Dietary fiber—or, more properly, dietary fibers, collectively—constitute a topic of greater health interest than ever, from the perspective of new and ongoing medical research; as protective elements of the diet; and from a formulation perspective. The latter concerns those ingredients in today’s better-for-you food and beverage products that have the potential to benefit health and enhance the organoleptic properties of the food. For many manufacturers, fiber simply is a component that contributes bulk to food. For consumers, fiber is an ingredient they vaguely recognize that aids the end stage of digestion.
According to the USDA, via data from the “National Health and Nutrition Examination Survey (NHANES) 2010,” the “mean dietary fiber intake of all individuals 2 years and older, excluding breastfed children” averages 16g per day, with intakes of males and females at 18g and 15g, respectively. African Americans have “significantly lower dietary fiber intake” at 13g, compared to Caucasians (16g) and Hispanics (17g). Yet the latest dietary guidelines set the average daily intake for fiber to 14g/1,000kcals, which translates to 25g/day for women and 38g/day for men.
The more detailed aspects and advantages of the many individual forms of fiber often are lost on both consumers and processors. For example, a shopper might see the word “inulin” on the side of a kefir carton and not know that it is considered a soluble fiber or that it has a number of health-promoting properties. (It also might have been included to enhance the creamy texture of the product itself.)
The reason for this differing perspective might be that fiber, as a topic, is presented as one item. The media generally refer to high-fiber diets or high-fiber foods without much specificity. Even food labels simply list one component—fiber.
Recommendations for dietary fiber still are relatively new and evolving as research continues to shed light on different fiber topics. Fiber as a health issue came to the attention of the general public in 1971 with the publication of the book, Don’t Forget Fibre in your Diet, by British physician Denis Burkett, MD.
The book became an international best seller, presenting its “fiber hypothesis” that stated that colon cancer was a deficiency disease and that dietary fiber was the deficient nutrient.
Burkett arrived at this conclusion as a result of his work in Uganda, where he compared the digestive health of British soldiers stationed there to that of the native Ugandan people. The native diet was far richer in dietary fiber than that of the British soldiers and, correspondingly, the digestive health of native people was greater, including lower rates of colon cancer.
Although some later studies failed to support a strong direct connection between fiber and colon cancer risk reduction, fiber was now on the map for two good reasons: The first is that Burkett himself was a well-known physician and researcher who first described a cancer common to children in Africa, now named Burkett’s lymphoma.
The second reason is that further research from multiple sources did support the lower risk of many types of cancer and other diseases with a fiber-rich diet that contained staples of fruits, vegetables, and whole grains.
A great deal of research has gone into defining exactly what is meant by the term fiber, and how different fibers might act in the prevention of, or at least lowered risk of, diseases like cancer, cardiovascular disease, and diabetes—as well as conditions that directly affect the digestive system.
In general, the term fiber refers to complex carbohydrates and other plant food components that contribute to the structure of the plant and cannot be broken down by human digestive enzymes to release sugar for use as energy.
This is a very large group of food components. In fact, it’s easier to define which carbohydrates are not classed as fiber. That’s simple: sugars and starches. Sugars, like glucose, fructose, and galactose, can be used for energy. These very simple carbohydrates, called monosaccharides (one sugar) can be coupled to form disaccharides (two sugars). For example, glucose and fructose make sucrose, aka, table sugar. Glucose and galactose form lactose, or milk sugar.
Disaccharides are broken into their constituent monosaccharides in preparation for absorption. Only one polysaccharide (many sugars) can be used for energy, and that is starch. A starch consists of hundreds or more glucose molecules linked together in chains. In the plant, starch is an important storage form of energy. For people, it’s the most dominant fuel on the planet. The glucose molecules are separated systematically before absorption.
Fiber consists of all the other carbohydrates, the ones that cannot be broken into constituent sugars and used for energy, along with “woody” plant-cell components like lignin and substances like chitins, found in fungi and that form the exoskeletons of insects. Cellulose, like starch, is made from very long chains of glucose molecules. However, the glucose units are linked together in a manner that does not allow the human digestive system to break them apart.
Humans can’t get useful energy from grass; cows can, because they have a different digestive system. The bottom line is that if a food component, generally carbohydrate, cannot be broken down to a monosaccharide and absorbed in the small intestines, it is transported to the large intestines, mostly intact—where it is considered fiber.
