How Insoluble Fiber Can Lower Your Risk Of Type 2 Diabetes And Cancer

Insoluble fiber supports bowel regularity, but plant sources may provide wider benefits through bioactive compounds associated with lower risks of type 2 diabetes and certain cancers. A 2023 review identified 64 bioactives in fiber-rich foods such as whole grains, legumes, fruit, vegetables, nuts and seeds, although it did not prove that insoluble fiber alone prevents disease.  

How Insoluble Fiber Can Lower Your Risk Of Type 2 Diabetes And Cancer
Medically reviewed

Dr. Chrysoula I. Liakou MD, PhD Google Scholar LinkedIn

Internal Medicine Specialist
Cancer Immunology Researcher

What Research Shows About Insoluble Fiber, Plant Bioactives and Type 2 Diabetes and Cancer Risk

  • Getting adequate fibre, mainly found in plant-based foods known as insoluble fibre, is essential for keeping your digestive system in good shape and maintaining a healthy gut.

  • Some fresh research from the University of Minnesota has revealed that insoluble fibre in plants contains unique compounds that might reduce the chances of developing type 2 diabetes, heart disease, and cancer.

  • This recent study indicates that incorporating these beneficial compounds from fibre into your diet or supplements could benefit your overall health.


Many people still need to consume more fruits, vegetables, and grains, even though the health benefits of these foods are well-known. For example, less than 10% of Americans meet the daily recommendations for whole-grain intake.


Numerous studies have shown that including plant-based foods in your diet can help reduce the risk of chronic diseases like type 2 diabetes, cardiovascular problems, and certain cancers.


Researchers at the University of Minnesota in St. Paul, MN, have recently uncovered additional advantages of insoluble fibre beyond its role in improving digestion and stool regularity.


In their recent review, experts found that insoluble dietary fibre (IDF) contains bioactive compounds that can support health in various ways. Moreover, these fibres can enhance the nutritional value of different foods by adding them to them.


In addition to peels, pulp, and pomace, which are rich in fibre and bioactive substances, this type of fibre can be easily obtained from byproducts of food production.


The research highlights the importance of recognising fibre and the bioactive components found in plant-based sources as valuable contributors to human health.



Comparing Different Foods’ Bioactive Content

 

Researchers thoroughly searched the Ovid Medline, Ovid Agricola, and Scopus databases to gather information on bioactive compounds. They examined 30 different sources of insoluble dietary fibre (IDF), including rice, wheat, lentils, mangoes, beets, and berries. The assessment focused on measuring total phenolic content (TPC), total flavonoid content (TFC), and antioxidant activity (AA) in these sources.

 

In this study, 64 bioactive compounds were identified across the various IDF sources. These compounds belonged to phenolic acids, flavonoids, and non-flavonoid compounds. The researchers noted that bioactives tend to be concentrated in different parts of different plants. For example, fruits might have more in the pulp, while vegetables might have more in the skin. The researchers aimed to expand their investigation beyond dietary fibre and sought published information on bioactives from various plant-based foods. Most of this information was found in plant science journals rather than nutrition journals, leading to an uneven and non-representative sampling.



Extraction Methods May Affect Bioactivity.


It has been observed by the team from the University of Minnesota that IDF can be found in specific plant foods and various plant tissues. These different plant tissues can contain varying fibre types; some tissues have fibre while others do not.


It was also noted that the content of IDF and bioactives can differ depending on how the IDF sources are extracted, processed, and treated.


For example, different extraction methods resulted in varying amounts of carotenoids for sweet corncobs and Mexican apple pomace powders. Additionally, bioactivity was affected by processing temperatures.


Preserving the bioactivity of lentil-fortified pasta was difficult, as boiling caused a 30% loss in phenolic content.



“Opportunity For Fortification”


Although some IDF compounds weren't preserved in the foods examined, many still demonstrated improved nutritional value.


The researchers noted that most commercially available ready-to-eat foods are primarily baked goods with limited nutritional benefits.


However, when plant sources were incorporated into cookies, it not only raised the levels of IDF, TPC, and TFC in the cookies but also reduced carbohydrate content.



Can Adding Fibre To Foods Have Any Drawbacks?


While adding dietary fibre to foods can offer advantages by increasing bioactive content, it's important to note that it can also alter the texture of certain products.


In some cases, this alteration can be beneficial. For example, when apple pomace was incorporated into yoghurt during the study, it resulted in a firmer and more consistent product.


