Fiber is a type of carbohydrate that the body can’t digest. Though most carbohydrates are broken down into sugar molecules, fiber cannot be broken down into sugar molecules, and instead it passes through the body undigested. Fiber helps regulate the body’s use of sugars, helping to keep hunger and blood sugar in check.
Children and adults need at least 20 to 30 grams of fiber per day for good health, but most Americans get only about 15 grams a day. Great sources are whole fruits and vegetables, whole grains, and beans.
Fiber comes in two varieties, both beneficial to health:
- Soluble fiber, which dissolves in water, can help lower glucose levels as well as help lower blood cholesterol. Foods with soluble fiber include oatmeal, nuts, beans, lentils, apples and blueberries.
- Insoluble fiber, which does not dissolve in water, can help food move through your digestive system, promoting regularity and helping prevent constipation. Foods with insoluble fibers include wheat, whole wheat bread, whole grain couscous, brown rice, legumes, carrots, cucumbers and tomatoes.
Some tips for increasing fiber intake:
- Eat whole fruits instead of drinking fruit juices.
- Replace white rice, bread, and pasta with brown rice and whole grain products.
- For breakfast, choose cereals that have a whole grain as their first ingredient.
- Snack on raw vegetables instead of chips, crackers, or chocolate bars.
- Substitute beans or legumes for meat two to three times per week in chili and soups.
Fiber and disease
Fiber appears to reduce the risk of developing various conditions, including heart disease, diabetes, diverticular disease, and constipation. Despite these benefits, fiber probably has little, if any, effect on colon cancer risk.
High intake of dietary fiber has been linked to a lower risk of heart disease in a number of large studies that followed people for many years. (16) In a Harvard study of over 40,000 male health professionals, researchers found that a high total dietary fiber intake was linked to a 40 percent lower risk of coronary heart disease. (17) A related Harvard study of female nurses produced quite similar findings. (18)
Higher fiber intake has also been linked to a lower risk of metabolic syndrome, a combination of factors that increases the risk of developing heart disease and diabetes. These factors include high blood pressure, high insulin levels, excess weight (especially around the abdomen), high levels of triglycerides, and low levels of HDL (good) cholesterol. Several studies suggest that higher intake of fiber may offer protective benefits from this syndrome. (19,20)
Type 2 diabetes
Diets low in fiber and high in foods that cause sudden increases in blood sugar may increase the risk of developing type 2 Diabetes. Both Harvard studies—of female nurses and of male health professionals—found that this type of diet more than doubled the risk of type 2 diabetes when compared to a diet high in cereal fiber and low in high-glycemic-index foods. (21-23) A diet high in cereal fiber was linked to a lower risk of type 2 diabetes.
Other studies, such as the Black Women’s Health Study (24) and the European Prospective Investigation Into Cancer and Nutrition–Potsdam, have shown similar results.
Read about what you can do to help prevent type 2 diabetes.
Diverticulitis, an inflammation of the intestine, is one of the most common age-related disorders of the colon in Western society. Among male health professionals in a long-term follow-up study, eating dietary fiber, particularly insoluble fiber, was associated with about a 40 percent lower risk of diverticular disease. (25)
Fiber and constipation
Constipation is the most common gastrointestinal complaint in the United States, and consumption of fiber seems to relieve and prevent constipation.
The fiber in wheat bran and oat bran is considered more effective than fiber from fruits and vegetables. Experts recommend increasing fiber intake gradually rather than suddenly, and because fiber absorbs water, beverage intake should be increased as fiber intake increases.
Studies have largely failed to show a link between fiber and colon cancer. One of these—a Harvard study that followed over 80,000 female nurses for 16 years—found that dietary fiber was not strongly associated with a reduced risk for either colon cancer or polyps (a precursor to colon cancer). (26)
Following the Scientific Trail: The Story on Fiber and Colon Cancer
Because science is such a dynamic process, you can never exactly tell where it is going to lead you. Conclusions that once seemed logical and fairly solid may be revised—or completely overturned—as more and better research is done on a particular topic. One example of this is the relationship between fiber and colon cancer.
Starting about 30 years ago, a high fiber intake was regularly recommended as one way to lower the risk for colon cancer. This recommendation was largely based on observations that countries with a high fiber intake tended to have rates of colon cancer lower than the rates found in countries with a low fiber intake.
But such descriptive studies don’t provide the most definitive information. While they are often good points to start a scientific journey, they only take a broad look at large groups of people. Descriptive studies generally can’t address all of the factors that might account for differences in rates of disease. Fiber intake could indeed have something to do with the differences in colon cancer rates, but those differences could also involve many other things that differ between countries, including other diet or lifestyle factors.
