Can You Eat Too Much Fiber? The Good and Bad

When It Comes to Gut Health, Fiber Can Be a Double-Edged Sword

• When Fiber Is Good and When It Isn’t|
• Fiber Types and Sources|
• How to Approach Fiber With SIBO or IBS|
• Increase Your Fiber Tolerance|
• Heal Your Gut|

What happens if you eat too much fiber? It all depends on the state of your gut and what “too much” means for you. If you have a healthy gut — meaning you digest food well, you don’t regularly experience symptoms like gas, bloating, constipation, or diarrhea, and you feel energetic — then you can probably handle a higher amount of fiber in your diet. 

But if you do currently experience gut symptoms, which often come from conditions like IBS and SIBO, then you may only be able to handle small amounts of dietary fiber right now. And that’s okay. 

The good news is that you can improve your gut health and train your gut to process larger amounts of fiber over time. We see this often at our clinic with SIBO and IBS patients as their gut health improves. 

There are a number of factors that influence the answer to: Can you eat too much fiber? In this article we’ll discuss when fiber is good and when it can be harmful, different types and sources of fiber, how to approach fiber when you have IBS or SIBO, and how to increase your fiber tolerance. 

When Fiber Is Good and When It Isn’t

You typically only hear about fiber as a magical, health-promoting substance that we all need more of every day. And when you have a healthy gut, fiber is great. It helps push waste through the digestive system, binding and sweeping out toxins and the metabolic junk left behind after you absorb the nutrients you need from your food. 

But there are cases when fiber can actually be harmful to us, primarily when the gut can’t move it along the small and large intestine (using peristalsis, involuntary muscle contractions along the colon that push waste through and eventually cause you to have a bowel movement). In this case, you can eat too much fiber, and even small amounts may cause problems.

When the gut isn’t moving properly, any kind of fiber will become trapped and typically ferment in the gut, causing symptoms like gas and bloating. This quickly leads to constipation and reabsorption of the toxins and water that are now sitting in the colon, making it even harder to have a bowel movement without some outside assistance (magnesium, aloe, cascara sagrada, or manmade laxatives like polyethylene glycol). 

Fiber can also be irritating to the gut lining for those who have an inflamed colon, such as inflammatory bowel disease patients (Crohn’s disease and ulcerative colitis). If you want to introduce more fiber into a healthy diet to help with inflammatory bowel disease, you should do so very slowly and ideally under the guidance of a practitioner. 

Signs You May Be Eating Too Much Fiber 

You know you’re eating too much fiber when [1, 2, 3, 4]:

• You experience symptoms like gas, bloating, abdominal pain/stomach pain, cramping, heartburn, or nausea after eating fibrous foods
• You’re constipated (less than one BM per day)
• You feel a sense of heaviness in your abdomen after or between meals
• You experience an increase in symptoms if you have IBS or IBD

These signs mean that you probably need to do a little work on your gut health before you can add in more daily fiber to your diet. 

Scientific literature backs up how too much fiber can become problematic in some individuals. Two randomized controlled trials found that switching from a typical low-fiber American diet to a high-fiber diet significantly increased bloating in people with elevated blood pressure [5, 6].

Another study found that increased fiber intake led to increased retention of gas in healthy people [7].

Additionally, one literature review found that a high-fiber diet can worsen constipation and IBS symptoms, and that a low-fiber diet may be beneficial for certain GI conditions such as IBD, diverticulitis, bowel obstruction, and infectious GI disease [2].

Fiber Types and Sources

You’ve probably heard about two main types of fiber: soluble and insoluble. 

Soluble fiber absorbs water, while insoluble fiber does not. They each have their benefits and uses for gut health, but many people find that soluble fiber is easier to digest, while insoluble fiber can be more difficult. 

For example, a systematic review and meta-analysis of 14 studies wanted to see if fiber was effective for improving IBS symptoms. A total of 906 patients were included in the analysis and the results showed that soluble fiber significantly improved IBS symptoms, but insoluble fiber (in this study, bran was used) did not [8].

However, most foods contain some combination of both soluble and insoluble fiber. In addition to soluble and insoluble fiber, we can also categorize fiber by:

• Fermentation ability
• Viscosity or non-viscosity

Fermentable fiber is used by our gut bacteria to produce metabolites like short-chain fatty acids, which are health promoting. But, the fermentation process can also create gas, causing bloating. These are the types of fibers we avoid when following a low FODMAP diet for SIBO or IBS (see more on this below). 

Viscosity refers to fiber that turns into a gel when mixed with water. These include fibers like beta-glucans found in oats, guar gum, pectin (think applesauce), and psyllium. 

Fiber Foods

Some foods high in soluble fiber include [9, 10]:

• Legumes
• Pulses
• Nuts
• Seeds
• Wheat
• Rye
• Onions
• Garlic 
• Artichoke
• Kashi
• Millet
• Oats
• Cooked and cooled pasta
• Potatoes
• Rice

Foods high in insoluble fiber include: 

• Wheat bran
• Whole-grain cereal
• Rye
• Brown rice
• Whole-meal pasta
• Potatoes
• Quinoa
• Flaxseed
• Nuts 
• Seeds

Some foods, like whole-grain pasta, nuts, seeds, and many fruits and vegetables, overlap both categories. If you have trouble with large amounts of fiber, focus on small amounts of the foods from the soluble category first. Soluble fiber is more likely to improve IBS symptoms as well as produce beneficial short-chain fatty acids [8].

Can You Eat Too Much Fiber? How to Approach It With SIBO or IBS

You can consume fiber with SIBO or IBS, but some types of fiber may be easier to digest than others. When you’re first starting on a diet for SIBO or IBS, you may want to restrict most fiber, especially fermentable fibers, temporarily while your microbiome adjusts [11].

