There are a few facts about fiber on which everyone agrees. One: It's a crucial component of a healthful diet, in recommended amounts of about 25 to 30 grams per day. Two: With an average daily intake of only about II grams, most Americans aren't getting nearly enough. Three: Certain kinds of dietary fiber seem to reduce serum cholesterol levels or to promote "regularity."
Those are about the only assertions one can make safely with regard to dietary fiber. The jury is still out, albeit not for Iong, when it comes to the burning question of whether dietary fiber can actually reduce the risk of heart disease or certain cancers. The same goes for the equally important distinction: Is it the fiber that may possess these properties or is it the food?
Oat bran revisited
No food designer can forget the great oat bran controversy of the early '90s. After some 25 years of sporadic reports that oat bran possessed cholesterol-reducing properties, the ubiquitous grain became the ingredient de rigueur during the late '90s. Then, in 1990, the New England Journal of Medicine published a study showing that oat bran alone did not reduce cholesterol levels. The article, which generated a flurry of media activity, attributed those earlier findings to the elimination of saturated fat from the diet and its replacement with the oat bran.
Since then several studies have revived the reputation of oat bran -- or at least the soluble fiber it contains -- as a cholesterol-reducing agent, combined with other appropriate dietary measures.
Among these was a six-week study at the Chicago Center for Clinical Research at Rush-Presbyterian-St. Luke's Medical Center, in which six groups of patients received various doses of oat bran or oatmeal daily, and a seventh control group was given farina instead. All seven groups followed the National Cholesterol Education Program diet for reducing serum cholesterol levels, but only the groups receiving the equivalent of either 2 or 3 oz. of oat bran daily achieved marked reductions in total and LDL (low-density lipoprotein) cholesterol. Their cholesterol counts returned to baseline levels during a subsequent six-week period in which their oat bran was discontinued, even though they continued to follow the other diet modifications.
Another well-publicized study actually could be considered "Part Three" of a series of trials conducted by Northwestern University Medical School, Chicago. Five years earlier, in 1986, Northwestern researchers had achieved average reductions of 3% in blood cholesterol levels using two daily servings of oat bran or oatmeal and holding fat intake to a consistent, reduced level among their 238 subjects. In 1988, the Northwestern researchers were able to repeat that result in another trial in which they focused on oatmeal, having found a greater degree of adherence the first time around than with oat bran.
That 3% reduction is more significant than it might sound. According to Linda Van Horn Ph.D. and R.D., a professor of preventive medicine at Northwestern, a 1% reduction in blood cholesterol equals a 2% reduction in cardiovascular mortality when applied to the whole population. What's more, the Northwestern researchers discovered that the higher an individual's initial cholesterol level, the greater the reduction from the same amount of oatmeal.
Then, in 1991, the researchers discovered that even without strict dietary controls, people who increased their oat bran consumption adopted other heart-healthy eating habits, as well. In this third study, which focused on the public-health ramifications of their earlier findings, researchers provided their subjects with two packets of instant oats to be consumed daily.
The researchers found that to make room for the oats in their diets, people tended to cut back on high-fat foods that could increase their serum cholesterol levels. They also increased their consumption of foods containing beneficial nutrients, such as calcium, iron, potassium and magnesium -- nutrients found, for example, in the milk that many people added when they ate their oatmeal.
To many dieticians and nutritionists, this improvement in overall eating habits is almost as welcome as the cholesterol reduction itself. To Van Horn, these findings indicated that people could achieve positive results by adding very manageable amounts of oat bran or oatmeal to their diets.
"In other words, there are worse things a physician can do than to suggest that patients eat more oats," she says.
What if their patients don't care for oatmeal? Several studies have shown that some, but not all, sources of soluble fiber can work as well. Most of the evidence points to beta glucan as the cholesterol-lowering agent in oat bran and oatmeal, according to Van Horn Barley and legumes are other good sources of beta glucan.
"That's not to say that it's the only cholesterol-lowering agent," Van Horn notes, citing guar gum and fruit pectin as other agents that have proven effective.
Psyllium also has proven effective, but it might be a challenge for food designers to incorporate it as an ingredient. As Van Horn notes, whereas oats are a nutritious food, "psyllium has no additional benefits other than lowering lipids... I get less excited about something that works as a drug than something that is a food."
No one knows exactly how these agents perform their lipid-lowering work. Maybe soluble fiber from these sources interacts in the gut with bile acids, which are then excreted from the body along with the fiber. The liver may then extract blood cholesterol to replenish the bile acids that have been excreted.
Whatever the mechanism, it wouldn't hurt to add more of these foods to the diet -- especially if they, too, replace fattier foods. For instance, a bean burrito would be a good source of fiber, but it would have a greater impact on blood cholesterol if it replaced a cheeseburger. A bowl of barley soup would be a nutritional powerhouse, especially if it replaced a plate of fettucine Alfredo.
