Fiber Report Shows You Can’t Trust a Lone Study

First beta carotene helped prevent lung cancer.

Then it didn’t.

At one time, eggs were great for health. Then they fell out of favor, and now they’re OK again--in moderation.

Until last week, we thought that a diet high in fiber could protect against colon cancer.

Oops, maybe not.

It’s little wonder that consumers trying to eat healthily are frustrated with the advice they get from scientists these days.

“People feel like these nutritional studies are ridiculously inconsistent,” notes Bonnie Liebman, director of nutrition for the Center for Science in the Public Interest in Washington, D.C. “People think these scientists don’t know what they’re talking about. So they decide, ‘I’m going to eat whatever I want.’ ”

But experts eyeing the fallout from last week’s headlines, in which a study by Harvard researchers in the New England Journal of Medicine found no link between high-fiber diets and protection against colon cancer, see something positive amid all the waffling. Consumers, they advise, should follow overall dietary guidelines and not make changes in their diet based solely on: (1) findings from a single research study; (2) studies that examine a single food or nutrient; (3) studies on the effect of a single food on a particular disease.

In other words, avoid specific nutritional advice and follow the major recommendations, Liebman says.

“People feel like scientists keep flip-flopping. But what most people don’t understand is that this is how science works,” she says. “It’s rare that every single study reaches the same conclusion. Each study is a piece of the puzzle, and then scientists look at the big picture.”

And that goes for those single, headline-making studies that appear to show that a particular food or nutrient is beneficial, experts say.

“We are a society that goes to the extreme,” says Keith Ayoob, a spokesman for the American Dietetic Assn. and assistant professor of nutrition at Albert Einstein College of Medicine in New York. “It’s hard for people to wait until there is a lot of evidence” or to even know when the scientific community is in agreement.

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The fiber-colon cancer association is a perfect example of how positive studies become inflated into solid truths before they are, in fact, definitive.

The fiber-colon cancer belief began with a simple observation. In the 1960s, a researcher named Denis Burkitt noticed that in certain areas of Africa, consumption of high-fiber diets seemed to go hand in hand with low rates of colon cancer.

There is logic to the idea. Researchers have proposed that fiber dilutes harmful substances like carcinogens and bacteria as they travel through the bowel. Fiber also speeds up digestion, limiting the amount of time carcinogens come in contact with the bowel.

Much research has tested that hypothesis. Many of the studies were epidemiological studies, in which a large population is asked about various lifestyle and health habits, such as eating patterns, to look for correlations. But those studies do not show a cause-and-effect relationship, and imprecise memories or lack of candor about eating habits can be a weakness of such research, Liebman says.

Other studies lent credence to the fiber-colon cancer theory. A 1991 Harvard study that was widely publicized found men with high-fiber diets had one-third the rate of colon polyps of men with low-fiber diets. Most colon cancers arise from polyps.

“You can’t make the jump [from wheat bran] to colon cancer,” says Barbara O. Schneeman, dean of the College of Agriculture and Environmental Sciences at UC Davis. “And you don’t even know if it’s the wheat bran or the fiber, or is there some other protective factor?”

Business interests also played a role in promoting the fiber-colon cancer link. Cereal manufacturers, such as the Kellogg Co., advertised the health benefits of eating high-fiber products and even applied to the Food and Drug Administration for the right to make health claims on the boxes.

(Those claims, which FDA officials say need not be changed solely on the basis of last week’s study, say that fiber can reduce the risk of “certain types of cancers.” Fiber is still valued for its ability to lower the risk of heart disease and diabetes.)

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Ignored amid all the touting of fiber’s benefits were other less-publicized studies that had already questioned the fiber-colon cancer connection. In 1994, another Harvard epidemiological study following the health of more than 40,000 physicians found no link between a high-fiber diet and reductions in colon cancer. That study, and several others that have found no connection, received little media attention.

Because last week’s study, widely praised for being meticulously performed, was also a single epidemiological study, the relationship between fiber and colon cancer remains in doubt, Schneeman says.

“No one study is ever the definitive study. Epidemiological data can’t stand on its own. . . . You need the totality of research,” she says.

While controversies surround topics like sugar, alcohol, salt, oil, vitamin C and many other nutrients and foods, the major dietary recommendations remain solid.

During the last 10 to 15 years, “there has been a consistent message: Reduce your fat intake, and increase fruits, vegetables and grains,” Schneeman says. “One of the things we need to get away from is always breaking food down into its components. We are looking more at food dietary patterns. It may be the food pattern that is important.”