Some people with digestive problems find that a low FODMAP diet is good for this adjustment period. Once your symptoms have calmed down (and possibly you’ve added in probiotics and antimicrobials) then you can slowly begin trying more fiber-rich foods. 

Low FODMAP Diet

A low FODMAP diet restricts certain fermentable fibers that may contribute to gas and bloating for people with IBS and SIBO. 

A randomized controlled trial involving 37 IBS patients (19 low FODMAP, 18 high FODMAP) found that three weeks of a low FODMAP diet was associated with a significant 27% decrease (improvement) in IBS symptom scores compared to a high FODMAP diet, which had a 7% increase in symptoms [12].

Another study involving 15 IBS patients and 15 healthy subjects found that two days of a low FODMAP diet was associated with significantly lower breath hydrogen levels and fewer gastrointestinal symptoms and lethargy than a high FODMAP diet. All symptoms were significantly worse with the high FODMAP diet in IBS patients [13].

Psyllium

Psyllium fiber is the holy grail for those with sensitive guts; it’s soluble, viscous/gel-forming, and poorly fermented, meaning that it results in less gas formation and can even be soothing to the digestive tract because of the gel consistency [14]. Popular fiber supplements like Metamucil use psyllium powder in their formulation. 

Psyllium is great for those who need more fiber to address constipation or diarrhea, but can’t tolerate more fermentable fibers (such as beans, lentils, and whole grains). Psyllium also serves as a low-grade binder, so you should take it separate from medications and supplements. 

In a 2021 literature review on the role of fiber in gastrointestinal health and disease, psyllium was the only fiber recommended for IBS-C, IBS-D, and functional constipation (but not for IBD and diverticulitis) [15].

If you want to try psyllium, you can start with as little as half a teaspoon mixed in 8 ounces of water. It’s easiest to add the psyllium powder and water to a jar and shake it so that it gets well mixed. Drink immediately. If you find a benefit from taking it, you can increase your psyllium dose over several weeks up to 15 grams (about 3 teaspoons) [15].

Resistant Starches

If you have a healthy gut, resistant starches are another type of fiber you may want to prioritize in your diet [9]. However, resistant starches are easily fermented, so not the best for those with gut issues. But this type of fiber is semi-responsible for creating beneficial short-chain fatty acids (SCFAs) when broken down by your gut bacteria.

One of these SCFAs is butyrate, the energy source for colonocytes (the cells lining the colon). Butyrate also has systemic benefits for brain health and lowering inflammation.

Examples of resistant starches include: 

• Cooked and cooled oats, rice, beans, and potatoes
• Plantain flour
• Whole grains
• Apples
• Asparagus 

Helping our microbiome make SCFAs is one reason fiber may have systemic health benefits, including reduced risk of stroke, gastrointestinal disorders, obesity, and depression. 

How to Increase Your Fiber Tolerance

Fiber is best increased slowly over time, and you may find your tolerance lowered during periods of stress or illness. Everyone’s capacity to handle fiber is also a little different, depending on things like your sex and ethnicity [5, 6]. In general, the recommended dietary intake in grams of fiber per day is [16]:

• Men: 30-38 g/day
• Women: 21-26 g/day
• Children: 19-25 g/day

You may need more or less depending on your gut health and what makes you feel good. If you’re starting to increase your fiber tolerance from a low FODMAP diet, reintroduce only one food at a time and see how you respond. If your symptoms return, go back to low FODMAP for a day or two and reintroduce a different food. The idea is that you’re giving your gut and your microbiome time to adjust and break down the new food before adding another. 

You can also experiment with different methods of cooking to make certain fibers easier to digest. For example, soak beans and legumes in water overnight and cook on the stove for two to three hours to break down the starch. Eating cooled legumes and beans may also make them more tolerable. 

Motility Is Essential to Handle More Fiber

One thing you have to make sure is in place before attempting a greater fiber intake is good gut motility. Without good motility, you end up with constipation and bloating with every food you eat (common in SIBO). 

One reason for low motility is a lazy or low-functioning vagus nerve. The vagus nerve is the longest nerve in the body, enervating many organ systems including the heart, lungs, and digestive system. Low vagal tone (a measurement of how well the vagus nerve is functioning) has been observed in conditions like IBS, SIBO, and functional constipation. The vagus nerve needs to be active to move fiber along the digestive tract.

The vagus nerve is extremely sensitive to stress, so one way to help it function better is to lower your stress. Deep-breathing techniques, exercise, and practicing enjoyable hobbies are all ways to lower stress. But stress management may require a deeper look at your habits, diet, and trauma patterning to make lasting changes. 

Heal Your Gut to Handle More Fiber

To tolerate a higher amount of fiber in your diet, good gut health is essential. Our microbiome ferments certain kinds of fiber, producing gas alongside beneficial metabolites like SCFAs. With conditions like IBS and SIBO, too much fiber can exacerbate symptoms. 

However, fiber is an undeniable tool for health when used correctly, and has implications for managing heart disease, weight loss, and blood sugar levels. To improve your digestive system’s ability to handle fiber, make sure to: 

1. Go low and slow; start with a very small amount and increase over several weeks.
2. Make sure you’re drinking plenty of water alongside the added fiber in your diet.
3. Get your gut moving (good motility).
4. If you encounter digestive problems after eating certain fibrous foods, back off and reintroduce them later when your gut is more resilient. 

You can learn more about fiber and its potential effects on gut health in my book, Healthy Gut Healthy You. For more personalized guidance, reach out to our functional medicine clinic.

The Ruscio Institute has developed a range of high-quality formulations to help our patients and audience. If you’re interested in learning more about these products, please click here. Note that there are many other options available, and we encourage you to research which products may be right for you.