Fibers typically have been divided into two primary types, insoluble fibers—those that don’t bind water—and soluble fibers, those that do bind water to form a gel. While these two important characteristics affect the action of fibers in the digestive tract, this classification is falling out of favor due to the reality that most fiber forms overlap in these characteristics.
A more accurate classification is presented in a recent article, “New Horizons for the Study of Dietary Fiber and Health: A Review,” published in Plant Foods for Human Nutrition. The study divides dietary fibers into four basic yet different categories:
- High-molecular weight dietary fiber, which spans both soluble and insoluble fibers and includes cellulose, non-starch polysaccharides, cell-wall components made of various sugars referred to as hemicellulose (cereal grains), pectin, beta-glucan, gums, and mucilages (from fruits, vegetables, legumes, and nuts). Lignin, which technically is not a polysaccharide, also is in this category. It is derived from the outer layers of cereal grains.
- Low-molecular weight dietary fiber, such as resistant oligosaccharides. These are polymers around 3-10 sugar units long, and include galacto-oligossacharides (GOS), raffinose, stachyose, and verbascose, as well as inulin, a naturally-occurring polysaccharide derived from chicory root and artichokes, among other plants.
- Resistant starch. Resistant starches are polysaccharides that, for one reason or another, resist digestion. These, too, are not true fibers. They are starch forms that, in processing, function in much the same manner as a flour or starch; yet in the body, they behave like fiber, improving digestion and immunity.
- Synthetic analogues of polysaccharides. As evident by their name, these are man-made derivatives of cellulose and indigestible dextrins. The best example of these is polydextrose, a common food ingredient used to lend bulk to low- and zero-calorie sweeteners, which allows them to be used effectively in baking or other formulations.
For the purposes of describing fibers for food product development and manufacturing, the original two classifications still can be applied.
Insoluble fibers, best characterized by wheat bran and cellulose in vegetables, tend to reduce transit time through the colon—in effect, improving digestion. This, of course, remains one of the first things that comes to mind in consumers when dietary fiber is mentioned. But there is a subtler and equally important effect of insoluble fibers, and it relates to the structure of the colon.
The primary function of the colon is to absorb water, which is why the walls of the colon are relatively thin and weak compared to the walls of the small intestines.
There is a biological downside to this adaptation, in that the thin-walled colon is susceptible to pressure build-up from the inside that can form outpouching, a thinning and pushing out of portions of the colon wall, referred to as diverticulosis.
These deformations of the colon wall, called diverticula, can become infected, leading to diverticulitis, an inflammation that places the patient at a greater risk for perforation of the wall and a medical emergency. By reducing transit time in the colon, insoluble fibers prevent the pressure build-up against the walls.
The so-called soluble fibers, or fermentable fibers, bind water, which increases the viscosity of the food and slows its movement through the small intestines. This has the immediate effect of delaying the entrance of sugar into the blood; in other words, it lowers glycemic response of any food rich in these fibers. Much of the research in this area has been done on compounds like arabinoxylan, beta-glucans, oligosaccharides, synthetic carbohydrate analogues (such as dextrins), and resistant starches.
One of the most noted functions of soluble fibers is their ability to lower cholesterol by preventing the recirculation of bile. The gall bladder has one task: to squirt bile into the small intestine when fat is in the meal. Bile is an emulsifier for fat, meaning that it allows the fatty remnants of the meal coming from the stomach to interact with the fat-digesting enzymes secreted by the pancreas.
When soluble fibers from the diet are present in the mix, they bind the bile and prevent its re-absorption. Since bile is made from cholesterol, the loss of bile forces the liver to make more, which it takes from the bloodstream, thus reducing plasma cholesterol. This mechanism is generally true for most soluble fibers, although the role of fibers such as beta-glucan have been extensively studied.
According to the FDA and European Food Safety Authority (EFSA), at least 3g/day of oat or beta-glucan is needed for a reduction in blood cholesterol levels, especially lower low-density lipoprotein (LDL) cholesterol levels, sufficient to produce a decrease in the risk of coronary heart disease. Other fibers that have been shown to LDL cholesterol at similar doses include pectin, psyllium, and guar gum.
