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Many of the health benefits attributed to fruits, vegetables, nuts, legumes, and whole grains are due to the way the fiber in these foods impact the gut microbiome.
Fiber is the quintessential example of a nutrient that isn’t labelled as essential but that is absolutely fundamental for our health. What separates fiber from other carbohydrates is that the ways the constituent saccharides link together are not compatible with our digestive enzymes; our bodies just aren’t capable of breaking apart those types of molecular bonds. Instead, fiber passes through the digestive tract mainly intact. And once it reaches the colon, the magic begins: fiber serves as a substrate (i.e., food) for the wide range of bacteria and other microbes living in our digestive tract, collectively referred to as our gut microbiomes!
Fiber Feeds the Gut Microbiome
The term gut microbiome refers to the massive collection of bacteria and other microbes that inhabit our gastrointestinal tract, and their metabolic byproducts. And “massive” is far from hyperbole: An estimated 30-100 trillion bacteria comprise the microbiota, collectively weighing around 4.5 pounds! These bacteria include a mixture of commensal (neutrally existing), probiotic (mutually beneficial, also called symbiotic), and pathogenic (harmful to us) organisms, and can consist of any of 35,000 species known to inhabit the human gut. Every person’s gut contains approximately 400 to 1,500 different bacterial species of the possible 35,000 that are well adapted to survive in the human gastrointestinal tract, although thirty to forty species of bacteria will dominate an individual’s gut microbiota, accounting for about 99% of the microorganisms present in our gut.
We have a symbiotic relationship with our gut microbes—we are the host and they are symbionts. In exchange for food and shelter, they contribute to our wellbeing via metabolic, trophic (digestion and nutrient absorption) and immunologic functions that exceed our own physiological capabilities. In fact, the gut microbiome can be considered a virtual organ, owing to the fact that our health is dependent on its activities.
Our gut microbes perform many different essential functions that help us to stay healthy. These include digestion, vitamin production, detoxification, regulation of cholesterol metabolism, providing resistance to pathogens, immune regulation, neurotransmitter regulation, regulation of gene expression, and more! In fact, every human cell is impacted by the activities of our gut microbes. A healthy gut microbial community is essential for our health. And, the converse is also true: An aberrant gut microbiome has been linked to conditions as wide-ranging as cancer, obesity, diabetes, cardiovascular disease, anxiety, depression, neurodegenerative diseases, autism, autoimmune disease, ulcers, IBD, liver disease, gout, PCOS, osteoporosis, systemic infections, allergies, asthma, and more! A healthy diversity of the right kinds of microorganisms in the gut is one of the most fundamental aspects of good health.
The composition and metabolic activity of the community of microbes in our guts is influenced greatly by the foods we eat, most notably the amount and types of fiber.
Health Benefits of Fiber
Diets rich in fiber reduce the risk of many cancers (especially colorectal cancer, but also liver, pancreas, and others) and cardiovascular disease, as well as lower inflammation overall. Prospective studies have confirmed that the higher our intake of fiber, the lower our inflammation (as measured by C-reactive protein). In fact, a recent study showed that the only dietary factor that correlated with incidence of ischemic cardiovascular disease is low fiber intake (not saturated fat!); the more fiber we eat, the lower our risk. If someone has kidney disease, a high-fiber diet reduces their risk of mortality. If someone has diabetes, a high-fiber diet reduces their risk of mortality. A high fiber intake can even reduce the chances of dying from an infection.
Many of the health benefits attributed to fruits, vegetables, nuts, legumes, and whole grains are due to the way the fiber in these foods impact the gut microbiota. However, fiber has other effects, too, like regulating peristalsis of the intestines (the rhythmic motion of muscles around the intestines that pushes food through the digestive tract), stimulating the release of the suppression of the hunger hormone ghrelin (so we feel more full), and slowing the absorption of simple sugars into the bloodstream to regulate blood sugar levels and avoid the excess production of insulin. Fiber also binds to various substances in the digestive tract (like hormones, bile salts, cholesterol, and toxins) and, depending on the type of fiber, can facilitate either elimination or reabsorption (for the purpose of recycling, which is an important normal function for many substances like bile salts and cholesterol), both of which can be extremely beneficial—if not essential—for human health.
