FODMAP Explained: “O” Stands for Oligosaccharides

Published on: 03/12/2019

Welcome back to my “FODMAP Explained” Series! This is a set of articles I have dedicated to explaining the acronym FODMAP and giving practical tips to avoid symptoms while including as wide a variety of foods as possible. In this blog post, I will discuss the “O” in FODMAP, which stands for “Oligosaccharides”(GOS and FOS). Let’s find out what they are and how they fit into the low FODMAP diet.

What are oligosaccharides

Oligosaccharides are short-chain carbohydrates (or sugars) that can have 3 to 10 single sugar units. In Greek, oligos means “a few” and saccharide means sugar. In contrast, polysaccharides are long-chain carbohydrates – poly means “many” – that have more than 10 sugar units. Greek aside, what is important to remember is that we cannot digest oligosaccharides as they are. We need to break them down into their single sugar units (glucose, fructose, or galactose).

Humans, unfortunately, don’t have the enzymes that are able to accomplish this task. Therefore, oligosaccharides move through the GI tract undigested. When they reach the large intestine, they meet our good bacteria. And – you probably guessed it – they do have those enzymes that break down the bonds between sugar units. In fact, they use them for energy (they are their fast food!). Whereas healthy people only have a bit of gas as part of this normal digestive process, people with IBS have a hypersensitive gut. This fermentation process leads to excess gas, bloating, abdominal pain and perhaps altered motility (constipation or diarrhea).

The two main oligosaccharides (GOS and FOS)

There are two main oligosaccharides in our diet: galacto-oligosaccharides (GOS) and fructo-oligosaccharides (FOS), or “fructans”(including inulin, a longer chain of fructose molecules).

First of all, you need to avoid or limit the foods that contain a high amount of GOS or FOS only during the elimination phase of the low FODMAP diet. Some examples are:

Galacto-oligosaccharides (GOS)

  • Nuts (cashews, pistachios, high amounts of other nuts)
  • Legumes (many beans and lentils, soy milk)
  • Some vegetables (peas, butternut squash)

Fructo-oligosaccharides (FOS)

  • Grains and cereals (wheat bread and pasta, couscous, amaranth)
  • Some vegetables (garlic, onions, artichokes, Brussels sprouts)
  • Some fruits (dried fruit, persimmon, pomegranate)
  • Some beverages (strong black tea, chamomile tea, kombucha)

In addition, some foods have both FODMAPs (GOS and FOS). Black beans, cashews, barley, and beets, just to name a few.

Bottom line: what can you eat?

To sum it all up, if you have IBS and are trying to alleviate your symptoms, you don’t need to avoid oligosaccharides altogether: they can have beneficial effects. They act as prebiotics and encourage the growth of our good bacteria. Moreover, they are rich in fiber and many nutrients. Here are a few tips:

  • Avoid foods that have the highest amounts of GOS and FOS (see examples above). Check out more foods on the Monash University FODMAP Diet App).
  • Have small amounts of low-FODMAP nuts (almonds, hazelnuts), seeds (chia seeds, sunflower seeds), and canned chickpeas or lentils.
  • Choose gluten-free grains such as brown rice, millet, quinoa, and corn tortillas and small amounts of sourdough bread
  • Eat small amounts of fruits (½ cup per meal or snack is a good rule of thumb).
  • Eat mostly low-FODMAP vegetables and load up on those that have only trace amounts or none at all. Some examples are kale, carrots, and red bell peppers.
  • Drink weakly brewed black tea or herbal teas such as ginger and peppermint (these are all great to soothe a tummy ache).

Most importantly, stay on the elimination phase of the low FODMAP diet as little as possible! You need to stay on this phase only until your symptoms have mostly subsided. Afterward, move on to reintroducing FODMAPs with the help of an expert dietitian who can guide you and help you find out which foods you can safely re-introduce into your diet. At the end of this process, most people find they can reintroduce many of these foods, including GOS and FOS, and enjoy a more nutritious and varied diet.

Want to learn more?

Check out more Low FODMAP Diet Basics


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I’m a registered dietitian with a passion for helping women with IBS find their way back to eating without fear of painful gut symptoms and without unnecessary diet restrictions.

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