Fiber

What does it do? Fiber is divided into two general categories–water soluble and water insoluble. Soluble fiber lowers cholesterol.1 An analysis of many trials of soluble fiber reveals it has a cholesterol-lowering effect, but the degree of cholesterol reduction in many studies was quite modest.2 For unknown reasons, diets higher in insoluble fiber (mostly unrelated to cholesterol levels) have been reported to correlate better with protection against heart disease in both men and women.3 4

Soluble fibers can also lower blood sugar levels in people with diabetes, and some researchers find that increasing fiber decreases the body’s need for insulin—a good sign for diabetics.5 However, a research review reveals that just how much moderate amounts of soluble fiber really help people with diabetes remains unclear.6 As with heart disease, a clear mechanism to explain how insoluble fiber helps diabetics has not been identified. Nonetheless, diets high in insoluble fiber (from whole grains) associate with protection from adult-onset diabetes.7

Insoluble fiber softens stool, which helps move it through the intestinal tract in less time. For this reason, insoluble fiber is partially effective as a treatment for constipation.8 The reduction in “transit time” has also been thought to partially explain the link between a high fiber diet and a reduced risk of colon cancer as found in some studies,9 though anticancer effects unrelated to “transit time” have also been reported.10

The true relationship between fiber and colon cancer risk has recently been clouded by data coming from several directions. In animal research, wheat bran is proving to be more protective than other diets containing equal amounts of insoluble fiber, suggesting that fiber in wheat may not be the primary cause of protection sometimes associated with wheat.11 In human research, a recent well respected study found no significant link between fiber and colon cancer prevention.12 A trial from South Africa found that avoidance of meat and dairy, and not the presence of fiber, appears to be primarily responsible for a low risk of colon cancer.13 As a result of these negative findings some researchers and doctors have begun to question the idea that insoluble fiber protects against colon cancer, a concept that had arisen from a large body of older research.

Fiber also fills the stomach, reducing appetite. In theory, fiber should therefore reduce eating, leading to weight loss. However, at least some research has found increased fiber to have no effect on body weight despite decreasing appetite.14

Lignan, a fiber-like substance, has mild antiestrogenic activity. Probably for this reason, high lignan levels in urine (and therefore dietary intake) have been linked to protection from breast cancer in humans.15

Where is it found? Whole grains are particularly high in insoluble fiber. Oats, barley, beans, fruit (but not fruit juice), psyllium, and some vegetables contain significant amounts of both forms of fiber and are the best sources of soluble fiber. The best source of lignan, by far, is flaxseed (not flaxseed oil, regardless of packaging claims to the contrary).

Fiber has been used in connection with the following conditions (refer to the individual health concern for complete information):

Rating Health Concerns
3Stars Constipation
Diabetes
Diverticular disease
High cholesterol
2Stars Diarrhea
Hemorrhoids
High blood pressure
Weight loss
1Star High triglycerides
Irritable bowel syndrome (fiber other than wheat)
Kidney stones
Peptic ulcer
Premenstrual syndrome
3Stars Reliable and relatively consistent scientific data showing a substantial health benefit.
2Stars Contradictory, insufficient, or preliminary studies suggesting a health benefit or minimal health benefit.
1Star An herb is primarily supported by traditional use, or the herb or supplement has little scientific support and/or minimal health benefit.

Who is likely to be deficient? Most people who consume a typical Western diet are fiber-deficient. Eating white flour, white rice, and fruit juice (as opposed to whole fruit) all contribute to this problem. Many so-called whole wheat products contain mostly white flour. Read labels and avoid “flour” and “unbleached flour,” both of which are simply white flour. Junk food is also fiber depleted. The diseases listed above are more likely to occur with low-fiber diets.

The benefits of eating whole grains are largely derived from the beneficial constituents present in the outer layers of the grains, which are stripped away in making white flour and white rice. Preliminary research has found that women who ate mostly whole grain fiber had a lower mortality rate than women who ate a comparable amount of refined grains.16

How much is usually taken? Western diets generally provide approximately 10 grams of fiber per day. So-called “primitive societies” consume 40–60 grams per day. Increasing fiber intake to the amounts found in primitive diets may be desirable.

Are there any side effects or interactions? While people can be allergic to certain high-fiber foods (most commonly wheat), high-fiber diets are more likely to improve health than cause any health problems. Beans, a good source of soluble fiber, also contain special sugars that are often poorly digested, leading to gas. Special enzyme products are now available in supermarkets to reduce this problem by improving digestion of these sugars.

