How Does Dietary Fiber Prevent Bowel Cancer?

It is not often that Western industrialized nations have looked to Africa for solutions to their own health problems. But when it comes to bowel cancer, that is what has happened.

Bowel cancer is the second most common cause of death by cancer in the United States: 1 in 24 Americans will suffer from it at some stage of their lives. And according to United States cancer specialist Oliver Alibaster, studies have shown that diet alone is responsible for 35 percent of them.

That’s where Africa comes in.

In 1981 Dr. Alibaster realized, after years of treating cancer patients, how little medical training he had received about the effects of diet. Soon after, he met British cancer pioneer Denis Burkitt, who had provoked worldwide interest in dietary fiber after performing extensive work in Africa in the 1970s.

Dr. Burkitt noticed that nearly all diseases that frequently occurred in Britain and other industrialized countries were extremely rare among Africans. The reasons were attributed to, more than anything else, diet, and lifestyle. In particular, he noted that the African diet was low in fat and high in fiber.

Fiber, the outer coating of plants, is rugged, indigestible, and a rather modest substance. In fact, it was so minimally regarded by the upper classes of France that in the 18th century, when they discovered they could get rid of it with a new milling process, they promptly invented and adopted white flour as a symbol of their wealth.

In the ensuing years, we have all followed in their footsteps so that fiber has been gradually reduced in our Western diets from 80 percent to a current average of only 40 percent. Taking its place has been fat, mainly derived from meat and dairy products.

“Put simply, we eat too much fat and too little fiber,” says Dr. Ali baster, “and there’s considerable scientific evidence that a diet low in fat and high in fiber, especially insoluble fiber like that found in wheat bran cereals, can significantly reduce the risk of bowel cancer.”

The Australian Nutrition Foundation points out that the fiber content of certain common foods such as wholemeal products, fruit, vegetables, and beans significantly outweigh other more popularly eaten foods like white bread, corn flakes, cakes, meat, dairy products, and soft drinks. It recommends a daily minimum intake of at least 30g of fiber. Dr. Alibaster goes further and suggests 50g per day is the most effective intake against bowel cancer.

The exact way in which fiber prevents bowel cancer is still difficult to understand precisely, says Dr. Alibaster. However, four factors are known to be involved.

  • First, the insoluble fiber in wheat bran increases the bulk of intestinal contents, speeding the time spent going through the bowel and diluting the cancer-causing chemicals found there. Overall, this reduces the time the bowel is exposed to these carcinogens.
  • Second, insoluble fiber absorbs and removes some of the bile acids (which normally help the body digest fat, but also promote bowel cancer) that are secreted by the presence of fat in the intestine.
  • Third, based on medical studies on rats, less fat is absorbed by the bowel in the presence of a high-fiber diet.
  • Fourth, it is likely that bacteria naturally found in the bowel are stimulated to grow by fiber which helps neutralize cancer-causing substances.

Dr. Alibaster does not stand alone on the subject of fiber, but some American cancer specialists are not as passionate about the health benefits.

Professor Richard Taylor, of the NSW Central Cancer Registry, says, “We will probably find definite evidence that a diet high in fiber and low in fat can reduce rates of bowel cancer, but up until now the evidence is only circumstantial.

“We do know that despite the changes in the diets of Americans in the last decade or so, bowel cancer rates haven’t been affected yet. If anything, they’re going slightly up.”

“But it could be that the lag time for the development of cancer means that it could take another 20 years before we see the benefits or that we need to do more to increase fiber and decrease fat intake.”

Dr. John Levi, director of oncology at Sydney’s Royal North Shore hospital, is less convinced. He agrees that dietary fiber is presently the most important prevention strategy. “But that’s because most of the research has looked at fiber, rather than knowing for sure that that’s the case,” he says.

“We’ve still got a way to go with understanding cancer and developing prevention strategies,” he says. “First of all, there’s genetics. It could be that if you were born with a family history of bowel cancer, it wouldn’t matter if you ate dietary fiber until the cows came in, you wouldn’t necessarily reduce your risk.”

“We also need to understand more about diet than just fiber. For instance, it may be that certain types of vegetables have a greater influence than others. And then there’s some initial evidence that the quantity of beer intake may increase the risk.”

“Overall, I think there is no question that someone whose diet is lacking in fiber should eat more. But all we can say is that it has some beneficial effects which are difficult to quantify at the moment.”

Apart from prevention, Dr. Levi says that treatments are getting better. “In the immediate future we’ll have as much impact on cancer by improving treatments as we will with the prevention,” he says. “But somewhere further in the future, we will be able to prevent all cancers.”

With that aim firmly in mind, everyone, from the fiber true-believers through to the less-enthusiastic, will be paying great attention to a prospective cohort study underway in the US.

Under the guidance of the National Cancer Institute, researchers are closely monitoring a community of people with an above-normal dietary fiber intake and comparing their bowel cancer rates with a control population that eats an average amount of fiber.

The results should be available in the next 5 to 10 years. When they are, we will be wiser about its importance. But in the meanwhile, it seems advisable to keep increasing fiber and decreasing fat intake — it is better to be safe than sorry.

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