Fermentable Foods: A Practical Outline on Benefits, FODMAPs, and Gut Health

When we talk about foods that support gut health, we often hear about probiotics, fermented foods (used for thousands of years to preserve food) and fibre. But there is another important term to understand: fermentable foods.

Unlike fermented foods, which have already been transformed by microorganisms before we eat them, fermentable foods contain carbohydrates and other compounds that can be broken down, or fermented, by microorganisms living in the gut.

This process is a normal and important part of digestion and can produce beneficial compounds such as short-chain fatty acids, which play a role in maintaining the health of the gut and supporting the wider body.

However, not everyone responds to fermentable foods in the same way. Some types of fermentable carbohydrate, particularly FODMAPs (more on that later), can cause digestive discomfort and symptoms such as bloating, gas and abdominal discomfort in susceptible people. Particularly those with Irritable Bowel Syndrome or SIBO.

Understanding which foods are fermentable, how gut bacteria process them, and why they can have different effects on different people can help put dietary choices into perspective. This article looks at the potential benefits of fermentable foods, the role of FODMAPs, and what the science tells us about their relationship with gut health.

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What are Fermentable Foods?

Fermentable foods are foods that contain carbohydrates that can be broken down by the bacteria living in your gut.

This fermentation process is a normal part of having a healthy gut, and it is a metabolic process in which microorganisms convert carbohydrates into products such as organic acids or carbon dioxide, releasing energy and beneficial short-chain fatty acids that have several important roles in the body.

Importantly, fermentable doesn't mean unhealthy. Many types of dietary fibre are fermentable, and feeding your gut bacteria with these fibres can support a healthy and diverse gut microbiome.

Fermented Foods vs Fermentable Foods

Although the words sound similar, fermented foods and fermentable foods are not the same thing. Fermented foods have been transformed by microorganisms such as bacteria or yeast, often as a form of food preservation. Examples include yoghurt, kefir, sauerkraut and kimchi.

Fermentable foods, on the other hand, contain carbohydrates or fibre types that can be broken down and fermented by bacteria in your gut. These include certain fruits, vegetables, legumes and wheat products. Both fermented and fermentable foods can have a place in a healthy diet, but they affect your body in different ways.

What are FODMAPs?

You'll often hear people saying that fermentable foods cause gastrointestinal problems. That's an oversimplification and not always true. The confusion often arises because some fermentable carbohydrates can cause digestive symptoms.

These are known as FODMAPs, a certain group of fermentable short chain carbohydrates that are difficult to absorb. FODMAP stands for Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols. A bit of a mouthful. Now you can see why they shorten it to FODMAP, can't you?!

When you eat these FODMAPs, they pass through the small intestine without being fully absorbed, ending up in the large intestine to be fermented by bacteria.

Unfortunately, this process can produce gas, bloating, abdominal pain, abdominal distension and changes in bowel habits in people who are sensitive to them, such as anyone suffering from IBS or SIBO. (1)

SUMMARY: So, while all FODMAPs are fermentable, not all fermentable foods are FODMAPs. Many fermentable fibres and other carbohydrates can be beneficial for gut health, while certain FODMAPs may cause problems for people with digestive sensitivities.

In other words, fermentation itself isn't the problem. It's the type and amount of carbohydrate being fermented, along with how an individual's digestive system responds to it, that determines whether it is beneficial, neutral or uncomfortable.

A Word about Gut Health and Gut Microbiota (Gut Bacteria)

Before we talk more about the potential benefits - and drawbacks - of fermentable foods in more detail, we should make sure we know what the gut microbiome is and why it's so important.

You may be surprised to learn that there are trillions of bacteria living in your gastrointestinal tract (GI tract). This microscopic ecosystem can weigh up to 2kg and is crammed with bacteria, viruses and fungi that all play a role in our health. A diverse balanced microbiome offers many health benefits, while a lack of diversity can cause potential health issues. (2)

A healthy gut microbiome, for instance, is heavily involved in keeping our immune system working well, reducing chronic inflammation, and potentially helping with weight loss and gain as well as overall well-being. Microbial fermentation may also improve the bioavailability of key nutrients. (3)

Let's look in more detail at how our gut bacteria can influence our health. For instance:

• Excessive 'bad bacteria' and disruptions in the gut microbiota have been linked to gastrointestinal symptoms and conditions, such as irritable bowel syndrome and inflammatory bowel disease. (4)

• Research tells us that type 2 diabetes, obesity and metabolic diseases may also have a link to gut bacteria. (5)

