Low-FODMAP Diet: Everything You Need to Know

The first time I mentioned FODMAPs to a patient; he stared at me blankly and then said ‘Uh, nope I’m not really into fancy navigation systems.’ That was funny in a sad way. After all, FODMAPs are very trendy in the health and wellness world, but people who would benefit the most from understanding the FODMAP science have never even heard of them…

You see, FODMAPs have absolutely nothing to do with any device whatsoever. They’re actually foods we commonly eat. So why would someone consider trying a low-FODMAP diet?

Well, let’s say, you (or someone you know) suffer from irritating digestive or IBS symptoms. If there were a natural strategy that could help reduce these debilitating symptoms, wouldn’t you want to know more about it?

In this article, I’ll cover the fundamentals of FODMAPs, the art of the low-FODMAP diet and how to use the science to improve your digestion (after consulting your physician).

Could these commonly consumed foods be behind your digestive or IBS symptoms?

What exactly are FODMAPs?

In a nutshell, FODMAPs are short-chain carbohydrates that are partially absorbed in the gastrointestinal tract where they are rapidly fermented. FODMAPs stand for ‘fermentable oligosaccharides, disaccharides, monosaccharides and polyols’ – saccharides simply mean sugar.

To understand the low-FODMAP diet, we’ll need to cover some basic biochemistry. But don’t worry; I’ll keep it short and sweet (no pun intended).


‘Mono’ means ‘one’ – so monosaccharides are molecules with only one sugar. The monosaccharide of interest when it comes to digestion is fructose or the sugar found in fruits.

A food is considered a high FODMAP food if it contains more than:

  • 5g of fructose in excess of glucose per 100g serving.
  • 3g of fructose per serving.

Other monosaccharides include glucose and galactose.

carbohydrates on the low-FODMAP diet


‘Di’ means ‘two’ – as you must have guessed, disaccharides have two sugar molecules. The disaccharide that causes the most digestive issues is lactose or the sugar present in milk and dairy products. Lactose consists of one glucose molecule attached to one galactose molecule.

The other disaccharides are:

  • Sucrose or table sugar which consists of one glucose molecule attached to one fructose molecule.
  • Maltose, also known as ‘malt sugar’, is present in the starch that we get from tubers and grains. It consists of two glucose molecules attached together.

Oligosaccharides and Polysaccharides

‘Oligo’ means ‘few’ and ‘poly’ means ‘many’. As such, oligosaccharides refer to carbohydrates containing three to ten sugar molecules attached together whereas polysaccharides are those long chains of carbohydrates with 11 to 15 sugars bonded together.

Oligosaccharides are divided into two categories namely the prebiotics fructans and galactans which act as food for bacteria. Any food that contains more than 0.2g of fructan per serving is considered a high-FODMAP food.



In the FODMAP context, fructans can refer to both oligosaccharides and polysaccharides. Short-chain fructans are better known as fructo-oligosaccharides, or FOS, whereas inulin is a longer-chain fructan (polysaccharide). Both FOS and inulin exist naturally in vegetables and grains.


Those are sugar alcohols such as:

  • Maltitol
  • Mannitol
  • Sorbitol
  • Xylitol

In general, humans absorb these sugar alcohols poorly  – this is why products containing polyols will often carry a warning message saying that the product may cause digestive upset or diarrhea.

Take home message: FODMAPS are highly fermentable carbohydrates that are the most likely to cause digestive distress. These include:
    • Fructose (monosaccharide)
    • Lactose (disaccharide)
    • Fructans (these include the oligosaccharide FOS and the polysaccharide inulin)
    • Galactans (oligosaccharides)
    • Sugar alcohols (polyols)

    Should you care about FODMAPs?

    Do you suffer from any of the following?

    • Irritable bowel syndrome (IBS)
    • Crohn’s disease
    • Celiac disease
    • Ulcerative colitis
    • Recurrent digestive issues such as bloating, abdominal pain, diarrhea, constipation or gas
    • Heartburn

    If despite eating a real food diet (one that is free from processed foods and industrial oils), you answered yes to any of the above questions, you may want to try a low-FODMAP diet for a while.

    The same goes for individuals who have never had any digestive problems but end up constipated while transitioning to a Paleo diet.

