Fiber is a type of carbohydrate that the body can’t digest. Though most carbohydrates are broken down into sugar molecules called glucose, fiber cannot be broken down into sugar molecules, and instead it passes through the body undigested. Fiber helps regulate the body’s use of sugars, helping to keep hunger and blood sugar in check. Show Children and adults need at least 25 to 35 grams of fiber per day for good health, but most Americans get only about 15 grams a day. Great sources are whole grains, whole fruits and vegetables, legumes, and nuts. Types of FiberFiber comes in two varieties, both beneficial to health: Soluble fiber, which dissolves in water, can help lower glucose levels as well as help lower blood cholesterol. Foods with soluble fiber include oatmeal, chia seeds, nuts, beans, lentils, apples, and blueberries. Insoluble fiber, which does not dissolve in water, can help food move through your digestive system, promoting regularity and helping prevent constipation. Foods with insoluble fibers include whole wheat products (especially wheat bran), quinoa, brown rice, legumes, leafy greens like kale, almonds, walnuts, seeds, and fruits with edible skins like pears and apples. Further defining fiberUnder the umbrella terms of insoluble and soluble fibers, you may see fiber described in other ways. It can be viscous with a gel-like quality, or fermentable because it acts as food for gut bacteria that break down and ferment it. Fibers that are not broken down by bacteria, called nonfermentable, travel intact to the colon and can add bulk and weight to stool so it is easier to pass. These properties offer health benefits such as slowing down digestion, delaying blood sugar rises after meals, promoting healthy colonies of bacteria, or having a laxative effect. In addition, there are many subtypes of soluble and insoluble fibers, some of which occur naturally in plant foods and others that are synthetically made. The National Academy of Medicine defines fiber as: 1) dietary fibers (nondigestible carbohydrates and lignans) that occur naturally in plants, and 2) functional fibers that are extracted from plants or synthetically made and are nondigestible with a beneficial health effect in humans. [1] Some types of fiber fall into both categories, such as oligosaccharides and resistant starches that may be naturally occurring or synthetically made. Naturally occurring plant fibers:
Manufactured functional fibers, some of which are extracted and modified from natural plants:
Fiber and DiseaseFiber appears to lower the risk of developing various conditions, including heart disease, diabetes, diverticular disease, and constipation. Fiber’s beneficial role in the gut microbiome may produce anti-inflammatory effects that alleviate the chronic inflammation associated with these conditions. [2] Heart disease Soluble fiber attracts water in the gut, forming a gel, which can slow digestion. This may help prevent blood glucose surges after eating and reduce hunger. Control of blood glucose and weight is important because these are risk factors for diabetes, a condition which doubles the risk of developing heart disease. Soluble fiber may also lower blood cholesterol by interfering with bile acid production. Cholesterol is used to make bile acids in the liver. Soluble fiber binds to bile acids in the gut and excretes them from the body. Because of this reduced amount of available bile acids, the liver will pull cholesterol from the blood to make new bile acids, thereby lowering blood cholesterol. [3] A meta-analysis of 67 controlled trials found a modest benefit of dietary soluble fiber in lowering total and LDL cholesterol. [4] Epidemiological studies find that a high intake of dietary fiber is associated with a lower risk of heart disease and deaths from cardiovascular disease. [5-7] In large cohorts of male and female health professionals, researchers found that higher intakes of cereal fibers were associated with a lower risk of heart disease and heart attacks. [8,9] Keep in mind that cereal fiber doesn’t necessarily refer to the aisle of boxed breakfast cereals in your local supermarket. “Cereals” in these studies referred to the seeds of minimally refined whole grains that include the germ, bran, and endosperm. Examples are steel-cut oats, quinoa, brown rice, millet, barley, and buckwheat. A higher fiber intake has also been linked to a lower risk of metabolic syndrome, a combination of factors that increases the risk of developing heart disease and diabetes: high blood pressure, high insulin levels, excess weight (especially around the belly), high triglyceride levels, and low HDL (good) cholesterol. [10,11] Type 2 diabetes Diets low in fiber, especially insoluble types, may increase the risk of type 2 diabetes (T2DM). Large cohort studies of women found that a diet low in fiber (especially lacking cereal fibers) but containing foods with a high glycemic index (causing blood glucose surges) increased the risk of developing T2DM. [12,13] Other large cohorts of male and female health professionals have found that high-fiber whole grains (brown rice, rye, oats, wheat bran) are most strongly associated with lower diabetes risk. [14,15] Fibers from fruits and vegetables do not appear to have as strong an association. [16] Read about what you can do to help prevent type 2 diabetes. Breast cancer