A consensus on cholesterol
Although the mechanism is not exactly understood, the scientific community has approached a consensus during the first part of this decade that soluble fiber can help reduce blood cholesterol levels, when combined with a reduction in dietary fat -- especially saturated fat. Just as important, it appears that most people can see some results with the relatively manageable addition of only about 3 to 5 grams of soluble fiber.
Many animal, clinical and epidemiological trials have confirmed those benefits in the past few years. In an analysis published last year in the American Journal of Clinical Nutrition, James Anderson, M.D., director of the Metabolic Research Group at the University of Kentucky, Lexington, wrote that some clinical trials have achieved reductions of up to 15% in total cholesterol and 16% in LDL cholesterol with the addition of soluble fiber.
One significant epidemiological study published this year in the American Journal of Clinical Nutrition is the first study to establish a link between soluble fiber intake and a reduction in blood pressure, independent of other factors. In this three-month observational study of 850 Yi, an ethnic Chinese minority, researchers observed that consumption of oats and buckwheat correlated inversely to both high blood pressure and high blood cholesterol levels. The more of these grains the individuals consumed, the greater the reduction in both of these risk factors for cardiovascular disease.
Still other studies are exploring the role of soluble fiber in slowing the absorption of glucose. This improves glucose tolerance, which in turn may help prevent non-insulin dependent diabetes. Other current studies are exploring the relationship between lipid and glucose metabolism. In short, dietary soluble fiber might turn out to be one ingredient that can help reduce the risk of diabetes, as well as the risk of coronary heart disease.
A weapon against cancer
Dietary fiber has been receiving even more press lately for its possible role in preventing cancers than for its usefulness in the fight against heart disease. According to Anderson, dietary factors contribute to 35% of all cancers. But the scientific community is considerably further from reaching a consensus on the cancer connection, beyond the fact that insoluble fiber -- found in grains such as corn, rice and wheat, as well as most vegetables and fruits -- serves as a bulking agent that may protect against colon cancer.
Several studies have found that wheat bran could help prevent precancerous polyps in the colon among subjects who have already had a polyp removed. Researchers at the Arizona Cancer Center at the University of Arizona, Tucson, have already found that wheat bran with calcium reversed precancerous changes in the lining of the colon. Under the direction of David Alberts, M.D., the researchers are now conducting large-scale polyp trials among more than 1,400 randomized subjects.
Another major trial is underway at the National Cancer Institute, Bethesda, MD, where 2,000 polyp patients are being compared over the course of four years. Half the subjects are being placed on a low-fat diet that includes 18 grams of fiber per 1,000 calories daily; the control group will consume the standard American diet. The dietary fiber will be drawn from a variety of sources -- half from grains, and half from vegetables and fruits.
According to Perla Vargas, M.A., the coordinator of the University of Arizona studies, environmental and dietary factors have been implicated in 85% to 90% of all colon cancers. Specifically, she cites excess consumption of fat and calories as culprits, and increased consumption of fiber and calcium as possible inhibitors.
In the November 1993 Oncology, Vargas suggested the mechanisms by which dietary fiber and calcium may work. Fiber, she wrote, "may reduce carcinogenic exposure by increasing fecal bulk and decreasing concentration of mutagenic compounds in the stool. Ionized calcium is believed to inhibit the effects of both fatty and bile acids by binding with these lipids to form insoluble calcium soaps."
Among the most recent -- and the most definitive -- studies linking consumption of wheat bran to a decreased incidence of colon polyps is the Australian Polyp Prevention Project. This study began in 1985 and the findings were reported in May 1995 to the American Gastroenterological Association conference by chief investigator Finlay Macrae, M.D.
The Australian researchers divided 424 subjects randomly into four dietary groups. Among their most significant findings was that those in the group that added a 25-gram serving of wheat bran to their daily diet and reduced their fat intake had developed no large polyps either two years later or four years later. Those in the group that added the bran without reducing the fat had a 40% lower rate of polyp occurrence than the group that did neither.
The Australian subjects all received a beta carotene supplement regardless of which dietary group they were in, but the researchers found no statistical effect.
On the other hand, an animal study published in the January 1995 Carcinogenesis did show a protective role for both wheat bran fiber and beta carotene. Under the direction of Oliver Alabaster, M.D., researchers at George Washington University's Institute for Disease Prevention, Washington. D.C., divided 30 rats into two groups. One group was fed a high-fat diet with little wheat bran; the other was given a high-fat diet with a large amount of wheat bran. Both groups received beta carotene. The equivalent dosage in humans would have been 40 grams of fiber and 20 mg of beta carotene per kilogram of body weight.
Despite the high-fat diet, the researchers found that both the fiber and the beta carotene protected against the development of precancerous cells and tumors, with the preventive effect of beta carotene and a low-fiber diet being about equal to the effect of a high-fiber diet without the beta carotene.