Problems From Low-Fiber Diets
Given how important fiber is for supporting the gut microbiome, it’s no surprise that low-fiber diets have been associated with a wide range of health problems, including heart disease, diabetes, colorectal cancer, and a variety of gastrointestinal illnesses (such as constipation, IBS, ulcerative colitis, and hemorrhoids). Scientists have been exploring the mechanisms behind how low-fiber diets exert their effects on disease risk via the gut, helping us understand exactly what happens when our microflora get deprived of this important food source!
Even for conditions where low-fiber diets are sometimes advised, such as Crohn’s disease, the wisdom of reducing fiber isn’t always backed up by science. In one study of 1619 patients with Crohn’s disease or ulcerative colitis, people who avoided high-fiber foods had a significantly higher risk of disease flares than people who didn’t reduce their fiber intake. Likewise, a randomized, controlled trial involving Crohn’s disease patients found that the group consuming a high-fiber diet reported significantly better health-related quality of life and gastrointestinal function than the lower-fiber control group.
Types of Fiber
Most of our dietary fiber intake comes from the cell walls of plants, where it acts like a skeleton and helps to maintain the plants’ shape and structure. However, special forms of fiber are also found in edible insects and fungi. There are many, many different types of fiber consisting of different length of carbohydrate strings composed of different saccharides, some with branches and some without.
Fiber can be broadly categorized as either soluble or insoluble:
Soluble fiber forms a gel-like material in the gut and tends to slow the movement of material through the digestive system. Soluble fiber is typically readily fermented by the bacteria in the colon, producing gases and physiologically active by-products (like short-chain fatty acids and vitamins).
Insoluble fiber tends to speed up the movement of material through the digestive system. Fermentable insoluble fibers also produce gases and physiologically active by-products (like short-chain fatty acids and vitamins). Insoluble fibers, such as cellulose and hemicellulose, are typically fermented more slowly by gut bacteria, and consequently move through the colon and add bulk to the stool (which is beneficial for regulating bowel movements and managing constipation).
Depending on the food in question, some foods have more insoluble fiber types and some have more soluble. While soluble fibers have the reputation of being the fermentable fibers, all types of fiber are fermentable at least to some extent by our gut microbiota.
Fiber can be more sophisticatedly classified based on the molecular structure. The major classes of fiber are as follows:
Cellulose is the main component of plant cell walls. Celluloses are identical to starch in the sense that they are long straight chains of glucose molecules (anywhere from several hundred to over ten thousand glucose molecules long), however the links between the glucose molecules are different than starch (they are in what is called a beta configuration), which make cellulose indigestible to humans. Celluloses are insoluble dietary fibers.
Our gut bacteria cannot ferment most cellulose particularly well (although cellulose is partially fermentable); however, cellulose is still very beneficial! One study showed that when combined with resistant starch, cellulose from wheat bran was able to shift resistant starch fermentation throughout a greater portion of the gut, nearly doubling the amount of resistant starch being fermented between the proximal colon and feces, and significantly increasing the concentration of butyrate in the distal colon. (Without the cellulose, the resistant starch was rapidly fermented in the caecum and proximal colon, leaving little left for further down in the colon.) A major benefit of this is the increased delivery of butyrate to regions of the colon where tumors commonly occur.
Cellulose is found in all plants, but foods that contain particularly large amounts of cellulose include bran, legumes, nuts, peas, root vegetables, celery, broccoli, peppers, cabbage and other substantial leafy greens like collards, and apple skins.
Hemicellulose is a common component of the cell walls of plants. In contrast to cellulose, hemicellulose is made of several types of sugar in addition to glucose, especially xylose but also mannose, galactose, rhamnose, and arabinose. Rather than forming long straight chains like cellulose, hemicellulose may have side chains and branches. Because of these variations, some hemicelluloses are soluble in water and some are insoluble, plus some forms are fermented by bacteria while others are not. Some non-digestible oligosaccharides (which are similar to resistant starch in that they resist digestion in the small intestine but can be degraded by bacteria in the colon) are classified as hemicellulose, such as the galacto-oligosaccharides present in legumes.
