What type of fiber increases bulk for digestive health?

November 2, 2025 •

3 min read

According to the Mayo Clinic, most Americans consume only about 15 grams of fiber per day, far less than the recommended 25 to 38 grams, which can lead to digestive issues. Understanding what type of fiber increases bulk is essential for improving bowel regularity and overall gut health.

Quick Summary

Insoluble fiber, found in whole grains and vegetables, and soluble fiber, found in oats and legumes, both increase stool bulk. Insoluble fiber adds mass and speeds transit time, while soluble fiber forms a gel that softens stool and aids in passage.

Key Points

Insoluble Fiber is the Primary Bulking Agent: It adds mass to stool and speeds up its passage through the digestive system, effectively preventing constipation.
Soluble Fiber Forms a Bulking Gel: It dissolves in water to create a gel-like substance that softens stool and aids in regularity, while also helping firm up loose stools.
Foods Contain Both Types: For the best results, consume a variety of whole foods like fruits, vegetables, and grains, as they often contain a mix of soluble and insoluble fibers.
Psyllium is a Powerful Soluble Fiber: This ingredient, often found in supplements, is highly effective at absorbing water to form a viscous gel that bulks and softens stools.
Increase Fiber Intake Gradually: A sudden increase in fiber can cause bloating and gas, so it's best to introduce it slowly alongside an increased water intake.
Drink Plenty of Water: Staying hydrated is essential for fiber to work correctly; without enough fluid, bulking fiber can worsen constipation.

Understanding the Two Main Types of Fiber

Dietary fiber, often called roughage, is the indigestible part of plant-based foods. It passes through your system relatively intact, performing vital functions for your digestive tract. Fiber comes in two main varieties: soluble and insoluble, both of which play a crucial role in adding bulk to your stool, though they do so in different ways. For optimal digestive health, a balance of both types is recommended by healthcare professionals.

The Role of Insoluble Fiber

Insoluble fiber, as its name suggests, does not dissolve in water. Instead, it acts as a bulking agent by absorbing water and speeding up the passage of food through the stomach and intestines. This 'sweeping' action helps to move waste through the system more quickly and efficiently, preventing constipation. By increasing the weight and size of your stool, insoluble fiber makes it softer and easier to pass.

Sources of insoluble fiber include:

Whole Grains: Wheat bran, whole-wheat flour, brown rice, barley, and quinoa.
Nuts and Seeds: Almonds, walnuts, and flaxseeds.
Vegetables: The skins of root vegetables, green beans, and cauliflower.
Fruits: The skins of apples and pears.

The Action of Soluble Fiber

Soluble fiber dissolves in water, forming a gel-like substance in the digestive tract. This gel, which is viscous and sticky, also contributes significantly to stool bulk, particularly in cases of loose, watery stools where it helps solidify the waste. This gel also slows digestion, which helps control blood sugar and can keep you feeling full longer. One of the most effective soluble, bulk-forming fibers is psyllium husk, widely used in supplements.

Sources of soluble fiber include:

Legumes: Peas, lentils, and beans.
Fruits: Apples, citrus fruits, berries, and pears.
Grains: Oats and barley.
Seeds: Psyllium husk, chia seeds, and flaxseeds.

Comparison of Soluble vs. Insoluble Fiber for Bulking

While both soluble and insoluble fiber increase bulk, their mechanisms and primary benefits differ. The following table highlights the key distinctions:


Feature	Insoluble Fiber	Soluble Fiber
Dissolves in Water?	No	Yes
Primary Bulking Method	Passes mostly intact, absorbing water and adding mass	Forms a gel that absorbs water, adding mass and softening
Digestive Speed	Speeds up the transit of food and waste	Slows down the digestion process
Best for Constipation	Excellent for promoting regularity by moving waste quickly	Also effective, especially for softening hard stools and solidifying loose stools
Other Benefits	May lower the risk of diverticular disease	May lower cholesterol and help control blood sugar
Example Sources	Whole wheat, wheat bran, vegetable skins	Oats, beans, psyllium, fruits

Combining for Maximum Benefit

For the best results, consuming a variety of fiber-rich foods from both categories is ideal. Many plant foods naturally contain a mix of both. Eating a whole apple with the skin, for instance, provides both insoluble fiber from the skin and soluble fiber from the fruit's flesh. Incorporating a wide range of fruits, vegetables, whole grains, and legumes into your diet ensures you receive the full spectrum of benefits.

How to Gradually Increase Your Fiber Intake

Increasing fiber too quickly can lead to discomfort, such as gas, bloating, and cramps. To avoid these side effects, introduce fiber-rich foods gradually over several weeks. Simultaneously, it is crucial to increase your water intake. Fiber works best when it can absorb liquid, and drinking plenty of fluids prevents the fiber from causing or worsening constipation.

Conclusion

Understanding what type of fiber increases bulk is a key step toward improving your digestive health. While insoluble fiber acts as a natural "scrub brush" for your intestines, speeding up transit and adding mass, soluble fiber forms a gentle, bulking gel that helps regulate bowel movements and softens stool. For optimal gut health, it is best to include a diverse range of plant-based foods that contain both types of fiber. By making small, consistent changes to your diet and ensuring adequate hydration, you can effectively use fiber to promote regularity and enjoy a healthier digestive system. A great source for further information on fiber can be found on the Harvard School of Public Health's website: Fiber - The Nutrition Source.

Frequently Asked Questions

Insoluble fiber passes through the digestive tract largely intact. It increases the weight and size of your stool by absorbing water, which helps promote regular bowel movements and prevents constipation.

Soluble fiber dissolves in water to form a gel-like substance. This gel both adds bulk and softens the stool, making it easier to pass. It can also help solidify loose or watery stools.

Excellent sources of insoluble fiber include wheat bran, whole-wheat flour, nuts, seeds, brown rice, and many vegetables like carrots, celery, and the skins of fruits.

Foods high in soluble fiber include oats, barley, beans, lentils, peas, and fruits such as apples, pears, and berries. Psyllium husk is a well-known supplement rich in soluble fiber.

While many foods contain a mix of both soluble and insoluble fiber, consuming a variety of plant-based foods is the best approach to ensure you get a full spectrum of fiber types and their associated nutrients.

Introducing too much fiber too quickly can cause uncomfortable side effects like bloating, gas, and abdominal cramps. A slow increase allows your digestive system to adjust over time.

Yes, drinking plenty of water is essential. Fiber absorbs water to work properly, and without sufficient fluid intake, it can exacerbate constipation and cause blockage.

Psyllium husk is a soluble, bulk-forming laxative derived from the seeds of the Plantago ovata plant. It soaks up water in the gut to form a gel, which softens and bulks stool, making it easier to pass.

While supplements like psyllium are effective for increasing fiber, whole foods are generally considered superior. They provide not only fiber but also a variety of other beneficial nutrients, vitamins, and minerals.