As promising as such animal data may seem, though, the scientific community is unlikely to come to a consensus on a relationship between insoluble fiber consumption and a reduction in colon cancer mortality -- at least until the results are in on the ongoing polyp trials.
Tenuous breast cancer link
Even less conclusive is the evidence pertaining to breast cancer. Several studies have shown an apparent link between high intake of fiber-rich foods and a reduced risk of breast cancer, although the highly regarded Harvard Nurses' Health Study did not.
Possibly the fiber reduces the level of estrogens in the bloodstream -- either by blocking the liver's production of enzymes that promote the absorption of estrogens or by binding the estrogens in the intestine.
The evidence in favor of fiber seems preliminary at best. According to the National Cancer Institute, it remains unclear whether any risk reduction shown so far should be attributed to the fiber or to other components of high-fiber foods, such as vitamins, antioxidants or phytoestrogens.
"Fiber is not a major hypothesis for breast cancer now," says Regina Ziegler, Ph.D. and M.P.H., a nutritional epidemiologist with NCI. "People are looking at pesticides, phytochemicals, and also different pathways of estrogen metabolism."
What is clear is that foods high in fiber -- whole grains, fruits and vegetables -- are high in other cancer-fighting substances as well, not to mention low in fat. Few researchers would dispute that a diet based on these foods -- indeed, the foundation of the U.S. Department of Agriculture's food guide pyramid -- can help protect against both cancer and heart disease.
That is the type of diet that most of the epidemiological data are based on," says David Kritchevsky, Ph.D., professor at the Wistar Institute in Philadelphia and co-editor of Dietary Fiber in Health and Disease. But when it comes to identifying which part of the diet is responsible for a particular result, "it's not always easy," he says.
There's also the danger that in their haste to bulk up on fiber, some consumers will overdo it. In fact, some studies have shown that excess consumption of soluble fiber alone may actually promote colon cancer. Food designers could help prevent such an overdose by combining soluble and insoluble fiber in the same products -- for instance, baked goods with both oat and corn fiber, or the aforementioned bean burrito in a whole-wheat tortilla.
For food designers and consumers, the bottom line remains: By all means focus on fiber-rich foods, but make them part of a balanced diet.
The Waiting Game on Fiber Health Claims
Officially, the U.S. Food and Drug Administration is sticking with its original position that health claims pertaining to fiber aren't yet supported by the evidence. Unofficially, industry sources maintain that FDA officials are nonetheless impressed with the body of evidence thus far and would look favorably upon more-definitive research in support of a health claim.
Even so, the agency has yet to approve a health claim specifically for dietary fiber -- although FDA's nutrition labeling regulations, which went into effect in May 1994, do require that it be listed on the Nutrition Facts panel.
At present, the only fiber-related claim that manufacturers can make is that foods that are good sources of dietary fiber -- such as fruits, vegetables and grains -- can help prevent coronary heart disease directly. A direct link between fiber and a reduction in blood cholesterol or heart disease risk is not permitted, nor are any claims linking fiber with a reduced cancer risk.
Some of the industry's top researchers into the benefits of dietary fiber acknowledge that the evidence supporting health claims for total and saturated fat far outweighs the evidence in favor of fiber's effects. Certainly, dietary fat reduction is the primary area of focus in preventing either cancer or heart disease.
But FDA was sufficiently intrigued by fiber to convene the leading researchers in May 1994 to hear all the evidence at once. The conference, entitled "Dietary Fiber and Cancer and Coronary Heart Disease," didn't yield any immediate, concrete results for food designers and marketers. However, it did show the industry and the research community what needed to be done.
"The conference participants seemed to be of the opinion that fiber itself is poorly defined," says Elizabeth Yetley, Ph.D., acting director of FDA's Office of Special Nutritionals. Specifically, she says, the conference showed "the need for better definitions of fiber sources, and maybe novel ways of defining them."
For example? "Maybe how well a fiber holds water, how viscous it is," she says. In other words, a structural definition -- as opposed to a chemical one -- may be more appropriate.
The concept of "structure/function claims" has been receiving a lot of attention as an alternative to outright health claims with the passage of the Dietary Supplement Health and Education Act of 1994, which endorsed such claims without defining exactly what they are. One example often cited is "dietary fiber helps promote regularity" which is a claim referring to how fiber functions in the body.
By contrast, wording such as "dietary fiber helps prevent colon cancer" would qualify as a health claim under the terms of the Nutrition Labeling and Education Act of 1990, and at present such a direct link to a specific disease has not been approved by FDA for dietary fiber.
FDA officials emphasize that they are no longer in the business of researching potential health claims themselves. Instead, they are now waiting for manufacturers to present their cases -- a scenario familiar to anyone who has submitted a petition for an additive approval.