Hemicelluloses encompass a wide variety of fibers with a variety of roles in shaping the gut microbiota! Hemicellulose in the form of oligosaccharides appear to enhance levels of the extremely important probiotics Bifidobacterium and Lactobacillus. A systematic review and meta-analysis of 64 studies found that galacto-oligosaccharide consumption (along with fructan consumption) was associated with significantly higher levels of bifidobacteria and lactobacilli compared to other fiber types.
Hemicellulose in general is particularly high in bran, nuts, legumes, and whole grains, as well as many green and leafy vegetables.
Pectin is soluble in water and highly fermentable (very little passes through to the colon, since it is so readily fermented by bacteria in the small intestine). Pectins are rich in galacturonic acid and can be found in several types of configurations (further subdividing this class of fibers by structure).
As with many fibers, the source of pectin (and subsequently, minor variations in its structure) matters in terms of usability by gut microbiota! For example, the extremely beneficial Faecalibacterium prausnitzii has been shown to grow on pectin derived from apples, but not pectin derived from citrus. In rats, a diet containing 6.5% citrus pectin enhanced the abundance of Bacteroides species, while a different study showed that a diet containing 7% apple pectin decreased the abundance of Bacteroides (while also decreasing levels of Alistipes and increasing levels of species belonging to Anaeroplasma, Anaerostipes, and Roseburia). The best way to benefit from pectin is to consume a wide range of food sources of it.
Pectins are found in all fruits and vegetables but are particularly rich in certain fruits, including apples and citrus fruits, and are also found in legumes and nuts.
Lignin is a type of fiber with lots of branches made of chemicals called phenols (rather than sugar molecules). Phenols are currently being studied for a variety of health-related effects, including antioxidant actions (for example, it is the phenolic compounds in olive oil that appear to be responsible for its cardiovascular health benefits). Lignin is unusual because it lacks an overall defining structure. Instead, it consists of various types of substructures that appear to repeat in a haphazard manner. Lignins are insoluble and can be metabolized by the gut microbiota into various bioactive metabolites, such as enterolignan. One study used lignin-rich fractions from Brewer’s Spent Grain and found that, using an in vitro metabolic model of the colon, the microbiota were able to successfully metabolize the lignin into several phenolic compounds without inhibiting the growth of beneficial microbes.
Most commonly a component of wood, food sources include root vegetables, vegetable filaments (like the stems of leafy greens and the strings in celery), many green, leafy vegetables, wheat, and the edible seeds of fruit (such as berry seeds and kiwi seeds).
Chitin is similar to cellulose in the sense that it is made of long chains of glucose (in the case of chitin, it’s actually long chains of a particular derivative of glucose called N-acetylglucosamine) and also has amino acids attached. Chitins are insoluble in water and are fermentable, albeit weakly.
In studies, chitin from mushrooms and insect exoskeletons has been shown to support the growth of important probiotic species from Bifidobacterium (including Bifidobacterium animalis), Lactobacillus, Akkermansia, and Bacteroides genera while also decreasing the abundance of the inflammatory microbe Desulfovibrio. In mice, chitin oligosaccharides (produced from chitin digestion) are also able to modulate the gut microbiota to combat diet-induced metabolic syndrome in mice, inhibiting the destruction of the gut barrier, restoring the Firmicutes to Bacteroidetes ratio to what it was before high-fat feeding.
Chitin is interesting because this fiber is found not only in plants and fungi but also in the exoskeletons of insects and in the shells of crustaceans.
Chitosan is similar to chitin in the sense that is composed of a long chain of N-acetylglucosamine molecules, but it also contains randomly distributed D-glucosamine molecules (like cellulose, linked in a beta configuration). Chitosan is a very unique fiber. It is soluble in acidic environments so it starts its journey through the digestive tract as a soluble fiber in the stomach, but when the acidity of the chyme (stomach contents) is neutralized in the small intestine (by pancreatic secretions), it becomes insoluble. It is also fermentable (much moreso than chitin).
