First they told us to eat fiber. Now they're telling us to forget it. What's a colon to do? 

On April 20, the New England Journal of Medicine tied America's bowels in a knot with two new studies declaring that high-fiber diets do nothing to prevent the development of colon cancer, the second leading cause of cancer deaths in this country. For 30 years the experts had been telling us that stuffing ourselves with cereal with the taste and texture of Kitty Litter was the key to keeping our colons from killing us. Now we were free to fling our bowls across the breakfast nook.

According to most press coverage, the two studies were both powerful enough to induce the vapors (New York Times: "Experts on colon cancer said the research … left them stunned") and definitive (NBC: "The new studies are actually the first big ones to thoroughly test the fiber-colon cancer connection"). No less than Dr. Jerome Groopman of Harvard Medical School, wrote an op-ed in the New York Times about how he wished that a patient and friend who had died of colon cancer had lived to learn that his diet had had nothing to do with his disease. The friend, who was also a doctor, felt guilty for bringing on his condition by "eating salami and eggs" instead of "whole grains for breakfast."

But don't go on the Colon Smart All Salami diet just yet. It turns out the studies were limited and disputable. And they fail to make the case that there's no connection between diet and colon cancer.

Both studies started with this thesis: Put people in their 60s who've already been diagnosed with colon polyps on high-fiber diets, and they won't get colon cancer. But when the results came in, thesis had become anti-feces. In one study, led by Dr. David S. Alberts of the University of Arizona, 719 people in an intervention group were given a daily serving of high-fiber cereal, a sort of intestinal bran bomb. They had about the same number of polyps—benign colorectal growths that signal increased risk of colon cancer—at the end of three years as a control group who were given cereal with minimal fiber. In the other study, led by Dr. Arthur Schatzkin of the National Cancer Institute, 958 people in an intervention group extensively changed their diets, giving up chuck roasts for chickpeas and raising their fiber intake by 75 percent. At the end of four years their colons, too, were as efficient polyp factories as a control group who didn't change their diet at all.

The idea that what goes in one end affects what happens at the other grew in part out of international studies of colorectal cancer rates. Incidence is high in prosperous, developed parts of the world where people eat a lot of meat, processed foods, and sugar—diets high in fat and low in fiber. If that sounds like dinner, then it's probably no surprise that when it comes to cases of colorectal cancer, the United States leads the world. Rates are also high in Canada, Europe, and Australia. But incidence is low where diets are based on plant foods, from grains to roots to vegetables, such as in Africa, Asia, and parts of Latin America.

Underscoring the diet connection is what happens when people move from places with a low colon cancer incidence to places where it is high. Their colorectal cancer rates soar. Take a survey of Japanese-born women who moved to Hawaii. Within their lifetime their rates of colon cancer jumped almost fourfold and remained high in their children. Chinese men who moved from Shanghai to Los Angeles experienced a doubling of colon-cancer rates. (Yes, a lot of things change when people move to America, so there may be some other factor at work here. But the New England Journal of Medicine has yet to publish a study on the connection between colon cancer and the purchasing of minivans.)

There are also biological explanations for the belief that a healthy colon is a colon that moves with autobahn-like efficiency. Diets high in fiber are thought to eliminate carcinogens we eat, as well as binding with bile acids, diluting their possible damaging effects. The high-fiber eaters in the Alberts and Schatzkin studies each consumed about 30 grams of fiber a day, the federal government's recommended level for adults. (For comparison purposes one cup of Raisin Bran contains 8 grams of fiber; an orange, 3.) Alas, the average American eats only about 13 grams a day. But in parts of the world with low incidence of colon cancer, people can consume 60 to 80 grams a day, says Dr. T. Colin Campbell of Cornell University. Pity the Ex-Lax marketing manager assigned to this territory.

Since America is No. 1 in colon cancer (there's a catchy headline for USA Today!), let's accept the idea that it's connected to the American diet. But we still don't know the exact mechanism. Is it the high consumption of meat and fat? The low consumption of vegetables and fruit? The high intake of sugar and refined carbohydrates? Or some combination of these? Since the Alberts study simply told its participants to keep eating American style but sprinkle some bran on top, it seems an unlikely way to prevent the disease.

That's what Cornell's Campbell believes. "The [Alberts] study should never have been done. Anyone could guess it wouldn't work," he says. "As soon as one begins to focus so narrowly on one nutrient or food, particularly in a population consuming a high-risk diet to begin with, one is setting themselves up for failure."

Look at it this way: Colon cancer is not a deficiency disease like scurvy. The way to cure scurvy is simple—consume sufficient vitamin C. But the development of colon cancer is a complicated, possibly decadeslong series of cellular changes and not simply the result of a bran deficiency. A co-author of the Alberts study, Dr. Maria Elena Martinez, concedes as much, "Sometimes we're guilty of looking for a magic bullet. Sometimes we find it, sometimes we don't."