5 Findings of Intrinsic Dietary Fibers for Better Health

In recent years, intrinsic dietary fibers have emerged as pivotal components in promoting better health.

These fibers, integral to the plant cell matrix, play a crucial role in enhancing gut microbiome diversity, thereby improving overall gut health.

As we delve deeper into their complex structure, the importance of fiber digestibility and the impact of food processing becomes evident.

This article will explore five key findings about intrinsic dietary fibers, shedding light on their health benefits, contribution to nutrient absorption, and role in microbial fermentation.

We will also discuss how these fibers, as functional foods, fit into plant-based nutrition, offering a comprehensive understanding of their significance in our diets.

Main Findings

• Dietary fibers significantly impact human health, especially via the gut microbiome.
• Fibers in their natural form within plant cell matrices (intrinsic fibers) may have different and potentially greater health benefits than isolated fibers.
• Food processing can substantially alter the structure of intrinsic fibers, affecting their health benefits.
• The complexity of fiber structures contributes to diverse physicochemical properties like solubility, bulking, viscosity, and fermentability.
• Intrinsic fibers in plant cells are entangled in a complex network, impacting how they are digested and fermented by gut microbes.
• The breakdown and physiological behavior of intrinsic fibers differ from that of their isolated counterparts due to their complex natural form.

Introduction

In recent years, the focus on human health has shifted towards the impact of our diet, and a key player in this arena is dietary fiber.

These indigestible components found in plant foods have gained recognition for their remarkable potential in reducing mortality rates and safeguarding against ailments like cancer, type 2 diabetes, and heart diseases¹.

Interestingly, the significance of dietary fibers was highlighted as far back as the 1960s by Denis Burkitt, who proposed the dietary fiber hypothesis linking them to various diseases prevalent in Western societies².

Initially, the benefits of dietary fibers were believed to be largely independent of gut microbiota.

Physical properties such as water retention for increased stool bulk and bile-acid binding to lower cholesterol levels were the main drivers.

However, the last 25 years have seen a revolution in our understanding ³.

Since human enzymes cannot break down dietary fibers, they reach the lower gut, where the gut microbiota, mainly consisting of colon-dwelling bacteria, utilize them⁴.

This microbial activity generates essential compounds, notably short-chain fatty acids (SCFA), which play a pivotal role in the microbiota-driven effects of fibers ^{5 6}.

These effects can range from local interactions within the gut, impacting epithelial and immune cells, to distant effects on other organs ⁷.

Consequently, manipulating gut microbiota through fiber intake opens new therapeutic possibilities.

Researchers are now investigating various fiber types to uncover their specific effects on health ⁸.

This research aims to dissect the intricate fiber-microbiota interactions and unravel the molecular intricacies responsible for different health outcomes ^{9 10}.

However, current research tends to examine fibers in isolation, neglecting their natural form as part of whole, minimally processed foods^{11 12}.

This article emphasizes the need for a holistic understanding of dietary fibers to fully harness their potential in modulating gut microbiota for improved health.

It delves into the natural existence of dietary fibers, highlighting how this perspective differs from current approaches.

Additionally, it explores existing studies on intrinsic fibers’ impact on human health and discusses potential directions for future research¹³.

Dietary Fiber and the Gut Microbiome

Dietary fibers encompass a diverse group of polymers with distinct chemical structures, including various sugar molecules like glucose, xylose, mannose, galactose, arabinose, and rhamnose.

These sugars form linear or branched structures with different glycosidic bonds, some decorated with phenolic acids or linked to other compounds ^{14 15}.

For example, inulin and starch consist of linear repetitions of single molecules (fructose and glucose monomers, respectively), while rhamnogalacturonan, a type of pectin, is highly branched with various side-chains ¹⁶.

Their common feature is resistance to digestion by human endogenous enzymes.

The extensive structural variation among dietary fibers leads to physicochemical properties often categorized as “soluble versus insoluble.” However, this classification is based on fiber content analysis rather than functional behavior in the human gut ^{17 18}.