In mice, chitosan increases gut microbial diversity (along with a general increase in Bacteroidetes and a decrease in Firmicutes) and decreases levels of potentially pathogenic genera Escherichia and Shigella. In diabetic mice, chitosan has also been shown to reshape the microbiota to induce an anti-diabetic effect, relieving dysbiosis by raising levels of the extremely important probiotic Akkermansia muciniphilia and suppressing the growth of Helicobacter.
Chitosans are naturally found in the cell walls of fungi but are also produced as a functional fiber by treating shrimp and other crustacean shells with sodium hydroxide.
Gums are a diverse group of fibers that plants secrete when they are damaged. They are very complex molecules that contain a variety of types of sugars as well as acids, proteins, and minerals. Gums are soluble and highly viscous fibers and are also fermentable. Isolated (functional fiber) versions are used in food manufacturing as thickening and gelling agents (like guar gum and xanthum gum). Some gums used in food manufacturing increase intestinal permeability through an action on the tight junctions between epithelial cells (one of those cases of the isolated concentrated compound being a problem but the small amount naturally occurring in whole foods being fine).
Various gums have demonstrated prebiotic properties in studies. In a study using human fecal samples, partially hydrolyzed guar gum was able to stimulate the growth of Parabacteroides, a bacterial genus inversely associated with IBS and ulcerative colitis. An in vivo human study also showed that partially hydrolyzed guar gum stimulated the growth of probiotic Bifidobacterium and other beneficial butyrate-producing bacterial strains.
β-glucans (more technically β(1,3)-glucans) are closely related to gums and are also soluble (a minority are insoluble), viscous and fermentable.
β-glucan feeds anaerobic microbes in the gut and can significantly increase levels of butyric and propionic acids. Grains rich in β-glucan, such as barley, have been shown to increase levels of extremely important probiotic Bifidobacterium, Lactobacillus, Dialister, and Roseburia species. In an extensive review of the health effects of β-glucan, researchers concluded that this fiber’s actions upon the gut microbiome, including enhancing the production of short-chain fatty acids, contributes to its anti-cancer, anti-inflammatory, anti-diabetic, and immune-modulating effects.
β-glucans are found in some grains (mainly oats and barley, but also rye and wheat), fungi (yeast and mushrooms, particularly those mushrooms that are used medicinally like shiitake and maitake), and some types of seaweed (mainly algae). β-glucans are the fiber in oats that are largely responsible for their unique health benefits among grains.
Mucilages are rich in the simple sugars xylose, arabinose, and rhamnose and have very complex structures. They are soluble and very viscous fibers, forming a thick gluey substance, and are produced by nearly all plants and some microorganisms. They can also be found in relatively large amounts in a variety of fruits and vegetables, including plantains, bananas, taro root, cassava, and berries. While soluble, mucilages are not particularly fermentable (only partially degraded by bacteria in our digestive tracts).
Mucilage has been shown to act as a prebiotic, enhancing the growth of the very important probiotic Lactobacillus, increasing short-chain fatty acid production, and reducing the population of harmful species of bacteria.
Mucilages are particularly concentrated in cacti and other succulents (like aloe), many types of seaweed (like agar agar algae), flax, chia and psyllium.
Fructans are fructose-rich soluble and highly fermentable fibers with simple structures (long chains, some with branches–like the fructose equivalent of cellulose). Shorter chain fructans are called fructooligosaccharides, whereas longer chain fructans are called inulins.
In a systematic review and meta-analysis of 64 different fiber studies, fructan consumption (along with galacto-oligosaccharide consumption) was associated with significantly greater abundance of the very important probiotics Bifidobacterium and Lactobacillus compared to other fiber types. The microbial effects of inulin have also been studied extensively, with various research showing that inulin increases levels of the beneficial butyrate producing bacteria.
Inulin fiber is one of the most heavily studied functional fibers. They are naturally occurring in a variety of plants including chicory, onions, and Jerusalem artichoke.
