This is the second article of a third part series on fiber. This article examines if fiber is good for heart health and protecting against colon cancer. The first article examines if fiber is good for constipation and lowering cholesterol. The third article examines if fiber is good for Irritable Bowel Syndrome (IBS) and heartburn.

Can Fiber Improve My Heart Health?

Two studies, one based in Finland and one in Japan suggest that people who eat lots of fiber (30-35 grams per day) may reduce their risk of coronary heart disease[i]. I asked Gary Taubes, the author of Good Calories Bad Calories, about these studies. Gary cautioned me about reading too much into them. Observational studies follow large groups of people (22,000 subjects in the Finnish case, and 58,000 in the Japanese study) over time to see how their health develops. Then they compare, for example, the subgroup that ate the most fiber with the subgroup that ate the least, measure the occurrence of disease in each, and then draw conclusions about cause and effect connections.

He pointed out that the problem with this kind of study is that fiber consumption may not be the only factor shared by members of the subgroups. For example, the subgroup that ate more fiber may also be more health-conscious in general, and a more careful look at them might reveal that they also drink less, smoke less and / or exercise more. If true, this means that you cannot make a cause-and-effect connection between fiber consumption and heart health based on these studies (At least not without a lot more information about the individuals).

Gary’s point is that many hypotheses generated by observational studies later turn out not to be true, and setting broad dietary guidelines without randomized, placebo-controlled clinical studies can lead to unintended and sometimes unhealthy consequences.

Can Fiber Protect Me Against Colon Cancer?

The idea that high dietary fiber may lower the risk of colon cancer came from studies in the 1970s that made a connection between low colon cancer rates in developing countries and diets higher in fiber[ii]. A more recent study of 88,757 women found no association between dietary fiber consumption and the risk for colon cancer[iii]. Similar results were obtained from the Fukuoka Colorectal Cancer Study [iv] and another large study of men[v].

Can Too Much Fiber Promote Colon Cancer?

Alberto Martin’s recent study in Cell provides evidence that at least in mice susceptible to colon tumors, too much butyrate (gut bacteria ferment fiber to produce butyrate) promotes colon cancer and a low carbohydrate diet or inhibiting the growth of gut bacteria is protective. About 1/5 of human colon cancers have the same type of mutation as the mouse model they studied. [vi]

Read Part I and Part III on fiber.

[i] Pietinen P, Rimm EB, Korhonen P, Hartman AM, Willett WC, Albanes D, Virtamo J. Intake of dietary fiber and risk of coronary heart disease in a cohort of Finnish men. The Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study. Circulation. 1996 Dec 1;94(11):2720-7. Eshak ES, Iso H, Date C, Kikuchi S, Watanabe Y, Wada Y, Wakai K, Tamakoshi A; JACC Study Group. Dietary fiber intake is associated with reduced risk of mortality from cardiovascular disease among Japanese men and women. J Nutr. 2010 Aug;140(8):1445-53.

[ii] Walker AR. Colon cancer and diet, with special reference to intakes of fat and fiber. Am J Clin Nutr. 1976 Dec;29(12):1417-26. Burkitt DP, Trowell HC. Dietary fibre and Western diseases. Ir Med J. 1977:70-272.

[iii] Fuchs CS, Giovannucci EL, Colditz GA, Hunter DJ, Stampfer MJ, Rosner B, Speizer FE, Willett WC. Dietary fiber and the risk of colorectal cancer and adenoma in women. N Engl J Med. 1999 Jan 21;340(3):169-76.

[iv] Uchida K, Kono S, Yin G, Toyomura K, Nagano J, Mizoue T, Mibu R, Tanaka M, Kakeji Y, Maehara Y, Okamura T, Ikejiri K, Futami K, Maekawa T, Yasunami Y, Takenaka K, Ichimiya H, Terasaka R. Dietary fiber, source foods and colorectal cancer risk: the Fukuoka Colorectal Cancer Study. Scand J Gastroenterol. 2010 Oct;45(10):1223-31.

[v] Giovannucci E, Rimm EB, Stampfer MJ, Colditz GA, Ascherio A, Willett WC. Intake of fat, meat, and fiber in relation to risk of colon cancer in men. Cancer Res. 1994 May 1;54(9):2390-7.

[vi] Belcheva A1, Irrazabal T1, Robertson SJ1, Streutker C2, Maughan H3, Rubino S4, Moriyama EH5, Copeland JK6, Surendra A6, Kumar S1, Green B1, Geddes K1, Pezo RC7, Navarre WW8, Milosevic M9, Wilson BC5, Girardin SE4, Wolever TMS10, Edelmann W11, Guttman DS6, Philpott DJ1, Martin A12. Gut microbial metabolism drives transformation of MSH2-deficient colon epithelial cells.  2014 Jul 17;158(2):288-299.