A more informative approach considers properties like bulking, viscosity, and fermentability ¹⁹.

Gut microbiota fermentation of these fibers relies on a set of enzymes capable of breaking down specific chemical linkages and molecules ^{20 21}.

Microbial communities demonstrate functional redundancy, with different gut microbiota members performing similar functions, including fiber breakdown and the production of short-chain fatty acids (SCFA) ^{22 23}.

Anaerobic bacteria, predominant in the gut microbiota, primarily contribute to fiber degradation, aided occasionally by methanogens that convert hydrogen to methane ^{24 25}.

Some bacteria have specialized roles, referred to as “keystone” species, critical in metabolic networks for compound breakdown or metabolite production ²⁶.

Numerous intervention studies have examined isolated fibers ^{27 28}.

These studies reveal that various fibers can diversify gut bacteria ²⁹ and complex fibers can target bacteria beneficial for human health ^{30 31}.

For instance, a study enriched wheat bread with diverse fibers and observed decreased cholesterol, insulin, and HOMA-IR levels, associated with an increase in gut bacteria capable of breaking glycosidic bonds ³².

Another study linked the fermentation of arabinoxylan, but not crystalline cellulose, to increased satiety in overweight individuals ³³.

In vitro assessments of fast-fermentable fibers indicated delayed fermentation rates when combined, a potential advantage in treating irritable bowel syndrome ³⁴.

While studying isolated fiber types elucidates trophic chain interactions between microbes and underlying mechanisms, it overlooks one aspect: how we consume fibers ³⁵.

Understanding the holistic context of dietary fibers, as they exist in whole foods, is equally crucial.

This broader perspective can shed light on how intrinsic dietary fibers impact our health and well-being.

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Intrinsic vs. Isolated Fibers

When we think of dietary fibers, we often picture them as individual components.

However, this perception doesn’t align with their natural state, which was recognized by Denis Burkitt as having crucial health benefits.

Most of the fibers we consume aren’t isolated entities but are part of plant-based foods like vegetables, fruits, seeds, nuts, legumes, and grains ^{36 37 38}.

These plant foods are made up of a matrix of plant cells, and at the core of this matrix are dietary fibers intricately woven into a three-dimensional network forming plant cell walls ³⁹.

These plant cells can also house other fibers stored in vacuoles, which are either fructans or starch serving as a growth reserve for the plant ^{40 41}.

Recent awareness highlights that this three-dimensional plant cell matrix has significant implications for both digestibility and health, potentially differing from the effects of isolated fibers.

To distinguish them from isolated fibers, these naturally occurring fibers are referred to as “intrinsic fibers” ⁴² as they are an intrinsic part of the plant cell wall.

Composition of Intrinsic Dietary Fibers

Plant cells share a common structural framework consisting of three main components: cellulose, hemicellulose, and pectin ⁴³.

Cellulose acts as the cell wall’s foundational scaffold, comprising linear chains of glucose units linked in a rigid microfibril structure.

Hemicellulose further reinforces these microfibrils, existing in various forms with glucose-xylose (xyloglucans), glucose (mixed-linkage glucans), xylose (xylan), or mannan backbones ⁴⁴

Additional sugar molecules and phenolic acids can attach to these backbones, as seen in the hemicellulose type arabinoxylan ^{45 46}.

Pectin provides stability and water retention, filling the spaces between cellulose and hemicellulose ⁴⁷

Pectin, like hemicellulose, comprises a diverse group of polymers with linear galacturonic acid backbones (homogalacturonans) or galacturonic acid and rhamnose backbones (rhamnogalacturonans), featuring complex branching ⁴⁸.

These polymers are physically entangled and, to some extent, chemically bonded, forming the three-dimensional structure of plant cells, held together by pectin ⁴⁹.

Plant cell walls also contain small quantities of proteins (arabinogalactan proteins) and minerals ⁵⁰.

