What is the difference between soluble and insoluble dietary fiber

For starters, there are actually two different kinds of fiber: soluble and insoluble. And both do different—but equally valuable—things for your body. In an effort to give fiber its full due, we broke it all down with the help of a few nutrition experts. Fiber, sometimes called dietary fiber, is a type of carbohydrate found in plant foods, according to the Food and Drug Administration FDA.

Its structure is formed by a bunch of sugar molecules, bound together in a way that makes it hard to readily break down and use as energy. So unlike sugar or starch, for instance, fiber is not actually a great source of fuel for the body. But it still plays a crucial role in a healthy diet.

Almost all plant foods which include vegetables, fruits, whole grains, legumes, seeds, and nuts contain a combination of both, according to the FDA. The flesh of the apple contains some soluble fiber, while the skin is full of insoluble fiber, Whitney Linsenmeyer , Ph.

Those often contain large amounts of added fiber, and often just one type or the other, Young notes. Soluble fiber is fiber that is able to dissolve in water. It is the main type of fiber found in grains like barley and oats , legumes like beans, lentils, and peas , seeds like chia seeds , nuts, and some fruits and vegetables like citrus fruits and carrots , according to the U.

National Library of Medicine. Eating too much fiber can cause bloating, gas, and constipation. Find out how much fiber is too much and how to relieve symptoms in this article.

A new meta-analysis of two large cohort studies finds a link between a high intake of fiber and painful osteoarthritis of the knee. Recently published research elucidates an unexpected connection between a high-fiber diet and maintaining a healthy set of lungs. Soluble and insoluble fiber: What is the difference? Medically reviewed by Natalie Butler, R.

Soluble versus insoluble fiber Benefits of fiber Getting enough fiber Dietary fiber, the indigestible part of plant material, is made up of two main types. Soluble versus insoluble fiber. Share on Pinterest Whole grains and cereals are a good source of fiber, particularly insoluble fiber.

Benefits of fiber. Share on Pinterest Regularly consuming good sources of fiber may help to stabilize cholesterol, blood sugar, and fat levels. Getting enough fiber. Share on Pinterest Choosing foods rich in fiber is preferable to relying on supplements. There are also suggestions that propionate can alter cholesterol synthesis [ 94 ].

It has also been shown to stimulate feelings of satiety, thus influencing food intake [ 92 ]. Associated with this function, it has been shown that butyrate influences metabolic pathways of the gut by changing cellular growth and metabolism [ 94 ].

By this means, it is thought that butyric acid is involved in the prevention of colonic cancer [ 95 , 96 ]. Protein fermentation refers to the bacterial breakdown of proteins to amino acids, as well as their further breakdown to ammonia and other potentially toxic compounds such as indoles, phenols, and amines [ 97 ]. This process normally increases when there is a shortage of fermentable carbohydrates available to the gut bacteria as a source of energy.

Health benefits of reduced protein fermentation are related to the reduction of ammonia and other nitrogenous, phenolic and sulphurous compounds in the GIT [ 98 ], while increased protein fermentation is considered to be detrimental to GIT health [ 99 ].

Excess protein fermentation can lead to an increase of NH 3 and amines. NH 3 then moves from the GIT into the bloodstream and is detoxified in the liver or muscles, with a large amount converted to urea and excreted by the kidneys [ ]. Protein fermentation can also lead to end-products such as branched-chain SCFA, amines, phenols, sulphides and thiols [ 94 ]. However, if there is a constant supply of carbohydrates and sufficient saccharolytic bacteria, the detrimental effects of these metabolites can be significantly reduced [ 94 ].

Dietary fibre includes a wide range of mostly carbohydrate polymers ranging from soluble polymers such as pectins and various oligosaccharides to insoluble ligno-cellulosic materials and resistant starch [ ] as discussed previously. Basically, these compounds comprise varying numbers of monosaccharide units joined by glycosidic linkages. They differ according to the composition of the monosaccharides, the types of linkages, and the presence or not of branches on the backbone structure [ ].

From a nutritional perspective, Kumar et al. The solubility of polymers depends on several different factors and molecular properties, such as the conformational entropy [ 51 ]. This self-association tendency is strongest where the polymers can form side-by-side ribbon binding or co-axial multi-stranded helices, and tends to be more prevalent with less backbone substitution.

Broadly speaking, solubility of polymers seems to improve as polymer molecular structures become: i more branched and with a greater diversity of linkages, or ii smaller. High molecular weight coupled with solubility results in thickening of solutions [ 51 ]. Within the soluble DF, there are known to be substantial differences in their fermentabilities, with many of them promoting the proliferation of health-promoting bacterial species such as Bifidobacterium , Lactobacillus , and Eubacterium [ ].

These data suggest that the presence of AX led to a significant shift in the microbiota in the presence of a soluble DF. Purified soluble oligosaccharides have become very popular as potential prebiotics [ ] partly because they do not alter the viscosity or texture of foods due to their low molecular weight, and because they are usually highly fermentable.

However, they may be so readily fermentable that they may be completely utilized by the end of the terminal ileum [ 61 ]. It is to be recommended therefore, that they be fed in conjunction with more slowly fermentable DF, which can allow carbohydrate fermentation to continue in the LI [ 99 ].

