Beyond Soluble and Insoluble: What Fiber Research Actually Says

You might've been told by your healthcare provider, your parents or the internet that you should eat more fiber. If you've read our fiber nutrient spotlight blogpost you might also know the benefits that fiber intake provides. If you're a nutrition student or a nutrition professional, you might've learned in your introduction to nutrition course that dietary fiber can be classified into two categories: Insoluble and soluble. However, this 2019 review published by William et. al, in the Journal of Animal Science and Biotechnology presents reasoning for why we should be moving away from this simplistic classification. Based on data from studies that measured how different types of fibers are broken down (fermented) in the large intestine (LI), they argue that solubility is not the only determinant of fiber's functionality.

Dietary fiber describes the undigestible part of plants that cannot be hydrolyzed (broken down) by gastrointestinal (GI) enzymes and evade absorption in the small intestines. This nutrient is integral to gut health for this reason, among others. Insoluble fiber are those that don't dissolve in water, while soluble fiber readily dissolves in water. These properties allow fiber to contribute to gut health by directing GI transit time (rate at which food travels through the GI tract), stool bulking etc. However, this review goes on to discuss the fact that not all soluble fibers are created equal, meaning they might actually have varying solubility in water. It also points to the differences in the physiological effects of fiber from each of these groups.

A Better Framework: Four Ways to Classify Fiber

Williams et. al, propose that dietary fiber be evaluated across four dimensions:

Source: Where the fiber comes from matters. The review categorizes food sources into cereals, legumes, fruits and vegetables. Each of these contain distinct fiber profiles with different physiological effects. Fiber from oats behaves differently from fiber from lentils, even if they fall within the same soluble or insoluble category (which further highlights the sensiblity of moving away from categorizing fiber as either soluble/insoluble).

Solubility: The traditional measure. Still relevant but not sufficient on its own.

Fermentability: This is where the review makes its most compelling argument. Fermentability refers to how readily the gut microbiota (bacteria etc.) can breakdown fiber. The review emphasizes that two fibers can be classified as insoluble but their effect in the gut might differ if one is highly fermentable and the other is not.

Physiological effect: This refers to what the fiber is actually doing in the body. This includes its contribution to gut health enpoints like GI transit time, short chain fatty acid production, stool bulking and changes in the composition of the microbiota (micorbiome diversity is very important for gut health).

Why fermentability Matters

When fermentable fibers reach the LI, they are fermented by the bacteria in the gut to generate byproducts called short chain fatty acids (SCFAs). Examples of SCFAs include butyrate, acetate and propionate. These are incredibly important signaling molecules that help support the immune system, health of colonocytes (cells on the wall of the colon) and gut motility. Butyrate in particular serves as the primary energy source for colonocytes and has been shown to influence cell growth and metabolism. This mechanism has linked butyrate to colon cancer prevention.

If we consider the argument for fermentability, two individuals who consume the same amount of fiber might have significantly different gut health outcomes if one person's fiber intake consists of more highly fermentable fibers. For example, a diet high in cellulose (poorly fermentable) will help to add bulk to the stool and support GI transit time (still important) but will not  produce the same SCFA-driven benefits as a diet rich in arabinoxylans or pectins, which are highly fermentable.

This is why fiber diversity matters. Eating a wide variety of plant foods will provide all the benefits necessary for the maintenance of good gut health. Not only will your gut microbiota will be exposed to a wider array of fermentable substrates resulting in their diversity and resilience, you will also receive the benefits of GI transit time, cholesterol control and stool bulking from the less fermentable types.

Purified vs. Whole Food Fiber: A Critical Distinction

One of the most practically important points in this paper is the distinction between purified fiber and fiber consumed in whole food form. The review highlights why differences in physicochemical characteristics between purified and non-purified food components are important in terms of health.

When fiber is extracted and isolated from its whole food matrix, as happens in most fiber supplements, it loses the structural complexity of the plant cell wall. That complexity matters because it influences how quickly and where fermentation occurs, how the microbiota responds, and what physiological effects follow. This is one of the reasons why whole food sources of fiber consistently outperform supplements in the research literature.

What This Means For You

This paper does not suggest that the soluble/insoluble framework is useless, it is still a practical starting point. However, it does argue that we need to think more carefully about what we are asking when we say “eat more fiber.”

More fiber of what type? From what source? How fermentable is it? What does it actually do once it reaches your colon?

The practical takeaway is straightforward: prioritize fiber diversity over fiber quantity alone. Eating a wide variety of plant foods, whole grains, legumes, vegetables and fruits, gives your gut microbiota the diverse fiber substrates it needs to thrive, produce SCFAs, maintain the gut lining, and regulate transit time effectively.

Future Directions: What the Research Still Needs to Address

This review raises really good questions and provided some strong answers. However, there are still gaps that require investigation:

1. Standardized classification systems: The field urgently needs a consensus on how to classify dietary fiber beyond solubility. A fermentability-based framework is promising but requires standardized in vitro and in vivo testing methods before it can be widely adopted in nutrition guidelines and food labeling.

2. Individual microbiome variability: Fermentability is not just a property of the fiber, it is also a property of the person eating it. Individual differences in gut microbiota composition mean that the same fiber can produce different SCFA profiles in different people. Future research needs to account for this person-to-person variability in fiber response.

3. Whole food matrix effects: We need more research specifically comparing the physiological effects of whole food fiber versus isolated fiber supplements across different populations, health conditions and fiber types. The evidence strongly suggests whole food fiber is superior, but the mechanistic reasons for this are not fully characterized.

4. Long-term intervention studies: Most fiber research relies on short-term intervention studies or epidemiological data. We need longer-term randomized controlled trials that examine how sustained changes in fiber diversity, not just quantity, affect gut microbiota composition, SCFA production and downstream health outcomes including metabolic disease, inflammatory conditions and colorectal cancer risk. Read our recent discussion of research linking fiber intake to colorectal cancer risk here.

5. Processing effects on fiber functionality: Since processing can shift a fiber from insoluble to soluble or alter its fermentability, future research needs to systematically examine how common food processing methods example: cooking, milling, fermentation and extrusion change the functional properties of fiber and what that means for health.

6. Pediatric and aging populations: Most fiber research is conducted in healthy adults. We know very little about how fiber fermentability affects gut microbiota development in children or how aging-related changes in gut microbiota composition alter the response to different fiber types in older adults.

Conclusion

The science of dietary fiber is more nuanced than most nutrition messaging lets on. Williams et. al, have made a compelling case that the soluble/insoluble binary has taken us as far as it can go, and that a more sophisticated, fermentability-based framework is the next frontier. For those of us who work in nutrition, this paper is a reminder that the best dietary advice is always grounded in the most current science, and that sometimes the most important thing we can do is question the frameworks we inherited.

Read The Full Article

Williams, B. A., Mikkelsen, D., Flanagan, B. M., & Gidley, M. J. (2019). "Dietary fibre": moving beyond the "soluble/insoluble" classification for monogastric nutrition, with an emphasis on humans and pigs. Journal of animal science and biotechnology, 10, 45. https://doi.org/10.1186/s40104-019-0350-9

Author: Kaydine Edwards, PhD