Fiber is the indigestible structural material of plants that is found in all fruits, vegetables, whole grains, beans, nuts and other seeds. Before food can be absorbed from your intestines into your bloodstream, it must be broken down into basic building blocks. Since you lack the intestinal enzymes to break down fiber into its building blocks of basic sugars, you do not absorb fiber in your upper intestines. Fiber passes through your intestines into your colon where soluble and insoluble fiber are treated differently by the bacteria in your colon.
Soluble fiber dissolves in water and is readily fermented by enzymes produced by bacteria in your colon, so its breakdown products can be absorbed in your colon. Soluble fiber does all sorts of good things for you:
• It helps to control your weight by drawing water into your stomach to delay emptying and keep your stomach full, so it can decrease the amount of food that you eat.
• It helps to lowers blood sugar and insulin levels. It draws water into your stomach to keep food in your stomach longer, and therefore slows the rate at which your body absorbs sugar from your intestines. This helps to control blood sugar levels in diabetics and non-diabetics.
• It binds to sugars and starches in fruits, vegetables and seeds, which prevents much of the sugar from being absorbed in the intestines so it passes to the colon. There bacteria break down the soluble fiber, releasing the sugars so they can be absorbed. This delayed absorption markedly reduces the rise in blood sugar that follows eating fruits, vegetables and grains, which helps to prevent diabetes or to control blood sugar in people who are diabetic.
• It is readily fermented in the right side of the colon to form short-chain fatty acids that help to reduce risk of colon cancer
• It helps to prevent heart attacks by being fermented by bacteria in your colon to form short chain fatty acids that are absorbed into your bloodstream and travel to your liver to help prevent the liver from making the bad LDL cholesterol. Soluble fiber is the only food component we know of that lowers blood cholesterol when you add more to your diet.
• It adds water to the stool to help prevent constipation.
Insoluble fiber can absorb water but does not dissolve in it, so it is generally not fermented by bacteria or absorbed in your colon. Insoluble fiber adds bulk to your stool so can pass out from your body so readily that you will not suffer from constipation.
How Much Do You Need?
I do not recommend trying to count grams of fiber in your diet, but you should get at least 30 grams a day. Good sources of soluble fiber include dried peas, soybeans, beans, oats, rye, barley, figs, avocados, plums, prunes, berries, bananas, apples, pears, broccoli, carrots, sweet potatoes and onions. Good sources of insoluble fiber include wheat bran, beans, lentils, dried peas, nuts and other seeds, potato skins and most whole fruits and vegetables.
The average North American eats only 11 grams of fiber a day. There is very little fiber in the typical diet of hamburgers, pizza, fried chicken and coke. Foods made from animal products never have any fiber, and processed foods made from grains, vegetables or fruit frequently have much of the fiber removed.
Don't worry about whether you are getting soluble or insoluble fiber; you need both kinds, and both are found in fruits, vegetables, whole grains and beans. If you're not getting enough fiber, don't try to correct the situation by adding fiber supplements, lots of bran cereal or foods made with added ground-up fiber. When you eat whole fruits, vegetables, whole grains and beans, you get all of the vitamins, minerals and phytochemicals nature packages with the fiber. Introduce more high-fiber whole foods into your diet gradually to avoid digestive discomfort.
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