Does Fiber Relieve or Cause Constipation? A Critical Look

Bowl of Oatmeal in a Blue BowlConstipation is a common problem that affects up to 20% of people every year (1, 2).

It’s a difficult condition to define, as bathroom habits vary considerably from person to person.

However, if you have less than three bowel movements a week and your stools are hard, dry and difficult to pass, you’re likely constipated.

One of the most common pieces of advice for people who are constipated is to eat more fiber.

But does this advice actually work? Let’s have a look.

Fiber Is Generally Good for Digestion

Dietary fiber is the name given to the non-digestible carbohydrates in plants. It can be found in all plant foods, including fruits, vegetables, grains, nuts and seeds.

It’s usually categorized into two groups, based on solubility:

  • Insoluble fiber: Found in wheat bran, vegetables and whole grains.
  • Soluble fiber: Found in oat bran, nuts, seeds, beans, lentils and peas, as well as some fruits and vegetables.

That said, most fiber-rich foods contain a mixture of insoluble and soluble fiber in varying proportions.

Even though your body can’t digest fiber, eating enough of it is thought to be very important for your gut health. This is partly because dietary fiber increases the size of your stools and makes them softer.

Larger, softer stools help keep you regular, as they move more quickly through your bowels and are easier to pass (3).

These two types of fiber help with this in slightly different ways.

Insoluble fiber bulks up your stool and acts like a brush, sweeping through your bowels to get everything out and keep things moving.

The soluble variety absorbs water and forms a gel-like substance. This helps your stool pass smoothly through your bowels and improves its form and consistency.

The fermentation of one type of soluble fiber, known as prebiotics, in the large intestine can also help maintain a healthy gut by increasing its number of good bacteria (4).

This could also improve your health by decreasing your risk of type 2 diabetes, heart disease and obesity (5).

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Bottom Line: Eating enough fiber can help keep you regular. It can also improve the balance of good bacteria in your gut. This may reduce your risk of various diseases, such as heart disease, obesity and diabetes.

It Can Relieve Constipation for Many People

Scoop of Flaxseeds

If you’re constipated and have a low fiber intake, eating more of it could help.

Studies have shown that increasing the amount of fiber you eat could increase the number of stools that you pass (6).

In fact, a recent review showed that 77% of people with chronic constipation found some relief by increasing their fiber intake (7).

Moreover, two studies have found that increasing dietary fiber intake could be as effective as the laxative lactulose for relieving constipation in children (8, 9).

This means that for many people with constipation, simply eating more fiber could be enough to fix the problem (10, 11).

It is generally recommended that men eat 38 grams of fiber per day, and that women eat 25 grams (12).

Unfortunately, it’s estimated that most people eat less than half of this amount, only reaching between 12–18 grams per day (3, 13, 14).

Bottom Line: Most people don’t eat enough dietary fiber. Those who lack fiber in their diet may experience relief by increasing their intake.

In Some Cases, Eating More Fiber Makes Constipation Worse

In theory, fiber should help prevent and treat constipation.

However, the evidence shows that this advice does not work for everyone.

While some studies show that adding fiber to your diet may improve your symptoms, other studies show that reducing your intake is best (15).

Also, a recent review found that although fiber was effective at increasing the number of bowel movements, it didn’t help with other symptoms of constipation like stool consistency, pain, bloating and gas (6).

Granola in a Jar and Spilled on the Table

To find out if increasing your fiber intake will help your constipation, try to determine its cause. You can become constipated for a number of reasons, including:

  • Lifestyle factors: Low dietary fiber intake, inactivity and low fluid intake.
  • Medications or supplements: Examples include opioid painkillers, antidepressants, antipsychotics and some antacids.
  • Disease: Examples include diabetes, irritable bowel syndrome, inflammatory bowel disease and neurological conditions like Parkinson’s.
  • Unknown: The cause of some people’s chronic constipation is unknown. This is known as chronic idiopathic constipation.

If you already eat plenty of fiber and your constipation is caused by something else, then adding more fiber may not help and could even make the problem worse (15).