In certain specialized cell types, lignin reinforces the structure, a dietary fiber composed of phenolic phytochemicals rather than sugars.

While these aspects are common to all plants, variations exist based on the plant type (monocots vs. dicots), maturity, and food processing ^{51 52}.

Even within a plant, differences exist between different tissues (leaves, roots, fruits, stems, seeds) and within those tissues ⁵³.

Hemicellulose and pectin, being heterogenous polymers, distinguish plant types in the composition of their cell walls ⁵⁴.

Dicots, including vegetables, fruits, nuts, seeds, and legumes, have type I cell walls, rich in xyloglucan and pectin, constituting a third of the cell wall weight.

In contrast, monocots like grains possess type II cell walls, which lack pectin and feature abundant β-glucan and arabinoxylan ^{55 56}.

Variations also occur in the outer coatings of grains, consisting of layers like the aleurone layer, seed coat, and pericarp, collectively known as “bran” ⁵⁷.

Bran isn’t uniform but contains various cell types with different hemicellulose ratios compared to the starchy endosperm ⁵⁸.

Complexity of Plant Cells

The three-dimensional capsules created by the cell wall fibers encase plant cells with their vacuoles, containing nutrients like lipids, proteins, carbohydrates, and phytochemicals ⁵⁹.

The content of these vacuoles varies depending on the plant type.

For instance, plants store reserve carbohydrates as fructans or starch granules.

Starch can be partially resistant to digestion due to its physical state, qualifying it as a dietary fiber.

Many vegetables and fruits have cells filled mainly with water, maintaining turgor pressure and a crisp texture.

These vacuoles also contain other molecules such as sugars and phytochemicals ⁶⁰.

In legumes and nuts, vacuoles consist of a protein matrix enclosing starch granules and lipid bodies, respectively.

Additionally, starch granules in grains are embedded in a protein matrix within the cell walls.

The three-dimensional structure of plant cells, along with physical and chemical interconnections and the encapsulation of other fibers and nutrients, impacts how gut microbes access these individual fibers.

Food processing can significantly alter and disrupt these intrinsic structures, often used to extract, isolate, and purify single fibers.

Unlike isolated fibers, during digestion in the upper gut, cell wall fibers don’t simply dissolve and become freely available as isolated components.

Instead, they remain to a considerable extent within their intertwined matrix.

Therefore, the composition of the plant food cell matrix, food processing, and digestion collectively influence how intrinsic dietary fibers are modified in the gut and fermented by the gut microbiota.

Understanding this complex interplay can shed light on the impact of intrinsic dietary fibers on digestion and gut health, opening doors to a deeper comprehension of the vital role they play in our overall well-being.

Food Processing and Fiber Structure

The classification of a fiber as intrinsic and its digestive fate are significantly influenced by food processing methods.

Intrinsic fibers are inherently part of the plant cell wall, found in whole, unprocessed foods like raw fruits and vegetables ^{61 62 63}.

Any form of processing, whether industrial or domestic, can alter this three-dimensional structure, making it crucial to evaluate whether a fiber retains its intrinsic status ^{64 65}.

The Impact of Food Processing

Food processing serves various purposes, including food preservation, enhancing digestibility, or extracting compounds for new food products ^{66 67}.

These processes encompass mechanical (physical), chemical, thermal, and enzymatic techniques, as well as high-pressure, ultrasound, and microwave technologies.

While industrial processing often focuses on extracting single fibers for specific uses, domestic processing tends to be gentler, preserving the intrinsic fiber structures to some extent.

Effects of Different Processing Techniques

Drying, Freezing, and Freeze-Drying: These methods aim to preserve food products.

While they can cause some cell tissue shrinkage or cracks in cell walls due to water crystals during freezing, they generally maintain the overall tissue structure ⁶⁸.

Hydrothermal Processing: This technique can maintain tissue structure but may induce damage, particularly to pectin ⁶⁹.