Oligosaccharides are also found normally in many plant tissues in the form of fructans [ 51 ]. Plant foods known to contain fructans include cereal grains, onions, chicory, and Jerusalem artichoke. They are generally known to be soluble [ ], and are readily fermented by the GIT microbiota [ ]. Arabinoxylans on the other hand, are heteroxylans which are abundantly present in the PCW of cereals and grasses, particularly wheat, and also within the genus Plantago [ ].

Arabinoxylans are generally highly viscous in aqueous solutions. It is also considered to be highly fermentable as has been shown in vitro [ ] using an inoculum of pig faeces. Pectins are structural polysaccharides present within the primary cell walls of many fruits and vegetables, which are extractable into a soluble, viscous form. They have an extremely diverse structure, sharing some common features such as the presence of galacturonic acid in the polysaccharide backbone [ 51 ].

Previous in vitro studies using pig faeces have shown pectin to be highly fermentable, both in the presence of chyme [ ] and also using both adult and unweaned piglet faeces [ ]. Cellulose is a major structural component of PCW from almost all plant foods. It is highly insoluble in water, and cannot be degraded by human digestive enzymes, but is fermented to varying extents by gut bacteria particularly in ruminant animals [ ], and also in pigs [ 25 , ], and humans [ — ].

Within plant cell walls, cellulose is also cross-linked with otherwise soluble pectin or hemicelluloses, rendering them insoluble. The isolated plant cell walls from apples, carrots and onions contain cellulose and a fraction of pectin that cannot be removed by washing and is therefore insoluble, as shown in Fig.

All spectra are from samples with added water. The cell walls of many plants are also classified as insoluble, and vary greatly in their ability to be fermented.

At one extreme, the soluble and insoluble fractions of refined cereal flours or food products made from them, had essentially identical in vitro fermentation behaviour with a porcine faecal inoculum [ ]. Both fractions were mostly composed of AX, and while the insoluble fraction was difficult to extract, both had comparable fermentation characteristics once extracted. At the other extreme, the fibrous vascular tissue present in e.

A further example of insoluble fibre is resistant starch from certain uncooked starch granules [ 19 ]. Another type of resistant starch is that held within plant well walls. For example, starch within cells in banana, were slow to ferment as they were unavailable until the cell walls surrounding it had been fermented [ 18 ]. There are therefore examples of insoluble DF that are rapidly fermented e. This provides clear evidence that equating insoluble fibre with non-fermentable fibre is no longer a valid premise.

Modifications of some properties of DF may occur at the stage of mechanical processing such as the dehulling and milling of cereals [ 51 ] to make flour. Milling disrupts cell wall structure and alters particle size [ 51 ]. In terms of pig production, it is often wheat by-products, such as wheat bran, and wheat middlings which are important components of the diet.

Both of these products are higher in DF than the extracted flour [ ]. It is known to have a high level of insoluble lignified fibre, which is generally resistant to fermentation in the LI [ ]. It is well known that the chemical structure of starches can be markedly altered by heat treatments [ 51 ]. Additionally, cooking of plant tissues can also alter physical and chemical properties of PCW, such as cell separation and dissolution of the middle lamella, breakdown of pectins, and formation of cross-links between food components [ ].

Extrusion cooking has been shown to actually break PCW bonds, reducing insoluble fibre content and increasing soluble fibres [ ].

Raw plant tissues usually retain much of their cell-level integrity following mastication [ ]. Consequently, there will be less breakdown of PCW in the small intestine, and digesta viscosity will be lower, and less cell contents will be available for mammalian digestion.

However, upon reaching the LI, microbial fermentation can lead to a breakdown of the PCW, and consequent release of the cell contents for further fermentation. McDougall et al. For example, an in vitro study compared fermentability of chewed banana and mango tissue, and showed that differences in physical characteristics of the two plant tissues led to profound differences in the fermentability. In the study by Warren et al gelatinised starch within cell walls of cooked sorghum grains was still observed at the late stage of in vitro fermentation.

The molecular order was unchanged throughout the fermentation [ 19 ], as shown in Fig. The spectrum of cooked sorghum is dominated by starch.

In contrast, for carrots which were processed to obtain different particle sizes, larger particles cell clusters resulted in faster production of gas, and increased concentrations of SCFA after fermentation in vitro with a porcine faecal inoculum [ 33 ]. At least two possibilities could explain this. Firstly that junctions between cells, allowed bacteria to attach more readily to cells, allowing better access to the PCW, or secondly, that in the smaller particles, pectin between cells had been lost, and so this fraction was no longer available to be fermented.

Further studies are required to elucidate the mechanism responsible. The emphasis has been on work describing how the GIT microbiota e. Some of the characteristics of DF functionality arise directly from their molecular structure as determinants of the tendency to self-associate simpler, less-branched structures or ferment slowly complex, more-branched structures. However, much DF in both food and feed is in the form of plant tissue pieces.