Interestingly, studies have shown that some people with constipation eat similar amounts of fiber as those who don’t have the condition (16, 17).

One 6-month study in 63 people found that for people with chronic idiopathic constipation, a low-fiber or even a no-fiber diet drastically improved their symptoms. Removing the fiber basically cured them of the constipation (18).

This is also true for people who have irritable bowel syndrome (IBS), as many high-fiber foods are also high in FODMAPS, which worsen IBS symptoms (19, 20).

Nevertheless, given fiber’s potential health benefits, you shouldn’t adopt a low-fiber diet over the long term without consulting your doctor or dietitian.

Furthermore, there’s evidence that non-fermentable, soluble fiber supplements may benefit these individuals, even though they don’t tolerate other types of fiber well.

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Bottom Line: For people who eat enough fiber but are still constipated, eating more of it could make their problems worse. In some cases, reducing dietary fiber could help relieve constipation.

The Best Types of Fiber to Get Rid of Constipation

Open Green Bottle Filled with Capsules

Fiber supplements can help treat constipation, including for those who have chronic constipation or IBS (21).

However, if you have chronic constipation or are experiencing symptoms like pain, wind, bloating and gas, it may be best to go for a non-fermentable, soluble fiber supplement (22, 23, 24).

This is because fermentable fiber is used as food by the bacteria in your gut, resulting in the production of gases in your large intestine.

This could cause an increase in gas production in your gut, which might make your symptoms worse.

Examples of soluble fiber supplements include:

  • Psyllium: Psyllium husk and Metamucil
  • Methyl cellulose: Citrucel
  • Glucomannan: Glucomannan capsules or PGX
  • Inulin: Benefibre (Canada), Fiber Choice or Fibersure
  • Partially hydrolyzed guar gum: Hi-Maize
  • Wheat dextrin: Benefiber (US)

Psyllium is often considered to be the best choice.

Despite being classified as fermentable, studies have shown that psyllium can normalize stools and is well tolerated, even by people with IBS (25, 26, 27).

Bottom Line: If you don’t get enough fiber, gradually increasing the amount of high-fiber foods in your diet could help. People with chronic constipation may benefit from a non-fermentable, soluble fiber supplement.

Best Foods to Relieve Constipation

High Fiber Vegetables

If your fiber intake is generally low, try including more high-fiber foods like fruits, vegetables and whole grains in your diet.

This will increase both your soluble and insoluble fiber intake and could help relieve your problem.

It’s best to do this gradually, as dramatically increasing your intake in a short period could cause unwanted side effects like pain, gas and bloating.

Foods high in insoluble fiber include:

  • Whole grains
  • Fruits and vegetables with skins
  • Nuts and seeds

Foods high in soluble fiber include:

  • Oats
  • Flax seeds
  • Barley
  • Rye
  • Beans and pulses
  • Root vegetables
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Some high-fiber foods have been shown to be particularly effective for constipation. For example, flax seeds could help if your constipation is caused by IBS (28, 29).

If you want to try flax seeds, start by taking 1 teaspoon per day and gradually increase the dose up to a maximum of 2 tablespoons throughout the day.

To make them more palatable, you can put them in a drink or sprinkle them on your yogurt, salad, cereal or soup.

Prunes can also help relieve constipation. They’re high in fiber and also contain the sugar alcohol sorbitol, which is a natural laxative (30, 31).

Some studies have shown that prunes are more effective than fiber supplements at relieving constipation. The effective dosage is thought to be around 50 grams (or 7 medium-sized prunes) twice a day (32, 33).

However, if you have IBS, you should probably avoid prunes since sorbitol is a known FODMAP and can exacerbate your symptoms.

Bottom Line: Insoluble and soluble fiber are found naturally in many foods. Prunes may also be helpful, as long as you don’t have IBS.

Take Home Message

Eating plenty of fiber-rich foods is a good idea to optimize digestive health.

If you become constipated and don’t have much fiber in your diet, then you may benefit from eating more of it.

However, if you already get enough fiber or your constipation has another cause, increasing your fiber intake from foods may make things worse.

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