Do you know how much fiber you’re getting every day?
Do you know where it’s coming from?
Is it enough? Should you raise or lower it?
What foods are the best sources of fiber?
These are all questions I get asked fairly regularly, so I thought their answers would make for a good article.
So, let’s start at the beginning.
Table of Contents
Fiber is an indigestible type of carbohydrate found in many types of foods, including fruits, vegetables, legumes, and grains. It comes in two forms:
- Soluble fiber. This type of fiber dissolves in water, and tends to slow the movement of food through the digestive system. Research shows that soluble fiber is metabolized by bacteria in the colon, and hence has little effect on stool weight. However, it can increase fecal output by stimulating the growth of healthy bacteria and fatty acids, and is actually an important source of fuel for the colon.
Some common sources of soluble fiber are beans and peas; oats; certain fruits like plums, bananas, and apples; certain vegetables like broccoli, sweet potatoes, and carrots; certain nuts, with almonds being the highest in dietary fiber.
- Insoluble fiber. This type of fiber does not dissolve in water, and contributes to stool weight. It bangs against the walls of the intestines, causing damage, but research has shown that this damage and the resulting repair and cellular regeneration is a healthy process.
Some common sources of insoluble fiber are whole grain foods like brown rice, barley, and wheat bran; beans; certain vegetables like peas, green beans, and cauliflower; avocado; and the skins of some fruits like plums, grapes, kiwis, and tomatoes.
The importance of getting adequate fiber has been known for a long time. The ancient Greek physician Hippocrates, who famously said “let food be thy medicine, and medicine be thy food,” recommended whole-grain breads to improve bowel movements.
What else does it help with? What doesn’t it help with? Let’s find out.
For many years, we have been told to eat a high-fiber diet to reduce our risk of colon cancer.
Well, it turns out that this advice was based on relatively small studies. More recent, larger, and better-designed studies refute their findings.
For instance, a study conducted by Harvard University that followed over 80,000 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).
Another Harvard study combined the above research with several other large studies to pool over 20 years’ worth of data on 700,000 men and women, and found that a high intake of fiber did not protect against colorectal cancer.
Fiber Intake and Other Cancers
While fiber doesn’t seem to protect us against colorectal cancer, research suggests it does reduce the risk of other types of cancer.
For example, a study conducted by the Institute of Social and Preventive Medicine (Switzerland) found that the fiber in whole grains was associated with a reduced risk of mouth and throat cancer. Refined grains had no such association because the fiber is removed during processing.
According to research conducted by Imperial College, getting an adequate amount of fiber every day may also reduce the risk of breast cancer.
Heart disease is the leading cause of death in the United States.
This type of disease is caused by a buildup of cholesterol in the blood vessels that feed the heart (arteries), which makes them hard and narrow. This is known as atherosclerosis, and a total blockage of an artery produces a heart attack.
Studies have shown that fiber reduces the risk of heart disease.
A pooled analysis conducted by the University of Minnesota analyzed the data from 10 studies to investigate the association between fiber intake and heart disease. Researchers found that each 10-gram increase in daily fiber intake was associated with a 14% decrease in risk of all heart disease, and a 27% decrease in risk of death from such disease.
Research conducted by Harvard University supports these findings. After following 43,757 men for 6 years, researchers found that as fiber intake increased, the risk of heart disease decreased.
Further research from Harvard University demonstrated that soluble fiber decreases total and LDL (bad) cholesterol levels, which helps protect against heart disease.
Metabolic syndrome is a combination of disorders including high blood pressure, high insulin levels, obesity (with excessive weight in the abdomen area), high levels of triglycerides (particles in the body that carry fats), and low HDL (good) cholesterol levels. Among its many obvious dangers, metabolic syndrome markedly increases the risk of heart disease and diabetes.
Research conducted by Tufts University demonstrated that increasing whole grain intake reduced the risk of developing this syndrome. Researchers found that the fiber and magnesium in the whole grains were primarily, but not wholly, responsible for these benefits.
Type 2 diabetes is the most common form, and is characterized by chronically high blood sugar levels. It’s caused by a body’s inability to produce enough insulin to lower blood sugar levels, or by cells being unable to use the insulin properly.
Studies have shown that fiber reduces the risk of developing type 2 diabetes because it improves your body’s ability to use insulin and regulate blood sugar levels.
On the other hand, a diet low in fiber and high in simple carbohydrates (those which are quickly absorbed by the body) has been shown to increase the risk of developing type 2 diabetes and heart disease.
