Dietary fiber doesn’t contribute much in terms of energy or nutrients, but it benefits your health in many ways. Insoluble fiber such as cellulose promotes regular bowel movements and combats constipation, whereas soluble fiber is able to keep your blood cholesterol levels in check primarily by clinging to cholesterol-rich bile and preventing it from being absorbed in your intestines.
Dietary fiber comes in two basic types, water-insoluble and water-soluble. Although insoluble fiber doesn’t dissolve in water, it attracts water like a sponge while it’s in your intestines. Consequently, eating foods rich in insoluble fiber, such as crunchy veggies and whole grains, leads to soft bulky stool, which cleanses your intestines and promotes gentle contractions and easy elimination. In contrast, soluble fiber dissolves in water and becomes gel-like and sticky, which enables it to attach to various compounds in your intestines and carry them out of your body in waste. All dietary fiber contributes health benefits, but only soluble types have an impact on cholesterol levels.
Common soluble fibers include pectin, lignin and gums, which are found primarily in fruit, beans, peas, oat bran and flaxseed. The ability of soluble fiber to get sticky in water is important for your health because unwanted debris, heavy metals and some toxins cling to the fibers and are removed from your body via stool. Soluble fiber also attaches to unused bile, which is produced in your liver and concentrated in your gallbladder before being released into your small intestine. Bile is made of salts and cholesterol-rich compounds, which are effective at digesting dietary fat into fatty acids and glycerol. Preventing intestinal re-absorption of bile can have an impact on blood cholesterol levels.
The cholesterol in your body comes from the fatty foods you eat and from your liver. Dietary cholesterol certainly contributes to high blood cholesterol levels, although the manufacture, recycling and control of cholesterol in your liver has a larger impact. Cholesterol is important because it’s needed for building cell membranes and steroidal hormones, but an excessive amount in the bloodstream, especially the “bad” LDL type, is linked to higher risk of cardiovascular diseases such as atherosclerosis, heart attack and stroke. Eating less saturated and trans fats helps prevent blood cholesterol levels from rising too high, and including lots of soluble fiber in your diet also has a positive effect. More specifically, when soluble fiber drags bile out of your body, your liver attracts more LDL cholesterol from your blood in order to make more bile salts. Studies have shown that for every 3 grams of soluble fiber you eat in a day, your total blood cholesterol levels can drop by about 2 percent, although a 5 percent total reduction seems to be the limit.
Daily fiber recommendations for women are between 20 and 25 grams, depending on size, weight and age. The best ratio of insoluble-to-soluble fiber is not known, although aiming for at least 5 grams of soluble fiber per day is a good start. That’s not that difficult to accomplish, as 1 cup of dry oatmeal contains about 4.2 grams of soluble fiber. Other excellent sources of soluble fiber include apples, pears, most berries, grapes, oranges, kidney beans, green peas, eggplant and okra.
- Human Metabolism: Functional Diversity and Integration; J. Ramsey Bronk
- Human Biochemistry; Charles Dreiling
- Contemporary Nutrition: Functional Approach; Gordon M. Wardlaw et al.
- The Nutribase Complete Book of Food Counts; Art Ulene
Sirah Dubois is currently a PhD student in food science after having completed her master's degree in nutrition at the University of Alberta. She has worked in private practice as a dietitian in Edmonton, Canada and her nutrition-related articles have appeared in The Edmonton Journal newspaper.