Resistant starch is really starch and doesn’t fit the original technical definition of fiber, which was limited to plant cell wall constituents. Resistant starch is considered to be a fiber because amylase, the enzyme that breaks starch into individual glucose units, doesn’t work on this type of starch. Resistant starch is insoluble yet highly fermentable, and plays a major role in the generation of short-chain fatty acids by gut bacteria. Further, resistant starch is divided into four subgroups: RS1, which is physically inaccessible due to being bound within cell walls; RS2, which is tightly packed ungelatinized granules found in certain raw starchy foods; RS3 (retrograded amylose), which is formed when certain starchy foods are cooked and then cooled down; and RS4, which is formed via a manmade chemical process.
In general, resistant starch is famous for feeding short-chain fatty acid-producing bacteria and enhancing levels of butyric acid. But different forms of resistant starch can have significantly different impacts on the gut microbiome, due fermentation preferences of various bacteria and variations in the speed of fermentation of the resistant starch subclasses.
Sources of resistant starch include grains, legumes, and seeds for RS1; green bananas, green plantains, and raw potato starch for RS2; cooked and cooled potatoes and cooked and cooled rice for RS3; and enzymatically or chemically modified starches sold under various brand names for RS4.
Benefits of Whole Food Fiber Sources
Even among fibers of the same class, differences in the fine-level structure can impact the interaction with the microbiota. For example, the cellulose in an apple peel is different than the cellulose in cabbage, and this may have a slightly different effect in your digestive tract.
Researchers have pointed to the idea of unique chemical arrangements called “discrete structures” that exist within fiber molecules, and which align with encoded gene clusters in bacterial genomes. These discrete carbohydrate structures (which likely number in the thousands!) provide nutrient niches for bacteria, triggering the expression of bacterial enzymes depending on the “cues” contained within the unique fiber structure. The huge number of potential fiber structures is due to a combination of basic differences in fiber types, variations in genotype and growing environment of the plants, subsets of structures within polymers, linkage types, alterations in fiber taken from different anatomical parts of the plant, carbohydrate chain length, and particle size—among other factors! As a consequence, fibers that vary even in seemingly minor ways might favor fermentation by bacteria down to the strain level, creating highly complex interactions.
For this reason, it is vastly preferable to get your dietary fiber from whole and minimally-processed foods, which naturally contain a wide diversity of fiber types that feed a wide diversity of gut microbes. When a single fiber type is isolated and used as a supplement or food additive, you miss out on the ability to support a diverse community of microbes within your gut.
How Much Fiber Do We Need?
The recommended dietary intake for fiber is 14 grams per 1000 kcal. So, if you eat a 2,000 calorie per day diet, aim for at least 28 grams of fiber. Fiber is one of those “the more, the better” nutrients. For example, a 2015 meta-analysis calculated that, for every 10 grams of daily fiber you consume, risk of all-cause mortality decreased by 10%.
That being said, if you’re looking to up your fiber intake, it’s best to do so gradually! This is because a sudden increase in fiber intake can cause a variety of unpleasant symptoms, such as gas, bloating, constipation and diarrhea—your gut microbiome needs time to adjust to your dietary changes. It’s recommended to step up your fiber intake weekly, giving your digestive tract a full week to adjust to each incremental increase, until you hit your target fiber intake levels—the recommended amount to step up each week is 5 grams per day. So, if you’re starting out at 10 grams of fiber daily, go up to 15 grams for all of next week, the week after go up to 20 grams for the whole week, and the week after that, go up to 25 grams for the whole week. And, if a 5-gram incremental increase causes gastrointestinal symptoms, it’s okay to take smaller steps and give more time between each increase. It’s also helpful to make sure your fiber is coming from a variety of sources (not from supplements or fiber-fortified meal replacement products like protein bars) and that you’re incorporating some fermented foods, like sauerkraut, yogurt, kefir and kombucha. When in doubt, consult your doctor.
Good Food Sources of Fiber
The following foods are also excellent or good sources of fiber, containing at least 2.8 grams per serving.
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