What does it do?
Dietary fiber comes from the thick cell wall of plants. It is an indigestible complex
carbohydrate. 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):
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 medicines may interact with fiber. Refer to drug
interactions for a list of those medicines.
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. M�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, R�er 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.
Copyright © 2004 Healthnotes, Inc. All rights reserved.
www.healthnotes.com
Learn more about Healthnotes, the company.
Learn more about the authors of Healthnotes.
The information presented in Healthnotes is for informational
purposes only. It is based on scientific studies (human, animal, or in vitro),
clinical experience, or traditional usage as cited in each article. The results reported may
not necessarily occur in all individuals. For many of the conditions discussed, treatment with
prescription or over-the-counter medication is also available. Consult your doctor,
practitioner, and/or pharmacist for any health problem and before using any supplements or
before making any changes in prescribed medications. Information expires March 2005.
| 