From a food processing perspective, the ability to bind water makes many soluble fibers perfect for adding texture and mouthfeel to foods. This is especially valuable when the goal is to lower fat or sugar in the formulation. Inulin is a popular fiber used to enhance the texture of dairy products, like kefir and many other processed foods. It also tends to increase the absorption of minerals, such as calcium and magnesium.
Chain of Fructose
Inulin is a general term that refers to a group of plant-storage carbohydrates composed mostly of fructose. Oligofructose fibers, also known as
fructooligosaccharides (FOS), are a subgroup of inulin. They consist of short chains of fructose molecules, usually fewer than 10 fructose sugars long. They are a naturally-occurring soluble fiber found in chicory root, onions, garlic, bananas, and wheat. Since they are not digested in the small intestines, neither causes a rise in blood glucose, nor a corresponding insulin response.
Commercially available inulin and its derivatives may substitute for fat, sugar, of flour, while at the same time contribute fiber with its variety of health benefits. Certain proprietary blends of inulin have been shown in recent studies to aid prediabetics in weight loss, fat loss (including fat loss from the liver), and blood glucose control.
These and other prebiotics can be extracted from plants or created by controlled manipulation of polysaccharides using various enzymes. Oligofructose-enriched inulin is made by combining these fibers into proprietary blends adapted to the needs of the food processor for taste, texture, and stability of the final product.
Both inulin and oligofructose stimulate the growth of the Bifidobacteria family of beneficial intestinal bacteria once they reach the colon. (See sidebar “More Food for Thought.”) Just how bacteria in the gut can have a positive effect on so many different conditions is the subject of decades of past and ongoing research. For example, studies have shown that supplementation with both oligofructose and inulin increases bifidobacteria in the colon, leading to a potentially healthier microflora. One possible mechanism is increasing the colonization of favorable bacteria that compete with pathogenic microorganisms.
Inulin supplementation also has been shown to increase lactobacilli bacteria. Both lactobacilli and bifidobacteria feed off of fermentable soluble fibers, releasing short-chain fatty acids that can feed the colon cells and possibly have other health benefits. In fact, much of the soluble fiber in the colon is naturally fermented to form short chain fatty acids.
GOS and RS
Galactose is a monosaccharide that, along with glucose, makes up lactose—milk sugar. Galacto-oligosaccharides (GOS) can be produced through the enzymatic conversion of lactose. A recent GOS mixture, produced using a proprietary enzyme system derived from the Bifidobacteria bifidum strain, has been shown in research studies to be highly selective toward promoting beneficial gut bacteria. This bifidobacterium also is the predominant species in the gut of healthy breast-fed infants.
The result is a galacto-oligosaccharide that is similar in structure to human milk oligosaccharides. Studies have shown that GOS ingestion could positively affect immune markers of inflammation; reduce total cholesterol and blood triglycerides; and lower the ratio of total to HDL cholesterol.
Resistant starch is another soluble fiber that that has been well-studied, revealing multiple health benefits. While starch is a major source of energy in the modern diet, a portion of the starch in any meal resists digestion in the small intestines.
There are four types of resistant starch. RS1 is physically inaccessible, due to the structure of the seed, legume, or whole grain where the starch is found.
RS2 resists digestion, due to the conformation of the starch molecule itself. There are two types of starch molecules in any starch-containing food. These are amylose and amylopectin. The former is a relatively linear chain of glucose molecules, while the latter is highly branched.
Since starch-digesting enzymes begin at one end of the chain and work in a “Pac-man” fashion, amylose has naturally fewer starting points for the enzymes to work on, leaving more of the starch undigested. High-amylose ingredients, such as high-amylose maize, are excellent sources of RS2.
RS3 is starch that has gelled subsequent to cooking to its gelatinization point in heated water. When the product is cooled, as in pasta and potatoes in a salad, a greater amount of the starch is inaccessible to digestion. RS4 is starch that has been chemically altered to resist digestion.
Functionally, resistant starch acts as soluble fiber. Resistant starch also is found in potatoes, slightly green bananas, rice, barley, wheat, and other plant sources.
Resistant starch plays a role in increasing satiety. Studies that have compared different fibers have shown two types of resistant starch (types RS2 and RS3) were highly satiating to participants. This is consistent with the satiety index that rates boiled potatoes at the top of the list, when it comes to foods that promote satiety. It must be noted that boiled starch contains far more digestible starch than resistant starch.