A high-fiber diet was also associated with a lower risk of benign breast disease, a risk factor in adolescents for the later development of breast cancer. [34] Colorectal cancer Earlier epidemiological studies show mixed results on the association of fiber and colorectal cancer (CRC). [27] One reason may be due to differing effects of fiber on specific subtypes of CRC. When accounting for this, fiber was found to be protective with certain subtypes. [28] A meta-analysis of prospective cohort studies found fibers from fruits, vegetables, and legumes to offer some protection from CRC, but cereal fibers showed a stronger association with CRC prevention. [29] Other later meta-analyses have also found significant associations between a high-fiber diet and lower risk of CRC. [30,31] Constipation Constipation is generally defined as having three or fewer bowel movements a week, difficulty or pain passing bowel movements, or small hard “pebbly” stool. Occasional bouts of constipation are common, but chronic constipation that does not resolve can lower quality of life and lead to symptoms of bloating, cramping, and even nausea. Chronic constipation increases the risk of diverticular disease and hemorrhoids. Lifestyle behaviors that help relieve constipation include eating more fiber from fruits, vegetables, and whole grains; drinking more water; and regular exercise. There are various reasons why fiber reduces constipation. Some types of soluble fiber bind to water, creating a gel that helps to soften and bulk stool. Insoluble fibers mildly irritate the intestinal lining, which stimulates the secretion of water and mucus to encourage movement of stool. [26] Certain fibers act as prebiotics, or food for gut bacteria, which ferments fibers into short chain fatty acids and increases water in the intestines to produce softer, easier-to-pass stools. [26] Because of the differing actions of various fiber types with constipation, a range of high-fiber foods from whole grains, fruits, legumes, and vegetables is recommended. It is suggested to increase fiber intake gradually, because a sudden significant increase in dietary fiber can cause bloating and cramping. Drinking more fluids while eating more fiber can also help lessen these side effects. Diverticular disease Diverticulosis is a condition in which small “pouches” called diverticula develop in the lower intestine. It is one of the most common disorders of the colon in the Western world, with the highest rates in the U.S. and Europe. [17] The risk of developing diverticulosis increases with age (more than half of people over age 60 have diverticula) and is usually silent, not causing noticeable symptoms unless the pouches tear or become inflamed, leading to diverticulitis. Diverticulitis can cause persistent abdominal pain (usually in the lower left side), nausea, vomiting, and fever. Treatment is typically a brief period of no food, drinking liquids only, and antibiotic medications. In severe cases where an abscess or perforation may develop, surgery may be needed. However, most people who develop diverticulosis will not develop diverticulitis, with more recent studies estimating only 5% progressing to diverticulitis. [18] Research shows that a Westernized diet low in fiber and high in red meat and ultra-processed refined foods is a major contributor. [19-21] An eating pattern such as this can lead to constipation, which over time weakens the colon muscles while increasing pressure when trying to defecate; small pouches can form within these weak spots. It can also lead to an increase in harmful intestinal bacteria, causing inflammation and further increasing the risk of diverticular disease. [17,20] Large cohort studies show a protective effect of fiber on diverticular disease, particularly fibers from fruits, cereal grains, and vegetables. [22,23] A cohort of more than 43,000 men from the Health Professionals Follow-up Study found a protective effect of dietary fiber from diverticulitis, especially cellulose, a type of insoluble fiber. [24] Cellulose is found in the skins of fruits, leafy vegetables, root vegetables, legumes, and wheat bran. A Nurses’ Health Study following more than 50,000 women found that those with the highest fiber intakes (25 or more grams daily) had a 13% lower risk of diverticulitis than those who ate the least (less than 18 grams daily). [2] The protective effect came especially from fibers from cereal grains and whole fruits, but not fruit juices. Other factors that increase the risk of diverticular disease are increasing age, smoking, lack of exercise, use of certain medications (NSAIDs, steroids, aspirin, opioids), family history, and history of irritable bowel syndrome. [18] Should I avoid nuts and seeds with diverticulosis?If you have diverticulosis, chances are you’ve heard that maybe you should avoid certain foods with small hard-to-digest particles: nuts, seeds, popcorn, corn, and fruits and vegetables with seeds like raspberries, strawberries, cucumber, or tomatoes. The reasoning is that these small undigested food particles might become trapped in the diverticular pouches and become inflamed from bacterial infection, causing the uncomfortable condition called diverticulitis. People who have experienced intense symptoms of diverticulitis often change their diets to avoid these foods in hopes of preventing a recurrence. However, evidence has shown this practice to be more of an urban legend than helping to reduce recurrences, and can deter people from eating foods that may actually help their condition in the future. Although the role of diet with diverticular disease has long been debated, a high-fiber intake with a focus on whole grains, fruits, and vegetables has been found to have a strong association with decreased risk of diverticular disease and diverticulitis. [18] When it comes to nuts and popcorn, research following more than 47,000 men from the Health Professionals Follow-up Study actually found a lower risk of diverticulitis when eating these foods. [25] Including these foods also did not increase the risk of developing new diverticulosis or diverticular complications. Bottom LineThere are many types of dietary fibers that come from a range of plant foods. It’s important to not hyperfocus on a particular fiber because of its specific proposed action, as each type offers some level of health benefit. Therefore, eating a wide variety of plant foods like fruits, vegetables, whole grains, legumes, nuts, and seeds to reach the fiber recommendation of 25-35 grams daily best ensures reaping those benefits. Some tips for increasing fiber intake:
References
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