• Around 70% of our immune cells live in our gut. These cells interact with the gut microbiota, meaning that our diet and lifestyle (which influence the gut microbes) also directly influence our immune system. Consumption of fermented foods has also been linked to reduced inflammation. (6)

• There is evidence that the gut also 'talks' to the brain and other organs. The gut-brain axis, for example, suggests that the state of our gut microbiome may influence stress, anxiety and depression. (7)

• When the microbes in our gut ferment fiber, they often product short chain fatty acids and more which have been shown to influence blood pressure and cholesterol. (8, 9)

• An imbalance in our gut bacteria means you may be less resistant to pathogens such as E.coli, C.difficile, Salmonella and more. (10, 11)

Needless to say then anything we can do to improve our gut health and encourage bacterial diversity is likely to be beneficial. That's where fermentable foods may come in...

Health Benefits of Fermentable Foods

So, where do fermentable foods fit into all this? Many types of dietary fiber and other fermentable carbohydrates provide food for the bacteria living in our gut. When these bacteria ferment them, they produce substances called short-chain fatty acids (SCFAs), including acetate, propionate and butyrate.

These compounds have a range of important roles in the body. For example, butyrate provides energy for cells lining the colon, while SCFAs can influence the gut barrier, immune function and metabolism. (12)

Fermentable fiber can help support a diverse and active gut microbiome by providing bacteria with the fuel they need to grow and function. Some fermentable fibres are also prebiotics, meaning they can be selectively used by beneficial gut microorganisms and support gut health

For most people, therefore, eating fiber-rich fermentable foods can be an important way of supporting a healthy gut microbiome. If you struggle to consume enough, you may want to consider a prebiotic supplement.

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Human research has found that daily inulin supplementation can significantly increase Bifidobacterium levels, although the size of the response varies between individuals and depends partly on their starting levels. (13)

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What About FODMAP?

I mentioned FODMAP earlier, didn't I, and promised to explain it. So how does it apply to fermentable foods?

Well, as I also said earlier, not every fermentable food will benefit everyone in the same way. Different types of fibre are fermented differently, and some fermentable carbohydrates can cause digestive symptoms in people who are sensitive to them.

That may particularly apply to FODMAPs - Fermentable Oligosaccharides, Disaccharides, Monosaccharides and Polyols, poorly absorbed sugars or carbohydrates found within fruits, vegetables, grains, dairy products and more.

Because they're poorly absorbed, they draw water into the intestine, which is why a diet high in FODMAPS may cause digestive issues for some people. That's especially true for anyone suffering from conditions such as IBS or SIBO, among other gastrointestinal disorders and functional gastrointestinal disorders.

Let's familiarize ourselves with these...

Irritable Bowel Syndrome IBS

IBS (Irritable Bowel Syndrome) affects approximately 10-15% of the population, two-thirds of whom are women. It is the most common gastrointestinal disorder in the world. Symptoms include cramping, bloating, and changes to bowel habits. Up to 80% of people with IBS may benefit from a low FODMAP diet. Up to 40% of IBS sufferers are also thought to have SIBO. (14 - 16)

Small Intestinal Bacterial Overgrowth (SIBO)

SIBO (Small Intestinal Bacterial Overgrowth) involves an abnormal amount of bacteria in the small bowel or intestine, often caused by something slowing the passage of food and waste through the gut. This creates a breeding ground for bacteria not normally present in the small intestine, causing malnutrition, weight loss and diarrhea. (17)

Often people suffering with these conditions are recommended to follow a low FODMAP diet.

What is a Low FODMAP diet?

Before I talk about the low FODMAP diet, I should point out that entire articles, even books, have been written on this subject and I can't possibly go into it in such detail here.

However, if you're new to FODMAP, it's my hope that this gives you a basic understanding of the issues surrounding fermentable foods and FODMAP for some.

In essence, the Low FODMAP diet is a way to reduce fermentable carbohydrates and, in some versions, limit high-fiber foods to reduce gut symptoms. It's short-term and highly restrictive, designed to weed out troublesome foods, tailored to your own body. You may also hear of the Low Fermentation Diet or the Cedars-Sinai diet, named after researchers at Cedars-Sinai. They're similar but not exactly the same, and both aim to lower bacterial fermentation in the gut.

More: The Essential Guide to the Low Fermentation Diet for Digestive Health

In the low FODMAP diet, foods are evaluated according to their FODMAP content. Classed as an elimination diet, it has three phases.

1. The elimination phase, where you reduce or stop eating high FODMAP foods for 2-6 weeks. This will give you a chance to reduce any symptoms and decrease abnormally high levels of microbes.