In this case, the cellular structure results in both insoluble DF and encapsulation of cellular components, sufficient to prevent digestion and absorption in the small intestine. This phenomenon provides a mechanism for intracellular contents such as starch, protein and secondary metabolites to be made available for fermentation in the LI after passing through the SI intact.

Purified DF, such as oligo- or polysaccharides extracted from whole plant foods, are not necessarily representative of those whole foods, but do provide insights into potential mechanisms by which DF has its beneficial effects in the gut. The classification of potentially fermentable carbohydrates into soluble and insoluble, while helpful, is no longer enough for the information required to elucidate mechanisms by which DF has beneficial effects on monogastric health.

BW wrote the first draft. Barbara A. Deirdre Mikkelsen, Email: ua. Bernadine M. Flanagan, Email: ua. Michael J. Gidley, Email: ua. National Center for Biotechnology Information , U. J Anim Sci Biotechnol. Published online May Williams , Deirdre Mikkelsen , Bernadine M. Flanagan , and Michael J. Author information Article notes Copyright and License information Disclaimer.

Corresponding author. Received Dec 2; Accepted Mar This article has been cited by other articles in PMC. Associated Data Data Availability Statement Data sharing not applicable to this article as no datasets were generated or analysed for this paper. Abstract This review describes dietary fibres originating from a range of foods, particularly in relation to their plant cell walls.

Introduction Dietary fibre DF is considered essential for overall human health. Table 1 Dietary fibre- physico-chemical characteristics and relationships to gut effects modified from [ 9 ]. Open in a separate window. Plant foods as sources of dietary fibre Plant cell walls PCW are essential to maintain plant structure and function [ 8 ]. Fruits and vegetables In human dietary recommendations around the world, fruits and vegetables are recommended to form a substantial part of the daily diet, given their known health-promoting properties.

Cereals and legumes Cereal grains are the most widely consumed, and an important source of energy in global nutrition, both of humans and monogastric production animals. Gut microbiota- activities and communities The GIT microbiota includes the entire microbial population within the GIT, from the mouth to the anus. Fermentation of dietary macronutrients Dietary components remaining undigested at the end of the small intestine can potentially be fermented within the LI.

Carbohydrates Bacterial utilisation of fermentable carbohydrates results predominantly in the production of SCFA such as acetic, propionic and butyric acids, but a range of other carboxylic acids can also be produced, including lactic acid [ 76 ].

Proteins Protein fermentation refers to the bacterial breakdown of proteins to amino acids, as well as their further breakdown to ammonia and other potentially toxic compounds such as indoles, phenols, and amines [ 97 ]. Molecular structure Dietary fibre includes a wide range of mostly carbohydrate polymers ranging from soluble polymers such as pectins and various oligosaccharides to insoluble ligno-cellulosic materials and resistant starch [ ] as discussed previously.

Soluble DF The solubility of polymers depends on several different factors and molecular properties, such as the conformational entropy [ 51 ]. Effects of processing Fractionation Modifications of some properties of DF may occur at the stage of mechanical processing such as the dehulling and milling of cereals [ 51 ] to make flour.

Cooking baking, toasting, roasting, extrusion etc. But do you know why fiber is so good for your health? Dietary fiber — found mainly in fruits, vegetables, whole grains and legumes — is probably best known for its ability to prevent or relieve constipation. But foods containing fiber can provide other health benefits as well, such as helping to maintain a healthy weight and lowering your risk of diabetes, heart disease and some types of cancer. Selecting tasty foods that provide fiber isn't difficult.

Find out how much dietary fiber you need, the foods that contain it, and how to add them to meals and snacks. Dietary fiber, also known as roughage or bulk, includes the parts of plant foods your body can't digest or absorb.

Unlike other food components, such as fats, proteins or carbohydrates — which your body breaks down and absorbs — fiber isn't digested by your body. Instead, it passes relatively intact through your stomach, small intestine and colon and out of your body. Fiber is commonly classified as soluble, which dissolves in water, or insoluble, which doesn't dissolve.

The amount of soluble and insoluble fiber varies in different plant foods. To receive the greatest health benefit, eat a wide variety of high-fiber foods. The Institute of Medicine, which provides science-based advice on matters of medicine and health, gives the following daily fiber recommendations for adults:.

If you aren't getting enough fiber each day, you may need to boost your intake. Good choices include:. Refined or processed foods — such as canned fruits and vegetables, pulp-free juices, white breads and pastas, and non-whole-grain cereals — are lower in fiber. The grain-refining process removes the outer coat bran from the grain, which lowers its fiber content. Enriched foods have some of the B vitamins and iron added back after processing, but not the fiber.

Whole foods rather than fiber supplements are generally better. Fiber supplements — such as Metamucil, Citrucel and FiberCon — don't provide the variety of fibers, vitamins, minerals and other beneficial nutrients that foods do. Another way to get more fiber is to eat foods, such as cereal, granola bars, yogurt and ice cream, with fiber added. The added fiber usually is labeled as "inulin" or "chicory root.

However, some people may still need a fiber supplement if dietary changes aren't sufficient or if they have certain medical conditions, such as constipation, diarrhea or irritable bowel syndrome. Check with your doctor before taking fiber supplements. High-fiber foods are good for your health.