of the above nutrients: the ones that pass through the small intestine largely undigested. The human genome doesn’t contain genes that code for the digestive enzymes that break down these substances; hence, they are not broken down by man, but rather by microbes. Gut microbes love FODMAPs.
FODMAPs are ubiquitous in nature: they are found in a broad range of different plant foods. Hence, it shouldn’t come as a surprise that they have been a part of the human diet for millions of years. Actually, our ancient ancestors took in a lot more fermentable carbohydrates than most contemporary humans do (5, 6). Some ancient people may even have taken in as much as 135 grams of inulin-type fructans every day (7). Contemporary African hunter-gatherers, such as the Hadza, are also known to consume a lot of fermentable carbohydrates (8, 9).
There’s absolutely no reason to think that a significant proportion of the human population has suddenly lost the ability to properly digest fermentable carbohydrates. There has to be something else that’s going on here.
Getting to the root of the problem
Gut dysbiosis – an imbalance in the microbial community of the gut – is a key feature of the pathophysiology of many diseases and health problems. Virtually all of the gastrointestinal problems that affect members of Homo sapiens have been linked with gut dysbiosis (10, 11). Patients with IBS – the condition most often treated with the low-FODMAP diet – typically harbor an imbalanced gut microbiota (12, 13, 14, 15). This has led some researchers to say that a better name for the cluster of symptoms that make up the condition we now call IBS would be the Dysbiotic Bowel Syndrome (DBS) (12). Gut dysbiosis is not the sole issue in IBS, but it’s surely an important one.
Given this, it shouldn’t come as a surprise that people with IBS have trouble digesting fermentable carbohydrates. We depend on the genetic capabilities of the microbes that dwell deep in our guts in order to digest the great variety of carbohydrates that are found in the plants we eat. By ourselves, we’re only capable of digesting starches and some simple sugars. The rest end up on the dinner table of the gut bacteria.
A broad range of genes are required to produce the broad range of enzymes that are needed to break down the broad range of carbohydrates that pass into the large intestine. If the microbial community of the lower gut is degraded and/or imbalanced, digestive processes will obviously be compromised. Some of the nutrients that pass into the gut may only be partially digested, due to the fact that microbes that are capable of completing the digestive processes are not present or have not been able to set up shop in the gut, and certain pathogenic bacteria, which may have been allowed to proliferate due to a low presence of beneficial bacteria, could make use of certain nutrients and produce various problematic substances. This in turn would promote symptoms of food intolerance, such as bloating, loose stools, and flatulence, which are stereotypical of IBS.
With this knowledge in mind, one can quickly understand why studies have found that many patients with IBS get better when they adopt a diet low in fermentable carbohydrates (e.g., the low-FODMAP diet). When a person with IBS dramatically reduces the amount of fermentable carbohydrates he consumes, he’s basically shutting down much of his colonic microbiota’s food supply. Not surprisingly, this will cause a reduction in some of his gastrointestinal symptoms, due to the fact that he’s no longer adding fuel to the fire that’s burning inside his gut.
There’s a big problem though. By cutting out fermentable carbohydrates from his diet, he’s able to keep his gastrointestinal symptoms at bay; however, he does nothing about the underlying problems of his illness. He’s not putting out the fire sort to say. He’s only suppressing it. Actually, he may, by dramatically reducing his intake of fermentable substances, make his condition worse over the long term, due to the fact that he’s not adequately feeding his beneficial gut bugs. When starved of food, his gut bacteria may wither and die, or they may end up digesting the mucus of the intestinal wall (16, 17, 18). Moreover, pathogenic bacteria may bloom in his gut.
Putting out the fire
Instead of trying to keep the fire in check with a diet low in fermentable carbohydrates, I would argue that it makes a lot more sense to try to turn out the fire completely, by fixing the microbial community of the gut. This strategy may not be effective in every case of IBS and IBD, due to the fact that some patients, because of genetic/epigenetic factors, have a severely inflamed gut that hinders them from developing a healthy, diverse microbiota; however, it should be effective in many cases.
This statement is supported by research showing that gut microbiota manipulation (e.g., via fecal microbiota transplantation or probiotic supplementation) is an effective treatment for many gut disorders, including IBS and IBD (12, 13, 19, 20). There is still a lot we don’t know with regards to what constitutes the best approach for repairing a damaged gut microbiota; however, we do know quite a bit. As long as we adhere to the evolutionary principles of Darwinian medicine, we should be on safe grounds.
The low-FODMAP diet, may, in some cases, be useful during the first stages of the treatment of certain gut disorder; however, it shouldn’t be relied on as a permanent solution, unless it has proven very difficult or impossible to repair the gut microbiota of the patient in question and improve his tolerance to fermentable carbohydrates.
A diet that limits the intake of naturally occurring fermentable carbohydrates is, from an evolutionary perspective, a novel human diet. It’s not the diet we evolved to eat. As a species, we are adapted to digest fermentable carbohydrates such as oligofructose and resistant starch. The fact that a growing proportion of the human species now seems to have trouble digesting these types of compounds can’t be explained by genetic changes within the human genome, but rather by changes within the human microbiome.
Via our modern lifestyle, saturated with antibiotics, processed foods, and other microbiota-disruptors, we’ve changed the microbial communities that colonize our bodies. These changes have made us sick and fragile. We’re at an increasing rate becoming allergic to pollen and other substances that are a normal part of the world around us, and we’re suddenly having trouble digesting nutrients that have been a natural part of the human diet for ages.
The solution to this problem is not to build a wall around ourselves in order to try and shield ourselves from the things in nature that our bodies now have trouble coping with. Rather, the solution must be to try to repair our microbiomes and restore our bodies to a fully functional state , so that we’re no longer allergic or intolerant to things that we are not supposed to be sensitive to. Again, this approach may not work in every single case. However, it should bear fruit in most cases. The removal of the offending substance(s) in question should only be a last resort that we go for when all other avenues have been explored.
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