DEAR DR. BLONZ: I understand that fiber is a carbohydrate, that it comes from plants and that it just passes through the digestive system. Does it provide any real nutrients? And if you have no problems with constipation, is fiber still needed? -- F.S., Anderson, South Carolina
DEAR F.S.: Dietary fiber is mostly carbohydrate, but it also includes non-carbohydrate plant lignins and waxes. You are correct that it is found in plants, such as vegetables, nuts, fruits and grains. While it is an essential component of a healthful diet, fiber does not contribute calories, essential vitamins or minerals to the body.
Nutrition research has associated diets naturally higher in fiber with lower risks of diabetes, diverticulitis, hemorrhoids, ulcerative colitis, coronary heart disease and certain forms of cancer. Fiber is also commonly used to treat constipation. All in all, an impressive set of talents!
If we think of food as a chain of nutrients linked together by padlocks, digestion is the process of opening the locks. Only after the chain is broken into smaller pieces can the body absorb and use the nutrients. Digestive enzymes are the keys that open the locks. Dietary fiber is unique because the body lacks the "key" to open it. This means that instead of being absorbed, the fiber becomes part of the bulk that traverses the entire digestive system.
It is true that most fiber "passes through," but it also becomes food for the flora in the large intestine -- our microbiome. The resulting output from the flora can be gas, which we are all familiar with, and it can also be a fermentation product like butyric acid, a short-chain fatty acid associated with beneficial effects. We are in the early stage of learning how the regular presence, or lack, of fiber-rich foods can impact health.
There are two types of dietary fiber: insoluble and soluble, with the important distinction being whether the fiber dissolves in water. The health benefits of the two differ.
Insoluble fiber is found in wheat bran, vegetables, fruits, and whole grains such as corn, rye, barley and brown rice. Insoluble fiber increases the bulk and weight of the stool, as well as the rate at which food travels through the digestive system. This makes for potential protections from certain cancers: Population studies have reported that the incidence of colon cancer decreases as insoluble fiber goes up. The thinking is that fiber can effectively dilute or even bind potential cancer-causing substances and quickly usher them out of the body. Insoluble fiber might also help lower the risk of heart disease, perhaps because it can bind the bile acids and cause more cholesterol to exit before being absorbed.
Soluble fiber is found in oat bran, rice bran, legumes (beans, lentils and peas), fruits and vegetables. Although these fibers dissolve in water, the body cannot break them down or absorb them.
Soluble fiber can't match the ability of insoluble fiber to add bulk. It can, however, improve conditions connected with diabetes, because it might slow the rate at which the body absorbs sugar. In addition, through a complex series of reactions, soluble fiber has a demonstrated ability to help lower blood cholesterol levels.
You'll notice that fruits and vegetables contain both soluble and insoluble fiber. This is further testimony to the wisdom behind including fiber-rich foods in the diet, and why it is essential to include them -- irrespective of your bowel habits.
Send questions to: "On Nutrition," Ed Blonz, c/o Andrews McMeel Syndication, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to firstname.lastname@example.org. Due to the volume of mail, personal replies cannot be provided.