The cooking process reduced some foods' bioactivity, but it remained higher than in the control food. As a result, the researchers concluded that incorporating insoluble dietary fibre (IDF) "may be useful as a supplement for consumers."


Kate Randall, a registered dietitian nutritionist at WellTheory who was not involved in the study, explained that isolated insoluble fibres from plant foods can offer various benefits, including improved digestive health, weight management, blood sugar control, and cardiovascular health.


However, she also cautioned that isolating insoluble fibre could be costly, labour-intensive, and may involve using chemicals or methods that could alter its natural properties.


She emphasised that in many cases, different components in whole plant foods work together synergistically to provide health benefits. Isolating a single component may overlook the combined advantages of the entire food matrix.



Evidence For Eating More Plant Foods


Researchers at the University of Minnesota aim to enhance the nutritional value of plant-based foods. Their work challenges previous beliefs regarding the benefits of dietary fibre.


While past thinking primarily favoured soluble dietary fibre for its physiological benefits and considered insoluble fibre mainly affecting bowel function, current research suggests a shift in this perspective.


This new understanding emphasises the importance of increasing our consumption of plant foods to meet recommended dietary fibre intake levels, highlighting the potential of insoluble dietary fibre and bioactive compounds in improving overall nutrition.


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Insoluble Fiber, Plant Bioactives and Chronic Disease Risk FAQs

The review found that insoluble-fiber-rich plant materials contain many potentially beneficial compounds beyond fiber itself. Across 30 sources, researchers identified 64 bioactives grouped mainly as phenolic acids, flavonoids and non-flavonoid compounds. It was a review of existing research rather than a clinical trial, so it did not show that any particular food or compound prevents diabetes or cancer.

Insoluble fiber is the part of plant food that does not dissolve in water and often increases stool bulk and supports regular bowel movements. Soluble fiber disperses in water and may form a gel, but solubility alone does not determine a fiber’s health effects. Viscosity, fermentability, food structure and accompanying bioactive compounds also influence how different fiber sources behave in the body.

Fiber-rich plant foods may support a lower type 2 diabetes risk by improving fullness, weight management, glucose regulation and the gut environment. Phenolic and other bioactive compounds attached to the fiber matrix could provide additional metabolic benefits. However, the strongest evidence concerns diets rich in whole plant foods and total dietary fiber, not insoluble fiber supplements as a proven diabetes-prevention treatment.

Evidence is strongest for colorectal cancer, with diets rich in whole grains and fiber-containing foods associated with a lower risk. Possible explanations include faster bowel transit, greater stool bulk, beneficial changes in gut bacteria and improved weight and insulin regulation. This does not mean insoluble fiber prevents every type of cancer, and the review did not directly measure cancer diagnoses.

Whole grains and bran, beans, lentils, nuts, seeds, vegetables and fruits with edible skins provide insoluble fiber alongside different phytochemicals. The review examined sources including wheat, rice, lentils, berries, mango and beet materials. Variety matters because no single food supplies every fiber type or bioactive compound, and substantial amounts of insoluble fiber can be concentrated in peels, hulls and seeds.

Researchers identified phenolic acids, flavonoids and non-flavonoid compounds, including substances such as quercetin, catechins, anthocyanins, lutein, lycopene and beta-carotene. These compounds are studied for possible antioxidant, anti-inflammatory and metabolic effects, but their amounts vary by plant, tissue, maturity and processing. Finding them in fiber-rich foods does not prove that an isolated compound prevents disease.

Processing does not have one predictable effect. Peeling often reduces fiber because skins contain substantial insoluble material, while blending does not remove dietary fiber. Heat, extraction and storage can reduce or alter certain bioactives, yet fortification studies often still increased the finished food’s fiber and bioactive content. The nutritional result therefore depends on the plant source and processing method used.

Not necessarily. Isolated fiber can increase dietary intake or support bowel function, but it may not provide the vitamins, minerals and bioactive compounds naturally packaged with whole grains, legumes, fruits, vegetables, nuts and seeds. The review cautioned that isolated insoluble fiber may offer limited additional value if processing removes its associated bioactives. Supplements should complement rather than replace a varied plant-rich diet.

EFSA considers 25 grams of total dietary fiber per day adequate for normal bowel function in adults, with higher intakes from fiber-containing foods associated with additional health benefits. Recommendations apply to total fiber rather than a separate insoluble-fiber target. Increase intake gradually and drink enough fluid, since a sudden increase can cause gas, bloating or abdominal discomfort.


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