5.3: Fiber and Health
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Whole grains, vegetables, and fruit, besides being rich in carbohydrate are also rich in fiber. Since fiber, by definition, is the indigestible parts of plants, it’s a not a required nutrient, per se. But fiber does add bulk to the diet without adding calories, and in this way can reduce the risk of becoming overweight.
Because fiber is indigestible, it passes down the digestive tract where it may reduce the risk of chronic constipation, hemorrhoids, diverticulosis, and colon cancer. Fiber’s possible relationship to diverticulosis and colon cancer will be discussed here; other conditions, e.g., fiber and heart disease, will be discussed in upcoming chapters.
Diverticulosis
Diverticulosis is the outpouching of the wall of the colon, much like a hernia (see Fig. 5-5). An estimated 35% of U.S. adults under age 51, 40% of those age 51-60, and 58% of those over age 60 have one or more of these outpouchings.3
The outpouchings (diverticuli) themselves ordinarily don’t cause problems, but in a few cases (5% or less) the outpouchings become inflammed, resulting in painful diverticulitis (inflammation of diverticuli) that can result in ulcers severe enough to require hospitalization.
With a low-fiber diet, the muscles in the colon have to squeeze hard to pass stool that’s too small or too hard. This increases the pressure inside the colon, which eventually may cause outpouchings at weak points in the colon wall. Fiber is thought to help prevent diverticulosis by providing bulk and softness to the stool, which helps the colon move things along with less pressure.
Colon Cancer
Colon cancer is the second leading cause of cancer death in the United States (lung cancer is first). There’s thus a tremendous interest in the possibility that diet may affect one’s risk of colon cancer.
In looking at colon cancer rates throughout the world, higher rates of colon cancer have been associated with diets low in fiber, high in fat and sugar, and animal protein. (All these dietary factors tend to be linked together; diets high in animal protein tend to be high in fat and sugar and low in fiber.) Other studies, such as those comparing vegetarians with non-vegetarians, also show an association between more fiber and less colon cancer.
But these studies show only associations rather than cause-and‑effect relationships. Studies with laboratory animals have had inconsistent results, with some showing fiber having protective effects, some showing no effect, and some even showing an increase in colon cancer.
The probable reason for these variable results is that the diets studied differed in the types and amounts of fiber. Oat bran, for example, isn’t the same as wheat bran. Even differences in how fine the various fibers are ground can change the outcome of a study
As can be seen from the array of high-fiber cereals on grocery shelves, the possible role of fiber in lowering risk has been well advertised. Although fiber might be helpful in lowering the risk of colon cancer, it could be that other dietary components are bigger factors.
Might the high fat and protein typical of low-fiber diets play a role in colon cancer? Fat and protein both stimulate the secretion of bile in the intestine, and suspected carcinogens (cancer-causing substances) include breakdown products of bile. Some kinds of colon bacteria have been shown to change bile acids into carcinogens.
Carcinogen: Any substance that causes cancer.
A study of over 88,000 nurses found that colon cancer occurred 2 1/2 times more often among those who ate beef, pork, or lamb as a main dish every day, compared to those who ate these meats as a main dish less than once a month.4 Most cuts of beef, pork, and lamb are high in both fat and protein, so this finding supports the hypothesis that a high intake of fat and protein increases the risk of colon cancer.
But the study doesn’t prove cause and effect. As discussed in Chapter 1, studies of population groups—in this case a big group of nurses—give clues to causes of disease. Selected aspects of the population (e.g., body weight, diet, exercise habits) are compared statistically to the occurrence of a disease. These studies only show links (people who differ in one way often differ in other ways that may or may not have been considered).
In this nurses study, a daily main dish of meat was linked with an increased occurrence of colon cancer. It doesn’t show that meat causes colon cancer. It could be, for example, that those who eat more meat also drink less milk. Dietary calcium may be protective against colon cancer, and milk is the main source of calcium in the American diet.
Fiber-rich foods, by making the stool softer and bulkier, are helpful in preventing constipation, diverticulosis, hemorrhoids, and perhaps colon cancer—good reason to eat more fruit, vegetables, and whole grains.
How might fiber be protective? Fiber might play a role in preventing colon cancer by moving substances faster through the colon. This would shorten the time that the colon is exposed to any carcinogen present. As a general rule, the length of exposure to a carcinogen has a bearing on whether cancer is induced in a cell (e.g., the more a person smokes, the greater the likelihood of developing lung cancer.). But if the time of exposure was the crucial factor, it would be expected that people with a history of constipation would be more likely to get colon cancer. This doesn’t appear to be the case.
It’s also generally true that the greater the concentration of carcinogens, the greater the risk of cancer (e.g., smoking unfiltered rather than filtered cigarettes raises the risk of lung cancer). By providing bulk, fiber might help prevent colon cancer by diluting carcinogens in the colon.
Fiber is also thought to promote a healthier microbial population in the colon which may, in turn, help prevent colon cancer.
Clearly, the possible effect of diet on colon cancer isn’t simple or straightforward. Based on current evidence, however, it appears that a diet of more fiber-rich foods, fewer fatty foods, and only moderate amounts of protein can help guard against colon cancer. This is consistent with the Dietary Guidelines (discussed in Chapter 18) that we eat more whole grains, vegetables, and fruits, and less animal fat.