Digestible starch is itself highly satiating. It does point out, however, that the case against potatoes being “fattening” is poorly thought out, especially since the vast majority of commercially cooked potatoes are high in fat from the cooking medium (think: French fries).
Starch is both a powerful energy source and of great benefit, even when is resists digestion. Fibers both insoluble and soluble in several categories have been linked epidemiologically and clinically to healthy weight control. Other fibers have demonstrated antioxidant capacity.
Although researchers are far from any definitive understanding of the relationship between resistant starch or other fibers and cancer, the epidemiological evidence that relates high-fiber diets to reduced risk of cancer is very encouraging. Some fibers have even shown to go beyond mere protection against cancer. For example, pectin and some saccharides have been shown to actually induce cancer cell death in in vitro culture studies.
Resistant starch also is thought to play an anti-cancer role by increasing intestinal bulk and reducing the concentration of potential carcinogens. While a direct link between lack of fiber and colon cancer still is not definitively established, there remains a great body of research that supports the role that some fiber types could play in reducing the risk of certain types of cancer.
The subject of fiber and the gut microbiota is highly complex. And the average consumer is not expected to learn or remember all the metabolic and biochemical details. In that case, fiber can come to the rescue.
A recent study in the journal Nutrients examined at the acute effects of consumption of oligofructose-enriched inulin on mood and cognition. The results showed that after consumption of inulin, volunteers felt happier, had less indigestion, and were less hungry. The most consistent behavioral effect was an improvement of episodic memory (recall and recognition). This is definitely high-fiber food for thought.
Originally appeared in the October, 2016 issue of Prepared Foods as Fiber Facts & Fiction.
Fruit fibers have a long history of being healthful. But they have great appeal for processors making better-for-you products. Multi-functional, clean label fiber ingredients derived from citrus peel are finding increased favor with developers who favor them for their unique water-binding capacities and their ability to boost product shelflife, yield and moisture retention in a variety of products, from bakery items to meat and dairy. Citrus fibers have excellent emulsification properties and have been used successfully in formulations calling for fat or egg replacement and phosphate reduction.
One of the main ways dietary fiber apparently helps counter obesity is through boosting satiety, the tendency to delay eating after a meal is finished. There are several possible mechanisms. The viscous nature of soluble fibers in the gastrointestinal tract, in addition to physically swelling and filling up the stomach, also contributes to slowing the glycemic response. This has been shown to prolong the elevation of cholecystokinin, a hormone that stimulates the release of pancreatic enzymes, thus prolonging satiety. The presence of fibers in the gut also stimulate the release of a peptide called peptide YY (PYY), which has been demonstrated to reduce appetite. Some fibers, such as resistant starch and others, also have been shown to trigger other chemical reactions that give the sense of fullness and reducing hunger—in some cases, from dinner through until breakfast the following day.
Decades of research still have revealed what might be only the tip of the iceberg with respect to the health benefits of fiber. The healthy bacteria that populate the intestines are collectively known as probiotics. These probiotics (and there are many different types) generally feed on soluble or fermentable fibers. When the soluble or fermentable fibers function as probiotic food, they are collectively known as prebiotics.
The probiotics that thrive off these prebiotic fibers have become so
interesting to the research world that they have taken on their own, holistic designation known as the gut microbiota. A second term commonly used is microbiome, although some scientists use that term to describe the “collective genomes of the microorganisms that reside in an environmental niche,” and use microbiota to refer “to the microorganisms themselves.”
Microbiota are composed of mostly bacteria, plus some strains of fungi, protozoa, yeasts, viruses, and bacteriophages. The interactions of these microorganisms are not fully understood, but there is ample evidence to suggest they play an indispensible role in human health. Just one example is the fact that healthy gut bacteria synthesize about half of the vitamin K the body needs.
This is so important that infants are given injections of vitamin K to cover them until their new, sterile intestines can produce the bacteria necessary to synthesize the vitamin. In addition, gut microbiota are believed to play a vital role in normal digestive functions; maturation of human immune system to defend against pathogens; and possibly contribute to brain development.
A healthy gut microbiota might be associated with the reduced pathogenesis of many diseases, including infectious diseases, allergy or asthma, inflammatory bowel disease, diabetes, and colon cancer. They even have been implicated in helping balance mood and emotional well-being.