Diverticulitis is an intestinal inflammation, and is one of the most common colon disorders in the Western world. It’s quite painful and especially prevalent in those over 45 years of age.
Harvard University conducted a study wherein they followed 43,881 men, and researchers found that eating adequate fiber, and insoluble fiber in particular, was associated with a 40% reduction in the risk of diverticulitis.
The evidence is pretty clear: eat enough fiber, and you’re more likely to live a long, healthy life.
How much is enough, though?
According to the Institute of Medicine, children and adults should consume 14 grams of fiber for every 1,000 calories of food eaten.
Here are some easy ways to make sure you hit your daily requirement:
- Eat whole fruits instead of drinking juices.
- Choose whole-grain breads, rice, cereals, and pasta over processed forms.
- Eat raw vegetables as snacks instead of chips, crackers, or energy bars.
- Include legumes in your diet (a fun way to do this is to cook some international dishes that use a lot of whole-grains and legumes, such as Indian or Middle-Eastern food).
If you’d like to see the fiber content of a wide variety of common foods, check out this helpful chart created by Harvard University.
Do you pay attention to your fiber intake? What are your favorite sources? Let me know in the comments below!
+ Scientific References
- Park, Y., Subar, A. F., Hollenbeck, A., & Schatzkin, A. (2011). Dietary fiber intake and mortality in the NIH-AARP diet and health study. Archives of Internal Medicine, 171(12), 1061–1068. https://doi.org/10.1001/archinternmed.2011.18
- Aldoori, W. H., Giovannucci, E. L., Rockett, H. R. H., Sampson, L., Rimm, E. B., & Willet, W. C. (1998). A prospective study of dietary fiber types and symptomatic diverticular disease in men. Journal of Nutrition, 128(4), 714–719. https://doi.org/10.1093/jn/128.4.714
- Liu, S., Willett, W. C., Stampfer, M. J., Hu, F. B., Franz, M., Sampson, L., Hennekens, C. H., & Manson, J. A. E. (2000). A prospective study of dietary glycemic load, carbohydrate intake, and risk of coronary heart disease in US women. American Journal of Clinical Nutrition, 71(6), 1455–1461. https://doi.org/10.1093/ajcn/71.6.1455
- Schulze, M. B., Liu, S., Rimm, E. B., Manson, J. A. E., Willett, W. C., & Hu, F. B. (2004). Glycemic index, glycemic load, and dietary fiber intake and incidence of type 2 diabetes in younger and middle-aged women. American Journal of Clinical Nutrition, 80(2), 348–356. https://doi.org/10.1093/ajcn/80.2.348
- Fung, T. T., Hu, F. B., Pereira, M. A., Liu, S., Stampfer, M. J., Colditz, G. A., & Willett, W. C. (2002). Whole-grain intake and the risk of type 2 diabetes: A prospective study in men. American Journal of Clinical Nutrition, 76(3), 535–540. https://doi.org/10.1093/ajcn/76.3.535
- Krishnan, S., Rosenberg, L., Singer, M., Hu, F. B., Djoussé, L., Cupples, L. A., & Palmer, J. R. (2007). Glycemic index, glycemic load, and cereal fiber intake and risk of type 2 diabetes in US black women. Archives of Internal Medicine, 167(21), 2304–2309. https://doi.org/10.1001/archinte.167.21.2304
- J Stevens. (n.d.). Does dietary fiber affect food intake and body weight? - PubMed. Retrieved June 25, 2021, from https://pubmed.ncbi.nlm.nih.gov/2840457/
- C M Ripsin, J M Keenan, D R Jacobs Jr, P J Elmer, R R Welch, L Van Horn, K Liu, W H Turnbull, F W Thye, & M Kestin. (n.d.). Oat products and lipid lowering. A meta-analysis - PubMed. Retrieved June 25, 2021, from https://pubmed.ncbi.nlm.nih.gov/1317928/