Fiber reduces the absorption of many minerals. However, high-fiber diets also tend to be high in minerals, so the consumption of a high-fiber diet does not appear to impair mineral status. However, logic suggests that calcium, magnesium and multimineral supplements should not be taken at the same time as a fiber supplement.

Bran, an insoluble fiber, reduces the absorption of calcium enough to cause urinary calcium to fall.17 In one study, supplementation with 10 grams of rice bran twice a day reduced the recurrence rate of kidney stones by nearly 90% in recurrent stone formers.18 However, it is not known whether other types of bran would have the same effect. Before supplementing with bran, people should check with a doctor, because some people—even a few with kidney stones—do not absorb enough calcium. For those people, supplementing with bran might deprive them of much-needed calcium.

People with scleroderma (systemic sclerosis) should consult a doctor before taking fiber supplements or eating high-fiber diets. Although a gradual introduction of fiber in the diet may improve bowel symptoms in some cases, there have been several reports of people with scleroderma developing severe constipation and even bowel obstruction requiring hospitalization after fiber supplementation.19

Are there any drug interactions? Certain medications may interact with fiber. Refer to the drug interactions safety check for a list of those medications.

References:

1. Todd PA, Befield P, Goa KL. Guar gum: a review of its pharmacological properties and use as a dietary adjunct in hypercholesterolemia. Drugs 1990;39:917–28.

2. Brown L, Rosner B, Willett WW, Sacks FM. Cholesterol-lowering effects of dietary fiber: a meta-analysis. Am J Clin Nutr 1999;69:30–42.

3. Jenkins DJA, Kendall CWC, Ransom TPP. Dietary fiber, the evolution of the human diet and coronary heart disease. Nutr Res 1998;18:633–52 [review].

4. Wolk A, Manson JE, Stampfer MJ, et al. Long-term intake of dietary fiber and decreased risk of coronary hart disease among women. JAMA 1999;281:1998–2004.

5. Anderson JW, Gustafson NS, Bryart CA. Tietyen-Clark J. Dietary fiber and diabetes. J Am Diet Assoc 1987;87:1189–97.

6. Nuttall FW. Dietary fiber in the management of diabetes. Diabetes 1993;42:503–8.

7. Salmeron J, Manson JAE, Stampfer MJ, et al. Dietary fiber, glycemic load, and risk of non-insulin-dependent diabetes mellitus in women. JAMA 1997;277:472–7.

8. Kritchevsky D. Protective role of wheat bran fiber: preclinical data. Am J Med 1999;106(1A):28S–31S.

9. Ausman LM. Fiber and colon cancer: does the current evidence justify a preventive policy? Nutr Rev 1993;51:57–63 [review].

10. Jacobs DR Jr, Marquart L, Slavin J, Kushi LH. Whole-grain intake and cancer: an expanded review and meta-analysis. Nutr Cancer 1998;30:85–96.

11. Müller-Lissner SA. Effect of wheat bran on weight of stool and gastrointestinal transit time: a meta analysis. Br Med J 1988;296:615–7.

12. Fuchs CS, Giovannucci EL, Colditz G, et al. Dietary fiber and the risk of colorectal cancer and adenoma in women. N Engl J Med 1999;340:169–76.

13. O’Keefe SJD, Kidd M, Espitalier-Noel G, Owira P. Rarity of colon cancer in Africans is associated with low animal product consumption, not fiber. Am J Gastroenterol 1999;94:1373–80.

14. Hylander B, Rössner S. Effects of dietary fiber intake before meals on weight loss and hunger in a weight-reducing club. Acta Med Scand 1983;213:217–20.

15. Adlercreutz H, Fotsis T, Hekkinen R, et al. Excretion of the lignans enterolactone and enterodiol and of equol in omnivorous and vegetarian postmenopausal women and in women with breast cancer. Lancet 1982;2:1295–9.

16. Jacobs DR, Pereira MA, Meyer KA, Kushi LH. Fiber from whole grains, but not refined grains, is inversely associated with all-cause mortality in older women: the Iowa women’s health study. J Am Coll Nutr 2000;19(3 Suppl):326S–30S.

17. Shah PJR. Unprocessed bran and its effect on urinary calcium excretion in idiopathic hypercalciuria. Br Med J 1980;281:426.

18. Ebisuno S, Morimoto S, Yoshida T, et al. Rice-bran treatment for calcium stone formers with idiopathic hypercalciuria. Br J Urol 1986;58:592–5.

19. Gough A, Sheeran T, Bacon P, Emery P. Dietary advice in systemic sclerosis: the dangers of a high fibre diet. Ann Rheum Dis 1998;57:641–2.