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The F-Word

Fiber, That Is

Fiber is the stepchild of nutrients. When most people think about fiber, they conjure visions of constipation or, more precisely, avoiding that condition. It wasn’t always like that. During the 19th century men such as the Rev. Sylvester Graham produced a cracker that he named after himself and that he said would cure most intestinal ailments. The secret ingredient was nothing more than fiber, and it seemed to work as advertised. If you’d checked in to the fabled Battle Creek Health Institute, the first question you’d likely be asked by health director J.F. Kellogg was about your bowel movements, or the lack thereof. Kellogg, who invented a cereal based largely on fiber, defined health in terms of regularity. Early advocates of a high-fiber diet were dismissed as health quacks. In fact, fiber has only recently gained some respect among medical researchers, mainly as a consequence of research conducted in Africa and first reported in professional journals 33 years ago. At that time scientists such as Dennis Burkitt noticed that Africans who ate the indigenous diet showed a lack of diseases common to more ‘civilized’ people’colon cancer, for example, was as rare among the native Africans as an honest used-car dealer in Los Angeles.

The lack of colon cancer was eventually traced to the African habit of eating relatively vast amounts of fiber. At that time no one realized fiber’s health benefits, although some assumed it had something to do with promoting bowel movement. Since then fiber has been associated with fat loss, as well as with preventing cardiovascular disease (CVD), several types of cancer and diabetes.

Dietary fiber is defined as the indigestible part of plant foods. It occurs naturally in fruits, vegetables, beans, nuts and whole grains. Large chains of carbohydrate that are so tightly bonded that digestive enzymes are unable to break them apart are what make fiber indigestible. Still, fiber supplies bulk to the food the body does digest.

Fiber is usually divided into two basic categories: soluble and insoluble. While both have a role in such processes as preventing constipation, each kind protects health in a different way. Insoluble fiber, which doesn’t dissolve in water and of which wheat bran is the richest source, is made up mostly of the indigestible carb cellulose. Common sources of fiber that does dissolve in water include fruits, grains, oatmeal and beans.

Health surveys report that the average total daily intake of dietary fiber is 15 grams, although the recommended intake is 25 to 35 grams. Like present-day Africans, Paleolithic-era Homo sapiens showed little or no incidence of the major degenerative conditions that plague modern man, such as cardiovascular disease, diabetes and cancer. The usual cause of death back then’as it is today in Africa’was bacterial or viral infection. Among the components of diet during the Stone Age were large amounts of fiber. Paleolithic man took in an estimated average of 77 to 120 grams of fiber daily, or five to eight times more than most American adults get. Paleolithic man was also lean and muscular. About 59 percent of Americans are fat, while less than 15 percent of people living in the African nations of Kenya, Uganda and Malawi are obese. While food is comparatively scarce in the Third World, the African fiber intake nevertheless averages 60 to 80 grams a day.

An example closer to home involves the Pima Indians of Mexico and Arizona. The Pimas living in Mexico are on a more traditional diet richer in dietary fiber than that of their Arizona peers. They average 50 grams of fiber intake daily, compared to 19 in the Arizona Pimas. The Arizona Pimas are also considerably more obese than their brothers in Mexico, with higher rates of diabetes and cardiovascular disease. So how can fiber help you in your quest for added muscle and maximum health? Read on.

Fiber: The Cardiovascular Connection

Water-soluble fiber is most associated with preventing cardiovascular disease. It appears to lower total blood cholesterol and low-density-lipoprotein (LDL) cholesterol, the kind linked to disease onset. As far back as 1963 a study of 21 men, aged 30 to 50, found that eating rolled oats’a form of soluble fiber’led to an 11 percent drop in total cholesterol levels over a three-week period.1 Another three-week study found that those who ate 17 grams of soluble fiber daily showed a 19 percent decrease in total cholesterol and a 21 percent drop in LDL. Other studies show that it takes a minimal daily consumption of three grams of soluble fiber to beneficially affect blood cholesterol levels.

Soluble fiber lowers blood fat levels because it forms a viscous gel after it gets into the body. For one thing, it binds bile acids in the intestine. The body can’t burn cholesterol, and the only way to eliminate excess cholesterol is by increasing the excretion of bile acids, which are largely made from cholesterol. When the body eliminates cholesterol through fiber-binding, the liver uses more cholesterol to make new bile acids, thereby lowering the level in the blood. One study showed that oat bran increases the excretion of secondary bile acids, which are made in the colon, by 240 percent.2

A high-fat diet usually leads to a depression of receptor sites for LDL cholesterol, so its levels increase in the blood. A study with hamster subjects, however, showed that even when the little critters were on a high-fat, high-cholesterol diet, their LDL-receptor levels were maintained if they were also eating a lot of soluble fiber.3 Moreover, the type of soluble fiber used in the study, psyllium, increased fourfold the rate-limiting enzyme in the liver used to synthesize cholesterol. Oat bran depresses that enzyme.