When studies that can take such things into account on an individual level began to look at the issue of fiber and colon cancer, the picture became much less clear. A number of case-control studies found that a high fiber intake was linked to a lower risk of colon cancer, but many did not. Given these wavering results—and because case-control studies are not an optimal way to assess food intake, relying as they do on participants’ recollections of what they ate in the past—more research using better methods was needed. In the meantime, many health professionals still regularly recommended a high fiber intake for people trying to lower their risk of colon cancer.
Not until the results of cohort studies came out did this recommendation begin to lose its backing. Because cohort studies observe a group of people over time, their findings are generally stronger than those of case-control studies, especially when it comes to something like diet and colon cancer. What most of these cohort studies found was that fiber intake had very little, if any, link with colon cancer.
Such findings were further bolstered by the results of randomized trials—types of studies that many consider the gold-standard of research. These studies took a group of people and randomly assigned individuals to one of two groups. One group was put on a high fiber diet, while the other group followed a lower fiber diet. After 3 to 4 years, the two groups were compared and no difference was found in rates of colon polyps—noncancerous growths that can turn into cancer. Of course, colon polyps are not cancer, but since it’s thought that all colon cancers start as polyps, it is strong evidence that fiber intake has no direct link with colon cancer.
In this case, the path of discovery led from widespread belief in a clear link between fiber and colon cancer to acceptance of the likelihood that there was no strong link between the two. As such, it’s an excellent example of how research can often develop. What may start as a clear connection based on findings from broad, descriptive studies can slowly unravel as more and better-quality research unveils the true nature of a relationship. However, keep in mind that a weak relationship is difficult to exclude altogether. Further studies might yet demonstrate a weak effect of fiber on colon cancer, although such a finding wouldn’t alter the conclusion that other means must be sought to prevent colon cancer.
A large-scale 2016 study (27) led by researchers at Harvard T.H. Chan School of Public Health showed findings that higher fiber intake reduces breast cancer risk, suggesting that fiber intake during adolescence and early adulthood may be particularly important.
- Women who eat more high-fiber foods during adolescence and young adulthood, including vegetables and fruit, may have significantly lower breast cancer risk than those who eat less dietary fiber when young.
16. Pereira MA, O’Reilly E, Augustsson K, et al. Dietary fiber and risk of coronary heart disease: a pooled analysis of cohort studies. Arch Intern Med. 2004;164:370-6.
17. Rimm EB, Ascherio A, Giovannucci E, Spiegelman D, Stampfer MJ, Willett WC. Vegetable, fruit, and cereal fiber intake and risk of coronary heart disease among men. JAMA. 1996;275:447-51.
18. 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.
19. McKeown NM, Meigs JB, Liu S, Wilson PW, Jacques PF. Whole-grain intake is favorably associated with metabolic risk factors for type 2 diabetes and cardiovascular disease in the Framingham Offspring Study. Am J Clin Nutr. 2002;76:390-8.
20. McKeown NM, Meigs JB, Liu S, Saltzman E, Wilson PW, Jacques PF. Carbohydrate nutrition, insulin resistance, and the prevalence of the metabolic syndrome in the Framingham Offspring Cohort. Diabetes Care. 2004;27:538-46.
21. Fung TT, Hu FB, Pereira MA, et al. Whole-grain intake and the risk of type 2 diabetes: a prospective study in men. Am J Clin Nutr. 2002;76:535-40.
22. Liu S, Willett WC, Stampfer MJ, et al. A prospective study of dietary glycemic load, carbohydrate intake, and risk of coronary heart disease in US women. Am J Clin Nutr. 2000;71:1455-61.
23. Schulze MB, Liu S, Rimm EB, Manson JE, Willett WC, Hu FB. Glycemic index, glycemic load, and dietary fiber intake and incidence of type 2 diabetes in younger and middle-aged women. Am J Clin Nutr. 2004;80:348-56.
24. Krishnan S, Rosenberg L, Singer M, et al. Glycemic index, glycemic load, and cereal fiber intake and risk of type 2 diabetes in US black women. Arch Intern Med. 2007;167:2304-9.
25. Aldoori WH, Giovannucci EL, Rockett HR, Sampson L, Rimm EB, Willett WC. A prospective study of dietary fiber types and symptomatic diverticular disease in men. J Nutr. 1998;128:714-9.
26. Fuchs CS, Giovannucci EL, Colditz GA, et al. Dietary fiber and the risk of colorectal cancer and adenoma in women. N Engl J Med. 1999;340:169-76.
27.MS, AH, , , , WC. Dietary fiber intake in young adults and breast cancer risk. Pediatrics 2016: 137(3).
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