2. Reintroduction phase, where you systematically add a high FODMAP food back into your diet every three days, keeping a close eye on how it makes you feel.

3. Maintenance phase or personalization, where you keep what works for you and scrap what doesn't. Identifying your triggers so you can personalize intake rather than permanently avoid whole food groups will help make the approach more sustainable. Using a food diary will help with this. (18, 19)

Given how restrictive the FODMAP diet is, you shouldn't follow the elimination phase long-term as it may cause damage to your gut health. This is a restrictive diet used short-term, not a permanent diet. It's also advisable to work with a dietician or speak to your doctor before starting. It is designed specifically for people suffering with IBS or SIBO, and everyone may have different triggers. It should not be used for weight loss.

High FODMAP Foods

High FODMAP foods include:

• Some dairy foods are high in lactose, such as ice cream, milk, yogurt, and evaporated milk, while aged cheeses are naturally lower in lactose.

• Wheat flour products such as bread, crackers and cereal, with wheat bran being a concentrated source of fermentable carbohydrates.

• Certain vegetables, such as onions, garlic, asparagus and artichokes.

• Lentils and beans and lentils.

• Certain fruits such as peaches, apples, pears and cherries.

• Some fruits, such as apples, cherries, pears, and peaches.

• Some processed foods, certain breakfast cereals, and products with artificial sweeteners can also be high FODMAP depending on their ingredients.

Low FODMAP Foods

Low FODMAP foods should make up the base of your diet. They include:

• Meat, eggs and some dairy foods, including aged cheeses such as Brie, Parmesan, and cheddar, which are lower in lactose

• Grains such as oats, rice and quinoa

• Certain vegetables, such as zucchini, cucumbers, tomatoes, eggplants, carrots, turnips and potatoes

• Low FODMAP fruits such as kiwi fruit, pineapple and oranges, though portion size still matters

• Almond milk

• Maple syrup as a sweetener option in moderation

Note that this is a small list; the full list will be much more comprehensive. Swapping options within the same food group can help maintain variety and balance, and choosing alternatives from the same food group may also make symptoms easier to manage. Your doctor or nutritionist should be able to provide you with a full list. (18, 19)

Other Elements of the FODMAP Diet

There are other aspects to the FODMAP diet that are as important as the food you choose. For instance, you should eat according to recommended serving sizes. Even foods low in FODMAP may cause issues if eaten in high quantities. With fruit, it is often best to have only one portion at a time to avoid triggering symptoms.

You should also space out your meals so they are 3-4 hours apart; this allows your intestine to 'clean up' and remove waste between meals, leaving less behind to encourage fermentation. Studies tell us, for instance, that frequent snacking correlates with IBS severity symptoms. (20)

Add a Quality Multivitamin to Your Diet

If you're following a low FODMAP diet, you may have eliminated certain food groups from your diet that are high in nutrients. Given how restrictive it can be, nutritional deficiencies can occur if the diet is not well planned, and nutrients such as B vitamins may need extra attention when intake is limited. You may want to add a quality multivitamin to your daily routine to make up any shortfalls.

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Final Thoughts

Fermentable foods are neither inherently good nor bad. For most people, fermentable fibre can provide valuable fuel for the gut microbiome and support the production of beneficial compounds such as short-chain fatty acids.

However, if you experience bloating, gas or other digestive symptoms, particularly if you have IBS or SIBO, certain fermentable carbohydrates may be harder to tolerate. FODMAPs are an important example.

The key is understanding that individual tolerance matters. Rather than avoiding fermentable foods altogether, paying attention to the types and amounts you eat can help you find an approach that supports your gut without causing unnecessary discomfort.

Frequently Asked Questions

What are fermentable foods?

Fermentable foods contain carbohydrates and other compounds that can be broken down by microorganisms in the gut. Many types of dietary fibre are fermentable and can support the gut microbiome.

Are fermentable foods the same as FODMAPs?

No. All FODMAPs are fermentable, but not all fermentable foods are FODMAPs. Many fermentable fibres can support gut health, while some FODMAPs may cause bloating, gas and other digestive symptoms in people who are sensitive to them.

Should I avoid fermentable foods if I have IBS?

Not necessarily. Some fermentable carbohydrates, particularly certain FODMAPs, may trigger gut symptoms such as bloating, abdominal pain, or altered bowel habits in people with IBS, but individual tolerance varies. A low-FODMAP diet can help identify personal triggers, but it is intended as a short-term elimination and reintroduction approach rather than a diet to follow indefinitely. Any strict low FODMAP diet should be short term and ideally guided by a dietitian rather than used indefinitely.