Diet Tips:What are the Oligos?

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Dr Jaci Barrett (Accredited Practising Dietitian)

You may have noticed the ‘oligo’ category of FODMAPs within the Monash University low FODMAP diet app. It combines two common oligosaccharides that are poorly absorbed and can trigger IBS symptoms, the fructans and galacto-oligosaccharides (GOS).

We have had several requests from the public asking us to separate these FODMAPs, so we wanted to take the opportunity to explain why they are grouped together.

Fructans are chains of fructose sugars joined together with glucose at the end.

In order to absorb fructans, we need to break the sugars down into single sugars (monosaccharides). But, we can’t. Humans do not produce any enzymes that can break down the bonds between these sugars, so the fructans move through the gut completely unabsorbed. 

When the fructans meet bacteria in the large intestine, they are fermented which produces gases. In healthy people this causes a bit of wind and is part of normal, healthy digestion. In IBS, where the gut is hypersensitive and motility disturbances are common, this results in bloating, abdominal discomfort and altered motility.

The same occurs with GOS. GOS are chains of galactose sugars joined together with glucose at the end.

Again there is no human enzyme capable of breaking down the bonds between the galactose sugars, so they move through the gut unabsorbed. Poor absorption of GOS leads to symptoms in IBS patients.

So these oligosaccharides (oligos) always pass through the gut and escape digestion. They have beneficial effects whereby they encourage the growth of good bacteria and act as prebiotics, but if you have IBS you might notice symptoms after ingesting foods rich in the oligos.

We list them together on the app because they have the same activity in the gut, but it is common that not all oligo-containing foods are triggers for everyone. This is not due to the type of oligos – fructans vs GOS – rather it is due to your individual response to different high FODMAP foods. You may find you can eat garlic but not onion, for example, both of which are high in fructans. Other people find they can eat chickpeas, but not red kidney beans, both high in GOS. 

The final long term low FODMAP diet is a modified version of the diet, individualised for you based on your reactions to food reintroductions. Keep trying new foods. Within each FODMAP subgroup you may tolerate some foods, but not others.

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