    What makes FODMAPs problematic?

    While FODMAPs are usually well tolerated by healthy individuals, issues crop up when the body has trouble absorbing these saccharides. These carbohydrates end up being excessively fermented by bacteria in the intestines and can cause digestive mayhem in susceptible people.

    lactose low-FODMAP diet

    You see, in order to absorb FODMAPs, the body must break down (digest) disaccharides and polysaccharides into monosaccharides. If (for reasons covered below) the body is unable to digest these FODMAPs, bacteria naturally present in the intestines will gladly take over the job.

    It is important to understand that bacteria will easily, and rapidly, ferment FODMAPs since these are short-chain carbohydrates. And when these bacteria have ‘access’ to disaccharides or oligosaccharides, they will produce enzymes to break down these molecules into monosaccharides so that they can feed on them. This fermentation process produces gas.

    That’s not all: when carbohydrates ‘sit’ in the intestines, they draw water in – this is called an osmotic effect. And this effect is enhanced by the small size of the carbohydrates.

    Now, the excess gas caused by fermentation and the excess water induced by the osmotic effect distend the lumen of the intestine. This swelling triggers the symptoms of flatulence, belching, bloating, constipation and/or diarrhea, nausea, acid reflux, pain or discomfort as well as fatigue.

    The following video illustrates how FODMAPs can affect our digestion.


    How does bacterial fermentation affect motility?

    When bacteria eat our food, they produce the following gasses:

    • Methane which causes constipation by interacting with the nervous system in the intestinal wall.
    • Hydrogen which promotes diarrhea. The exact mechanism behind this is currently unknown.

    Moreover, bacteria also produce short-chain fatty acids during their ‘feast’ – these can also increase motility.

    Take home message: FODMAPs can cause problems in susceptible individuals because some bacteria are rude! They eat OUR food and then fart in our intestines… Plus they also possess a strong osmotic (drawing water) effect.

    Why would anyone react to FODMAPs?

    One of the underlying cause is carbohydrate malabsorption. This malabsorption occurs when carbohydrates bypass digestion in the small intestine and are left in the lumen (the inside space) of the small intestine or the large intestine. As explained earlier, this will cause excess gas and osmosis.

    What causes carbohydrate malabsorption?

    Issues with carbohydrate absorption can be linked to:

    • A lack of enzyme

      – That’s the case with the fructans and the galactans: unlike bacteria, we do not produce enzymes to digest these oligosaccharides. So, the bacteria will eat these oligosaccharides and produce gas.

    • Insufficient brush-border enzyme production – Brush-border enzymes refer to digestive substances that are embedded in the microvilli, finger-like projections in the intestine. A damaged intestinal lining (as in celiac disease or in small intestinal bacterial overgrowth) or genetics (such as primary lactose intolerance) can lead to a decreased production of these enzymes. For instance, individuals who do not produce enough of the enzyme lactase will react to products that contain lactose.

    • Issues with transporters

      These transporters act as ‘doors’ that allow molecules such as fructose to cross the lining of the intestines and enter the circulation (that is, they are absorbed). In some individuals, the transporters are not working normally. For instance, they may get saturated easily (the ‘door’ is too small for all the fructose to go through). Or some people may have too few transporters. Any issue with the transporters means that the fructose will not be absorbed properly and will be left stranded in the lumen of the intestine where bacteria will feast on it.

    • The size of the substance

      Polyols are too large to be effectively absorbed so they stay in the intestines where they cause an osmotic effect that results in diarrhea.

    • Very fast transit

      If for some reason, the food you eat moves very rapidly through your digestive tract, it won’t have the chance to be broken down. When this happens, the food is rapidly delivered to the large intestine making the bacteria there very happy.

    Note: It is worth noting that individuals with small intestinal bacterial overgrowth (SIBO) who also have a reduction in brush-border enzymes will often be unable to digest all disaccharides and most of the polysaccharides. However, this is not covered by the low-FODMAP diet. So, if you suspect you have SIBO, you may want to work with a qualified healthcare professional to come up with a dietary protocol that is better suited to your condition.

    P.S. Take a look at the 5 veggies that boost female metabolism and burn off lower belly fat.

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