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People’s consumption of fruits, vegetables, and grains remains low despite the benefits of these foods being widely documented. For example, fewer than 10% of Americans meet the daily Dietary Guidelines for whole-grain intakes.

Multiple studies attest that consuming plant foods may reduce chronic illnesses such as type 2 diabetes, cardiovascular disease, and certain cancers.

Researchers at the University of Minnesota in St. Paul, MN, have now uncovered additional benefits of insoluble fiber beyond improving stools and laxation.

In their recent review study, the experts found that bioactives in insoluble dietary fiber (IDF) may support health in different ways. They also discovered that isolated fiber may be added to various foods to boost their nutritional value.

Such fiber may easily be derived from food production byproducts such as peel, pulp, or pomace—substances that are rich in fiber and bioactives.

Dr. Joanne Slavin, professor of nutritional sciences at the University of Minnesota’s Department of Food Science and Nutrition and co-author of the study, said:

“Fiber is the marker of health that is included in our dietary guidelines and found on product labels, but our research indicates that we need to ensure the other valuable components of fiber-containing plant sources —the bioactives —are also recognized as providing valuable benefits for human health.”

Comparing different foods’ bioactive content

Dr. Slavin and her colleagues searched the Ovid MedlineOvid Agricola, and Scopus databases for bioactive research.

The search yielded 30 IDF sources assessed for bioactive content, including rice, wheat, lentils, mango, beets, and berries. Dr. Slavin’s team evaluated the bioactive content of each according to total phenolic content (TPC), total flavonoid content (TFC), and antioxidant activity (AA).

The present study’s authors detected 64 bioactive compounds across the IDF sources appearing in their research. The compounds fell into the categories of phenolic acids, flavonoids, and non-flavonoid compounds.

“We believe that bioactives are concentrated in different parts of different plants—so generally including some pulp for fruits or skin for vegetables should increase bioactives. We wanted to go beyond dietary fiber and look for published information on bioactives in a wide range of plant foods. The information was mostly in plant science journals, not nutrition journals, so the coverage is not even or representative,” Dr. Slavin told Medical News Today.

Extraction methods may affect bioactivity

The University of Minnesota team noted that IDF is present in certain plant foods and within different tissues of the plants. They also said that “Many plants contain tissues that have different types of fiber, or one tissue type will contain fiber where the other does not.”

Moreover, they observed that the IDF and bioactive content varies according to the extraction, processing, and treatment of the IDF sources.

For instance, different extraction methods yielded more or fewer carotenoids with sweet corncobs and Mexican apple pomace powders. Temperatures during processing also made a difference with bioactivity.

Maintaining bioactivity was a challenge, as 30% of the phenolic content was lost after boiling lentil-fortified pasta.

“Opportunity for fortification”

Still, many of the foods analyzed had a heightened nutritional value, even though the IDF compounds could not be fully retained.

The study’s authors pointed out that most commercial ready-to-eat foods are baked goods with little nutritional value.

However, adding plant sources to cookies increased the IDF, TPC, AND TFC content of the cookies while decreasing the amount of carbohydrates.

Can adding fiber to foods have any drawbacks?

Adding dietary fiber can have drawbacks. While supplementing foods with IDF increases bioactive content, the researchers found that it can change the texture of some products.

Sometimes, the change was advantageous. For instance, apple pomace yielded a firmer and more consistent product when added to yogurt in the trial.

Cooking lowered the bioactivity of some foods, but the bioactivity remained higher than in the control food. The study’s authors concluded that IDF “may be useful as a supplement for consumers.”

MNT discussed this research with registered dietitian nutritionist Kate Randall, who was not involved in the present study.

Randall explained that isolated insoluble fibers from plant foods may promote benefits such as improved digestive health, weight and blood sugar management, and cardiovascular health.

However, she cautioned that doing so might not always be worth the effort due to the process involved.

“The process of isolating insoluble fiber can be costly, labor-intensive, and might involve the use of chemicals or methods that could potentially alter its natural properties,” she said.

“Often, different components in whole plant foods work synergistically to provide health benefits. Isolating one component may overlook the combined benefits of the whole food matrix,” she added.

Evidence for eating more plant foods

The University of Minnesota researchers hope their work can provide plant-based foods with higher nutritional value.

“The collection of literature we reviewed and the results of this research can serve as a paradigm shift in how the food and health industries, as well as consumers, view insoluble dietary fiber and bioactive,” said graduate student and lead author Madeline Timm.

“Past thinking that soluble dietary fiber has the most physiological benefits while insoluble fiber only alters bowel function is no longer accepted. Dietary guidance must continue to support increased consumption of plant foods to increase our total dietary fiber intake to the recommended levels,” Dr. Slavin and her team concluded.

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