- Appel, L. J., Moore, T. J., Obarzanek, E., Vollmer, W. M., Svetkey, L. P., Sacks, F. M., Bray, G. A., Vogt, T. M., Cutler, J. A., Windhauser, M. M., Lin, P.-H., Karanja, N., Simons-Morton, D., McCullough, M., Swain, J., Steele, P., Evans, M. A., Miller, E. R., & Harsha, D. W. (1997). A Clinical Trial of the Effects of Dietary Patterns on Blood Pressure. New England Journal of Medicine, 336(16), 1117–1124. https://doi.org/10.1056/nejm199704173361601
- Chandalia, M., Garg, A., Lutjohann, D., von Bergmann, K., Grundy, S. M., & Brinkley, L. J. (2000). Beneficial Effects of High Dietary Fiber Intake in Patients with Type 2 Diabetes Mellitus. New England Journal of Medicine, 342(19), 1392–1398. https://doi.org/10.1056/nejm200005113421903
- McKeown, N. M., Meigs, J. B., Liu, S., Saltzman, E., Wilson, P. W. F., & Jacques, P. F. (2004). Carbohydrate Nutrition, Insulin Resistance, and the Prevalence of the Metabolic Syndrome in the Framingham Offspring Cohort. Diabetes Care, 27(2), 538–546. https://doi.org/10.2337/diacare.27.2.538
- Brown, L., Rosner, B., Willett, W. W., & Sacks, F. M. (1999). Cholesterol-lowering effects of dietary fiber: A meta-analysis. American Journal of Clinical Nutrition, 69(1), 30–42. https://doi.org/10.1093/ajcn/69.1.30
- Rimm, E. B. (1996). Vegetable, Fruit, and Cereal Fiber Intake and Risk of Coronary Heart Disease Among Men. JAMA: The Journal of the American Medical Association, 275(6), 447. https://doi.org/10.1001/jama.1996.03530300031036
- Pereira, M. A., O’Reilly, E., Augustsson, K., Fraser, G. E., Goldbourt, U., Heitmann, B. L., Hallmans, G., Knekt, P., Liu, S., Pietinen, P., Spiegelman, D., Stevens, J., Virtamo, J., Willett, W. C., & Ascherio, A. (2004). Dietary Fiber and Risk of Coronary Heart Disease: A Pooled Analysis of Cohort Studies. Archives of Internal Medicine, 164(4), 370–376. https://doi.org/10.1001/archinte.164.4.370
- Aune, D., Chan, D. S. M., Greenwood, D. C., Vieira, A. R., Navarro Rosenblatt, D. A., Vieira, R., & Norat, T. (2012). Dietary fiber and breast cancer risk: A systematic review and meta-analysis of prospective studies. In Annals of Oncology (Vol. 23, Issue 6, pp. 1394–1402). Oxford University Press. https://doi.org/10.1093/annonc/mdr589
- Levi, F., Pasche, C., Lucchini, F., Chatenoud, L., Jacobs, D. R., & La Vecchia, C. (2000). Refined and whole grain cereals and the risk of oral, oesophageal and laryngeal cancer. European Journal of Clinical Nutrition, 54(6), 487–489. https://doi.org/10.1038/sj.ejcn.1601043
- Park, Y., Hunter, D. J., Spiegelman, D., Bergkvist, L., Berrino, F., Van Den Brandt, P. A., Buring, J. E., Colditz, G. A., Freudenheim, J. L., Fuchs, C. S., Giovannucci, E., Goldbohm, R. A., Graham, S., Harnack, L., Hartman, A. M., Jacobs, D. R., Kato, I., Krogh, V., Leitzmann, M. F., … Smith-Warner, S. A. (2005). Dietary fiber intake and risk of colorectal cancer: A pooled analysis of prospective cohort studies. Journal of the American Medical Association, 294(22), 2849–2857. https://doi.org/10.1001/jama.294.22.2849
- Fuchs, C. S., Giovannucci, E. L., Colditz, G. A., Hunter, D. J., Stampfer, M. J., Rosner, B., Speizer, F. E., & Willett, W. C. (1999). Dietary Fiber and the Risk of Colorectal Cancer and Adenoma in Women. New England Journal of Medicine, 340(3), 169–176. https://doi.org/10.1056/nejm199901213400301
- A A Rabassa, & A I Rogers. (n.d.). The role of short-chain fatty acid metabolism in colonic disorders - PubMed. Retrieved June 25, 2021, from https://pubmed.ncbi.nlm.nih.gov/1553927/
- Stephen, A. M., & Cummings, J. H. (1980). Mechanism of action of dietary fibre in the human colon. Nature, 284(5753), 283–284. https://doi.org/10.1038/284283a0
- J L Slavin. (n.d.). Dietary fiber: classification, chemical analyses, and food sources - PubMed. Retrieved June 25, 2021, from https://pubmed.ncbi.nlm.nih.gov/3040839/