Fiber is a complex carbohydrate that consists of cellulose, pectin, gum, mucilage, hemicellulose, and lignin. You don’t completely digest and absorb fiber, because you don’t have the enzymes required to disassemble it. For that reason, fiber accounts for less than the four Calories per gram of other carbohydrates. Fiber helps you maintain an ideal weight by absorbing water, slowing the emptying of your stomach, and adding volume to food so that you feel full longer. Foods high in fiber often require more chewing, so it takes more time to eat, and you can’t eat a large number of calories in a short amount of time. It helps to prevent diabetes by slowing the entrance of glucose into the bloodstream, reducing glucose and insulin spikes after meals. Fiber helps prevent deaths from coronary heart disease. Fermentation of fiber and resistant starch by bacteria in the large intestine helps to prevent colorectal cancers.
There are two types of fiber: soluble and insoluble. Fiber in cell walls is insoluble in water. These include cellulose, hemicellulose, and lignin. Insoluble fiber binds water as it passes through the digestive tract, making stools softer and bulkier. It also stimulates peristalsis—the rhythmic contractions that move food along the digestive tract, preventing constipation and hemmorhoids. Insoluble fiber also prevents irritable bowel syndrome and diverticulosis, a painful inflammation of the diverticula, which are pouches of the intestinal wall. Because fiber accelerates the transit of carcinogens in the gastrointestinal tract, colon cells are exposed for a shorter time to these toxins, and the likelihood of colon cancer is reduced. Insoluble fiber also helps to prevent gallstones in women.
Soluble fiber, also called viscous fiber, is found inside plant cells. Pectin, gum, and beta-glucan are soluble fibers. Soluble fiber increases the viscosity of food, which slows the movement of food through the intestines, preventing diarrhea. Your body uses cholesterol to produce bile acids, some of which are excreted daily. Soluble fiber binds to bile acids, reducing the amount of bile reabsorbed in the intestines, and increasing the amount of bile that is excreted in the feces. To make up for this loss of bile, the liver makes more bile salts, using more cholesterol to make them. In order to obtain the cholesterol necessary to make more bile salts, the liver increases its production of low-density lipoprotein (LDL) receptors. These receptors pull cholesterol out of LDL molecules in the bloodstream. Therefore, the more bile salts the liver makes, the more LDL cholesterol is pulled from the blood. One of the short-chain fatty acids produced by the fermentation of soluble fiber in the large intestines may also inhibit the amount of cholesterol produced by the liver. A high-fiber diet reduces total cholesterol, triglycerides, and Very Low Density Lipoprotein–the most dangerous form of cholesterol. This prevents the buildup of plaque in the arteries and improves cardiovascular health. It also lowers the risk of heart disease.
In addition to its beneficial effects on the digestive and cardiovascular systems, soluble fiber helps stabilize blood sugar levels by preventing blood sugar levels from rising rapidly after a meal. If you have insulin resistance, hypoglycemia, or diabetes, the soluble fiber in foods like lentils can help balance blood sugar levels while providing steady, slow-burning energy. Researchers compared two groups of people with type 2 diabetes who ate different amounts of high-fiber foods. People who consume 50 grams of fiber per day versus the standard 25 grams of fiber per day had significant improvements in glycemic control and lipid panels. Soluble fiber also provides a feeling of fullness, so it can potentially help with weight loss.
In addition, probiotic bacteria thrive on soluble fibers, including oligofructose and inulin. Oligofructose is a fructooligosaccharide, which refers to a short chain of fructose molecules. Inulins are a group of polysaccharides, which means a long chain of sugar molecules. The probiotic bacteria in your colon can metabolize these soluble fibers through fermentation, releasing significant quantities of carbon dioxide, hydrogen, and methane. This process can sometimes cause intestinal gas; however, if you eat these soluble fibers regularly, your body grows accustomed to them, and you experience fewer problems with gas.
The probiotic intestinal bacteria can metabolize the soluble fiber into short chain fatty acids (SCFAs). This process not only helps support healthy populations of friendly bacteria in your large intestine, but also provides a direct supply of energy (in the form of SCFAs) to the cells that line your large intestine. With this benefit of this extra SCFA energy supply, your intestinal cells can stay healthier and function at a lower risk of becoming cancerous.
Inulin and oligofructose are naturally present in many plant foods, and may help prevent constipation, promote enzyme activity and improve the pH levels in your colon. In addition, inulin promotes Lactobacillus acidophilus to produce butyrate, a beneficial short-chain fatty acid that helps inhibit inflammation in the intestinal tract. Beans, peas, and lentils contain the oligosaccharides, raffinose and stachyose, that feed bifidobacteria, which suppress the activity of putrefactive bacteria, such as Clostridium in the colon.
Beta glucans are sugars that are found in the cell walls of baker’s yeast, shiitake mushrooms, and cereal grains, like barley, oats, rye, and wheat. They increase the number of probiotic bacteria in the intestines, especially in people over the age of fifty. Beta glucans stimulate the activity of macrophages, which are immune cells that ingest and demolish invading pathogens and stimulate other immune cells to attack. Macrophages also release cytokines, chemicals that enable the immune cells to communicate with one another. In addition, beta glucans stimulate lymphocytes (white blood cells) that bind to tumors or viruses, and release chemicals to destroy it. Beta glucans also help to lower total and low-density lipoprotein (LDL or “bad”) cholesterol. Lentinan, a type of beta glucan found in shiitake mushrooms, is believed to reduce tumor activity and lessen the side effects of cancer treatment. Beta glucans also help your body fight bacteria resistant to antibiotic treatment and viruses that cause upper respiratory infections. They fight a form of Escherichia coli (ETEC), which cause traveler’s diarrhea. They also fight upper respiratory infections from colds and flu. Lentinan strengthens the immune system and helps combat illnesses that attack the immune system like AIDS.
The best sources of soluble fiber are oats, especially oat bran, barley, dried beans, soybeans, sweet potato and white potato, broccoli, asparagus, carrot, apple, pear, citrus fruits, berries, banana, almonds, psyllium, and flax seeds.
In fact, all dietary fiber is found only in plant foods: fruits, vegetables, nuts, and grains. Meat, milk, and eggs do not contain fiber. Canned and frozen fruits and vegetables contain just as much fiber as raw ones. Drying and crushing, however, destroy the water-holding qualities of fiber. Removing seeds, peels, or hulls also reduces fiber content. Whole tomatoes have more fiber than peeled tomatoes, which have more than tomato juice. Likewise, whole wheat bread contains more fiber than white bread.