Boiling or Steaming: These processes can lead to swelling of cell walls and pectin dissolution, weakening cell cohesion and potentially causing cell wall rupture ^{70 71}.

Dry Thermal Treatments: Techniques like baking, roasting, or popping can damage the cell wall but often preserve overall tissue structure ⁷².

Mechanical (Physical) Destruction: Milling or grinding can break cell walls, releasing encapsulated contents.

Finer milling can disrupt the three-dimensional cell wall structure ⁷³.

It’s important to note that “whole grain” products do not necessarily provide intrinsic fibers, as the components (bran, germ, and starchy endosperm) are separated, milled, and then reconstituted, altering the original grain structure ^{74 75}.

Effects of Processing on Intrinsic Fiber Structure

Mechanical processing, whether domestic or industrial, significantly impacts intrinsic fiber structures.

Even in powdered preparations, cell-wall parts may be present but with small particle sizes, potentially increasing accessibility for gut bacteria.

Extrusion, a high-pressure, high-shear, and high-temperature technique, can completely degrade even rigid cellulose fibrils ⁷⁶.

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Isolated Fiber Extraction

To produce single, isolated fibers, extensive extraction techniques are necessary to separate specific fibers from the plant cell matrix ^{77 78}.

Some fibers are water-extractable, termed “soluble,” while others require enzymatic, gravitational, and/or chemical treatments.

The integrity of these structures depends on the harshness of the applied processing.

Fiber products based on waste streams may or may not contain intrinsic fibers, impacting their sustainability ^{79 80}.

Understanding the Difference

Studying the isolated behavior of single dietary fibers typically involves fibers that have undergone extensive processing for extraction and purification ⁸¹.

This may not sufficiently reflect the effects and behaviors of intrinsic fibers ⁸².

In conclusion, food processing plays a pivotal role in altering the structure and properties of intrinsic dietary fibers.

Understanding these changes is essential for comprehending the impact of intrinsic fibers on digestion and overall health, shedding light on the complexities of dietary fiber processing and its consequences.

Impact of Intrinsic Fibers on Digestion and Gut Microbiota

Food processing has a significant impact on how fibers behave during digestion ⁸³.

While extensive research has focused on isolated single fibers, the digestion of plant tissues is not as well understood.

Few studies have followed intrinsic fibers’ digestion from the upper gut to the colon ⁸⁴, mainly relying on in vitro and animal in vivo models.

Here, we’ll highlight the main consequences of upper gut digestion for intrinsic fiber structures.

Digestion in the Upper Gut

When we consume plant tissues with an intact cell matrix, their constituents don’t readily dissolve as they are intertwined within the cell wall and not water-soluble under normal conditions ^{85 86}.

Encapsulated compounds are shielded from immediate digestion, delaying their breakdown.

Chewing degrades plant tissues into smaller sizes, and during swallowing, a mix of differently sized particles containing intact and broken cells arrives in the stomach.

Further particle breakdown can occur by mechanical gastric forces.

Smaller particles pass into the small intestine, potentially containing intact plant cells.

Larger structures are retained for further size reduction (gastric sieving) ⁸⁷.

Arrival in the Colon

In the colon, a mixture of broken and intact cells confronts gut bacteria.

Released fructans and resistant starches resist digestion.

Pectin leakage may increase cell wall porosity, but limited diffusion of enzymes into intact cells and nutrients out of the cell is believed to occur due to cell wall pore size limitations ^{88 89}.

Plant cell wall material can reduce enzymatic starch breakdown by adsorbing α-amylase ^{90 91}.

Impact of Mild Food Processing

In cases of mild food processing, the fractions of intact plant tissue we swallow likely arrive in the colon relatively intact, primarily affected by chewing and pectin dissolution.

Therefore, colon bacteria encounter a mix of plant tissue particles, intact plant cells, broken cell wall material, and their contents.

Dissolved fibers, like fructans or starch, can be directly used by gut bacteria.