A study involving guinea pigs, which process blood fats the way humans do, found that giving the pigs two types of soluble fiber, pectin and psyllium, led to a 32 percent drop in cholesterol, a 25 percent lower blood triglyceride (fat) level and decreased LDL oxidation.4 That’s important because oxidized LDL is what spells CVD danger.

A more recent study involving 2,909 young people, aged 18 to 30, found that fiber intake correlated with insulin levels, weight gain and cardiovascular risk factors more closely than total or saturated-fat intake.5 Fiber offered protection against obesity by lowering blood insulin levels.

In a longitudinal study examining the effects of fiber on CVD, researchers followed 10,000 subjects for 19 years.6 Those who ate the most fiber during that time showed a 12 percent lower risk of CVD than those eating the least. The CVD reduction among those getting soluble fiber was 15 percent.

Besides helping to rid the body of excess cholesterol through bile-binding, soluble-fiber is fermented in the colon by resident bacteria, which results in the release of short-chain fatty acids. One of these, propionate, is shuttled to the liver, where it inhibits cholesterol synthesis. Pectin also helps prevent CVD by reducing excess internal blood clotting.7 Most heart attacks and strokes start from the formation of a clot in an already occluded artery. ALL Fiber and Colon Cancer

For years fiber was considered protective against colon cancer. After lung cancer, colon cancer is the second leading cause of cancer death. Insoluble fiber was the hero here. Because insoluble fiber increases intestinal transit time, it decreases the contact between intestinal cells and the secondary bile acids produced in the colon that irritate the colonic lining and initiate a process that culminates in tumor formation.

Soluble fiber helps also because bacteria ferment it in the colon. When this occurs, the pH, or acid, concentration in the intestine is limited, thus helping protect the body against cancer. Short-chain fatty acids are also released. One of them, butyrate, acts as a direct energy source for colonic cells, as glutamine does for small-intestine cells, which helps prevent cancer. Butyrate also causes latent cancerous cells to kill themselves, a process known as apoptosis.

Even so, a 1999 study of 88,000 nurses found that fiber offered no protection against colon cancer.8 Two other studies published in 2000 also concluded that fiber just didn’t work as advertised. It seems that soluble fiber works best to prevent colon cancer when combined with fish oil or omega-3 fatty acids.9 Omega-3 fats lower the activity of an enzyme called COX-2, which promotes the synthesis of eicosanoids linked to tumor formation and prevents the self-destruction of cancerous cells; anything that blunts or blocks COX-2 helps protect against cancer. Butyrate promotes cancer-cell suicide but only when COX-2 is low. That explains why some studies say that fiber is ineffective against colon cancer; you have to get both soluble fiber and omega-3 fats for best results.

A study involving 400,000 people across nine nations recently found that those who ate the most fiber showed a 40 percent decreased risk of colon cancer. One difference between that research and studies showing no effects from eating fiber is that the subjects ate about three times as much fiber as subjects in the other studies.

As reported in the International Journal of Cancer in 2001, eating fiber helps prevent cancers of the mouth and throat. People who ate the most fruit, vegetables and whole grains showed half the cancer incidence of those who ate the least amounts of those foods.

High-fiber foods also contain substances collectively known as phytochemicals, most of which function as potent antioxidants and many of which block or inhibit the progress of cancer cells. For example, phytic acid, found in whole grains, appears to block colon cancer development.

Does Fiber Help You Lose Fat?

A high-fiber diet reduces food’s energy density. That means you feel fuller more quickly when eating a high-fiber food, so you eat fewer calories. Just chewing a high-fiber food makes you eat more slowly, enabling your brain to sense the food intake and tone down appetite. When you eat a soluble-fiber source with a high-glycemic-index carb, the fiber slows the entry of carb into the blood. That, in turn, reduces insulin release, making a fast-acting carb resemble a low-glycemic-index, or slow-acting, carb. Less insulin means better blood glucose control and less hunger, as well as less bodyfat synthesis.

Some studies show that soluble fibers promote the release of gut hormones, such as glucagon peptide 1, which slows gastric emptying and reduces hunger. Research on obese people shows that they secrete less GLP-1 than their thinner counterparts.

Fiber may even lock on to the fats in food, thereby reducing fat absorption to some degree. One study showed that when a drug called Orlistat, which works by inhibiting lipase, or fat-digesting enzymes, combines with psyllium, side effects such as excess gas, abdominal cramps and incontinence are prevented.10

Fiber may be useful to bodybuilders because it can slow down the absorption of high-glycemic carbs. That lowers insulin release, which fosters more rapid bodyfat loss. When athletes eat a high-soluble-fiber meal before training, they maintain a more stable blood glucose level throughout the workout. Adding some fiber to a protein drink slows gastric emptying, making you feel fuller and less hungry.

