The word FODMAP is an acronym used to describe rapidly fermentable foods that often trigger uncomfortable gut symptoms in people with IBS. Oligosaccharides (FOS or GOS) are one group of foods in the FODMAP family, the “O” in FODMAP.
As a registered dietitian whose specialty is the low FODMAP diet for the management of IBS, I know how confusing this diet can be! In this article, I will explain what oligosaccharides are and why they matter for people with IBS. I will give you examples of foods that contain them, and discuss how you can eat them without flaring up symptoms.
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.
Putting the Greek terminology aside, the key point is that humans cannot digest oligosaccharides in their original form. We must first break them down into individual sugar units like glucose, fructose, or galactose.
Meet FOS and GOS – The Two Main Oligosaccharides in FODMAP
There are two main types of oligosaccharides (the “O” in FODMAP): FOS (fructo-oligosaccharides or fructans) and GOS (galacto-oligosaccharides or galactans).
FOS are chains of fructose molecules linked together, usually with a glucose molecule at one end. They are found in garlic and onions, several fruits and vegetables, and grains like wheat.
GOS are chains of galactose molecules with a glucose molecule at the end. They are mostly found in legumes (beans, dried peas, and lentils), nuts, and seeds.
Why Do FOS and GOS Trigger IBS Symptoms?
As we don’t have the enzymes needed to break down oligosaccharides into single sugars, these chains pass undigested through the GI tract. When they reach the large intestine (or colon), our gut bacteria take over.
They do have the enzymes to break the sugar bonds and ferment the oligosaccharides for energy, producing gas in the process. This causes only minimal discomfort in healthy individuals. However, people with IBS, who have more sensitive guts, may experience bloating, abdominal pain, and changes in bowel habits like diarrhea or constipation.
Common Foods High in FOS and GOS
Here are just a few examples of foods that are high in oligosaccharides (FOS and GOS). For a more complete list, check out the Monash FODMAP App. Monash University regularly tests foods and keeps the app up to date.
In addition, FOS and GOS can often be hidden in packaged foods such as:
- Protein powders made with pea protein, soy protein, or legume flours (chickpea, lentil, or pea flours)
- Gluten-free crackers and cookies made with almond meal or legume flours
- Spice blends or processed meats like sausages containing garlic or onion powders
- Protein bars, granola, and plant-based yogurt that have chicory root fiber, also known as inulin, which is often added for additional fiber.
Navigating Oligosaccharides (FOS & GOS) on a Low FODMAP Diet
So, what does this all mean for someone with IBS? Do you need to avoid oligosaccharides? Not quite.
You need to avoid these foods only while you are following the elimination phase of the low FODMAP diet.
But this is just temporary: this phase usually lasts 2-6 weeks. When you have an expert dietitian to guide you, the low FODMAP diet can help you feel better in just a few weeks.
Tips for Including FOS and GOS Without the Bloat
While you are following a low FODMAP diet, you can still eat some legumes, nuts, and seeds, and wheat, and enjoy the flavor of garlic and onions without discomfort. Here are my best tips:
- Eat smaller portions of legumes: canned, rinsed chickpeas, cannellini beans, or lentils in small amounts (¼-½ cup); small portions of peanut butter (about 2 tablespoons).
- Include small portions of nuts and seeds: walnuts, macadamia nuts, pecans, chia seeds, pumpkin, and sunflower seeds.
- Use authentically-made sourdough bread for sandwiches and toast (up to 2 slices).
- Try infused oils: garlic or onion-infused olive oil to flavor your dishes or use the green tips of spring onions and leeks. You can find more tips in this article: Low FODMAP Garlic & Onion Substitutes.
- Drink weak black tea as opposed to strong tea (tip: infuse your tea for no more than 2-3 minutes).
Why You Shouldn’t Cut Oligosaccharides Out Forever
It’s important you include FOS and GOS in your diet as they feed the good bacteria in your gut. Your gut microbes play an important role in digestion, immune health, and even your mood. Oligosaccharides help keep them happy and thriving. If you avoid them for too long, it can reduce the diversity of your gut bacteria, which isn’t great for your long-term gut health.
That’s why it is so important to reintroduce FOS and GOS (and all the other FODMAPs) after the elimination phase of the low FODMAP diet, and identify those that trigger your gut symptoms. Slowly adding oligosaccharides back into your diet will help you figure out how much your body can handle.
You can learn more about reintroducing FODMAPs in this article: How the FODMAP Diet Works – Step 2: Reintroducing FODMAPs.
That’s A Wrap!
Oligosaccharides—specifically FOS and GOS—are types of fermentable carbohydrates found in foods like legumes, certain grains, vegetables, nuts, and seeds. While they’re an important source of fuel for your gut bacteria, they can trigger digestive symptoms in people with IBS.
Understanding which foods contain FOS and GOS, and how to manage portion sizes or use low FODMAP alternatives, can help you reduce symptoms without cutting them out completely. With the right guidance, you can reintroduce these foods while maintaining symptom relief.
Working with a dietitian like me can help you figure out which foods – oligosaccharides or other FODMAPs – are triggering your gut symptoms. If you find that certain foods don’t sit well with you, I can help you explore enzyme options that may make it easier to enjoy them again without the uncomfortable side effects.
If you want to learn more about how I can help you find relief from bloating, abdominal pain, and unpredictable bowel movements, reintroduce more foods into your diet, and feel good at the same time, book your complimentary IBS Clarity Session.
We will review your symptoms and history and discuss the best next steps for you! You will come out of this call feeling relieved and hopeful that you can take control of your IBS.
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Book a free IBS Clarity Call today!
Published March 12, 2019. Updated, June 9, 2025.
Antonella Dewell, MS, RDN | IBS Resolution Expert
Antonella Dewell is a registered dietitian and IBS specialist based in California. She has over 2 decades of experience working in clinical research at Stanford. Since 2018, she has been running a private practice. She helps people with IBS gain real, lasting relief from IBS symptoms in just 2-4 weeks without giving up all the foods they love, so they can stop feeling anxious around food and get back to fully enjoying food and life again.