However, as bacteria cannot diffuse into intact plant cells, they must spatially interact with the plant tissue to access intertwined polymers like pectins and hemicelluloses.

These complex digestive aspects highlight that the behavior of intrinsic fibers during breakdown and digestion cannot be entirely explained by the isolated behavior of single fibers ^{92 93}.

Consequences of Food Processing on Fiber Digestibility

Understanding how intrinsic dietary fibers interact with our gut microbiota is a fascinating field, yet surprisingly underexplored in human in vivo studies.

While we’ve learned much from in vitro research about the role of specific bacteria and enzymes in breaking down isolated fiber types ^{94 95}, there’s still much to uncover about how gut bacteria handle the formidable challenge of plant cell walls, which consist of various complex fibers ⁹⁶.

Diverse Enzymatic Arsenal of Gut Bacteria

Gut bacteria employ a range of enzymes categorized as carbohydrate-active enzymes (CAZymes) to break down fibers ⁹⁷.

The most common are glycoside hydrolases, which cleave sugar and non-sugar moieties.

Polysaccharide lyases focus on uronic acid moieties present in pectins, while carbohydrate esterases target esterified groups found in pectins and hemicelluloses.

This intricate enzymatic system varies among bacterial species ⁹⁸.

The Challenge of Cellulose

Cellulose microfibrils, a robust component of plant cell walls, pose a particularly intriguing puzzle.

While amorphous cellulose is believed to be partially utilized by bacteria with a complex enzyme system, crystalline cellulose remains largely unfermentable ^{99 100}.

Bacteria have been found adhering to cell walls in human feces, suggesting they might use the crystalline cellulose backbone for attachment ^{101 102}.

Research on the bacterial communities inhabiting these plant tissue fractions reveals unique phyla, distinct from those in the liquid fraction ¹⁰³.

Different particles impact the types of adhered bacteria ^{104 105}.

From Plant Cell Walls to SCFA Production

Plant cell walls are believed to be broken down by primary degraders, initiating the release of materials that other bacteria use to cross-feed ¹⁰⁶.

This intricate interaction remains unclear, but studies on plant cell-bacteria biofilms and bacterial enzymatic systems offer some insights ^{107 108}.

The Delayed Fermentation of Intrinsic Fibers

Fermentable fibers are generally thought to be rapidly metabolized in the proximal colon.

However, when these fibers are part of plant cell walls or encapsulated by them, fermentation rates slow down ¹⁰⁹.

In vitro experiments support this, showing that microbial enzymatic activity initially targets cell wall fiber degradation, followed by slower starch degradation and different microbial communities ¹¹⁰.

Intracellular material from broken cells also affects plant cell wall fermentation ¹¹¹.

The intrinsic structural complexity of plant cells leads to delayed fiber fermentation, creating a gradual release of short-chain fatty acids (SCFA) throughout the colon ¹¹².

This is significant because it benefits local mucosal health and likely has positive systemic effects, with distal SCFA infusion showing more pronounced impacts on biomarkers than proximal infusion ¹¹³.

Delayed fermentation is also considered beneficial for treating irritable bowel syndrome ¹¹⁴.

Human Studies on Intrinsic Fibers

To determine whether these in vitro findings hold in vivo and how they affect health outcomes, we analyzed human intervention studies.

These studies investigated the effect of intrinsic fibers on gut microbiota composition, activity, and related metabolic and bowel function outcomes, emphasizing whole foods or specifically processed food types.

Whole Foods and Health Outcomes

Whole food interventions, focusing on various intrinsic fibers from grains, fruits, vegetables, legumes, and nuts, have shown promising health outcomes.

These diets have been linked to improvements in cognitive function, inflammation, lipid, and glucose metabolism ^{115 116 117}.

Increased levels of gut microbial taxa involved in fiber breakdown and SCFA production are associated with these improvements.

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The Wheat Bran Mystery

Studies on wheat bran, although not strictly intrinsic fiber, have offered insights.