A study of 50 men and 94 women, aged 30 to 80, compared the effects of eating breakfasts having either a low or high fiber content.11 Subjects on the high-fiber breakfast reported less emotional distress, fewer cognitive (thinking) problems and less fatigue. Although the authors didn’t explain those effects, the likely mechanism was better glycemic, or glucose, control.

As for drawbacks to increasing fiber intake, too much too soon will likely cause a few undesirable gastrointestinal events, mainly excess gas, with some abdominal bloat. You have to increase fiber intake gradually, working up to a daily total of at least 35 grams and keeping in mind that the higher the fiber intake, the more health protection you get. On the other hand, a study of women who took more than 35 grams a day of fiber over the course of a year found no adverse gastrointestinal effects.12 Some studies show that soluble fiber taken in with carotenoid nutrients, such as lycopene and beta-carotene, may impede uptake of those important antioxidants. The solution is simple: Just eat your fiber separately from food or supplements containing them.

One form of fiber, called lignans, is found in the popular food supplement flax oil, among other sources. It locks on to steroid hormones, such as testosterone and estrogen, causing them to be excreted.13,14 The literature doesn’t show the amount you’d need to take in to have a significant effect in that situation, but I’d guess that it would be considerable. The only people who may be at risk are vegans, who get far higher levels of lignans than others. Sure enough, some studies do show lower testosterone levels in such people, although their lowfat diet may have more to do with it.

The good news is that lignans speed the elimination of estrogen from the body. That means less breast cancer, uterine cancer and any other cancer related to excess estrogen levels.15

So fiber does a lot more than just prevent constipation and related diseases, such as diverticulosis and diverticulitis. Besides aiding dieting and fat-loss efforts, it boosts protection against various types of cancer and heart disease, the two leading causes of death in the world. The F word to describe fiber’s effects can only be fantastic.

Food Sources of Insoluble Fiber

‘Wheat bran
‘Whole-wheat products
‘Bran-based cereals or shredded wheat
‘Crunchy vegetables

Food Sources of Soluble Fiber

‘Oat bran
‘Citrus fruit
‘Dried beans (prepared)


1 DeGroot, A.P., et al. (1963). Cholesterol-lowering effects of rolled oats. Lancet. 2:303-304.
2 Marlett, J.A., et al. (1994). Mechanism of serum cholesterol reduction by oat bran. Hepatology. 20:1450-57.
3 Horton, J.D., et al. (1994). Regulation of hepatic 7 alpha-hydroxylase expression by dietary psyllium in the hamster. J Clin Invest. 93:2084-92.
4 Vergara-Jimenez, M., et al. (1999). Pectin and psyllium decrease the susceptibility of LDL to oxidation in guinea pigs. J Nutr Biochem. 10:118-24.
5 Ludwig, D.S., et al. (1999). Dietary fiber, weight gain, and cardiovascular disease risk factors in young adults. JAMA. 282:1539-46.
6 Bazzano, L.A., et al. (2003). Dietary fiber intake and reduced risk of coronary heart disease in U.S. men and women. Archives Int Med. 163:1897-1904.
7 Veldman, F.J., et al. (1997). Dietary pectin influences fibrin network structure in hypercholesterolemic subjects. Thromb Res. 86:183-96.
8 Fuchs, C., et al. (1999). Dietary fiber and the risk of colorectal cancer and adenoma in women. N Eng J Med. 340:169.
9 Lupton, J.R. (2000). Is fiber protective against colon cancer? Where the research is leading us. Nutrition. 16:558-61.
10 Cavaliere, H., et al. (2001). Gastrointestinal side effects of Orlistat may be prevented by concomitant prescription of natural fibers (psyllium mucilloid). Int J Obesity. 25:1095-99.
11 Smith, A., et al. (2001). High-fiber breakfast cereals reduce fatigue. Appetite. 37:249050 12 Mceligot, A.J., et al. (2002). High dietary fiber consumption is not associated with gastrointestinal discomfort in a diet intervention trial. J Am Dietetic Assoc. 102:549-551. 13 Shultz, T.D., et al. (1986). In vitro binding of steroid hormones by natural and purified fibers. Nutr Cancer. 8:141-7. 14 Ross, J.K., et al. (1990). Dietary and hormonal evaluation of men at different risks for prostate cancer: Fiber intake, excretion, and composition, with in vitro evidence for an association between hormones and specific fiber components. Am J Clin Nutr. 51:365-70. 15 Rose, D.P., et al. (1991). High-fiber diet reduces serum estrogen concentrations in premenopausal women. Am J Clin Nutr. 54:520-25. IM

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