Coarse bran, which retains its structure, reduces transit time and intraluminal pressure ¹¹⁸.

In contrast, hydrothermal processing impairs bran effectiveness ¹¹⁹.

While some studies found limited effects of bran on gut microbiota composition, others discovered that the whole kernel of grains likely acts differently than the milled and reconstituted fractions in whole-grain products ^{120 121 122}.

Nuts: A Unique Source of Intrinsic Fibers

Nuts, rich in fat-filled cells, are a unique source of intrinsic fibers.

Studies have shown that they can moderate gut microbiota composition, though effects vary by nut type and processing method ^{123 124}.

Fresh and Dried Fruits

Fresh fruits, often studied dried, have been less explored in their intrinsic form.

However, some studies have examined their impact on gut microbiota composition, bowel function, and SCFA production ^{125 126}.

Exploring Avocado’s Potential

Avocado, rich in fat-filled cells, is another intriguing subject of study.

While its high fat content is believed to affect lipid metabolism, its impact on the colonic microbiota is not yet clear ^{127 128}.

Future research will provide valuable insights into the microbiota-mediated health benefits of these intrinsic fibers.

Unlocking the Potential of Intrinsic Fibers

Human intervention studies have confirmed that intrinsic fibers can modulate the gut microbiota, with effects ranging from small to moderate.

However, few studies have combined changes in gut microbiota with metabolic markers and bowel function.

Further research into the effects of processing and particle size is needed, and these studies hold the promise of unlocking the full potential of intrinsic fibers.

In conclusion, intrinsic dietary fibers are a complex and fascinating subject in the realm of gut health.

While there is much to learn, existing research indicates their significant impact on our microbiota and overall well-being.

As we delve deeper into the intricate world of intrinsic fibers, we are sure to uncover even more exciting insights into their role in promoting good health.

Summary and Future Directions

The world of dietary fibers and their impact on gut health is an exciting frontier, holding promise for new ways to enhance well-being.

For years, researchers have taken a reductionist approach, focusing on isolated fibers to understand their effects on human health.

However, reality is more complex.

During digestion, dietary fibers don’t simply dissolve into single components.

Instead, they arrive in the colon as part of plant tissues, with intricately woven cell walls and encapsulated fructans and starch polymers.

These are what we call intrinsic dietary fibers, and they appear to slow down bacterial fermentation in the colon.

This slowdown means that the production of short-chain fatty acids (SCFA), which have numerous health benefits, occurs throughout the entire colon, particularly in the distal colon.

However, we have limited understanding of the specific processes involved.

Most research on intact plant tissues and intrinsic fibers has focused on the upper gut or used in vitro or animal in vivo data.

Human in vivo data on how intrinsic fibers are utilized by the gut microbiota are lacking.

Future research should aim to uncover:

• How intrinsic fiber structures differ from isolated fibers in terms of fermentation kinetics.
• How the gut microbiota colonizes intrinsic fiber particles and collaborates with other bacteria in different environments.
• How intrinsic fibers from various plant sources and their processing impact microbial breakdown and human health.

Additionally, we should explore how food processing affects intrinsic fiber structures.

Different preparation techniques, such as cooking or steaming vegetables, can fundamentally alter the health impact of intrinsic fibers.

It’s crucial to remember that not all food processing is detrimental to health.

In some cases, processing is necessary, especially for populations with specific dietary needs.

However, in regions where highly processed foods dominate, increased digestibility has led to negative health outcomes like obesity and related diseases.

To promote healthier diets and reduce waste, we should rethink our approach to dietary fibers.

Instead of focusing solely on isolated fibers, we should harness the potential of the intrinsic plant cell matrix found in all plant foods.

This shift towards utilizing intrinsic plant cell matrix can reduce the extent of food processing, minimize waste, and create new, healthy products that align with sustainable and plant-based diets.

By doing so, we can unlock the true potential of intrinsic dietary fibers for better gut health.

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