Cancer Prevention Diet, RIP

It is time to reconsider the whole idea that the public ought to change its diet to help prevent cancer.

The cancer prevention diet was launched on inadequate scientific evidence almost 25 years ago. Now five different randomized clinical trials have evaluated the preventive power of various recommended foods or ingredients, and not a single one has found a measurable effect.

Rare is the adult who has not head the message from the National Cancer Institute that to lower the risk of cancer, people ought to eat five servings a day of foods with high fiber, low fat and containing fruit or vegetables. This dietary advice was publicized extensively with millions of dollars from the fruit and produce industry, which was hoping to increase market share for its products.

The larger lesson, however, is that the diet and cancer story proves how easy it is for health authorities to give dietary advice and the eagerness with which the public embraces it despite the shaky scientific foundations of the whole enterprise.

Of all the key aspects of health, diet may be the most difficult to study. We eat different foods at every meal, and diet varies greatly day by day. Most of the food we eat is either excreted or heavily processed in the human chemical factory and reduced to basic compounds such as sugars, trigylcerides and lipids.

To make matters more difficult, the exact same food item can have quite different effects when consumed as a liquid, a solid or mixed with other foods. With some ingredients such as sodium and calcium, the body tightly regulates concentrations within narrow limits using powerful biological mechanisms to dispose immediately of any surplus. Only dire shortages or gross excesses overwhelm this system--and these can be life threatening but are quite rare.

Because we eat a relatively fixed total quantity of food at each meal, it is particularly difficult to study any one item. For example, if people are persuaded to eat two oat bran muffins for breakfast, we have to consider not only the effect of additional oat bran fiber but also the impact of what got removed from the breakfast meal to make room for that bulky muffin. It might have been low-fat milk that was crowded out, bacon or fruit.

We are the sum total of what we eat, and this is inherently a poor target for a reductionist scientific method that relies on breaking down a complex problem into smaller and smaller parts to study in depth. All this was known, or should have been known to researchers looking for a link between diet and cancer.

However, the ethic of the diet advice industry was to make recommendations first, and do the research later. The cancer-prevention dietary advice dates back at least to a major scientific conference held in 1975. The National Cancer Institute gathered the nation's leading experts to set a research agenda to consider whether there was a link between diet and cancers and decide what studies should be performed to identify specific causes. A keynote speaker, Donald Hegsted of Harvard University, summed up by telling them not to wait for research before giving Americans dietary advice.

"I would support …the position that one does not need to know all of the answers before one can make practical recommendations," Hegsted told the scientists. "Indeed if we take that position we will never make recommendations." He urged them to promote a diet with "less fat, less meat, less cholesterol, less food and more fruits and vegetables, and cereals, especially crude cereals."

By 1981 little more had been proven, but diet had won a prominent role in the pantheon of cancer dangers equal to the peril of cigarette smoking. Behind this pivotal event was a study by two famed Oxford University epidemiologists, Richard Doll, an original discoverer of the link between cancer and cigarette smoking, and his younger colleague, Richard Peto.

The two examined all the causes of cancer, and all the kinds of cancers, and concluded air pollution, toxic chemicals, and contaminants in food were of minimal importance. However, a large majority of cancers were preventable, they concluded Doll and Peto concluded than as few as 10 percent as many as 70 percent of cancers were linked to diet. The only reason it wasn't a range of 1-100 percent was need to leave room for the toll of cigarette smoking, already blamed for 30 percent of cancer deaths. To this day the "one-third of cancers" supposedly linked to diet can be traced to the midpoint of Doll and Peto's broad estimate.

But even this was only a guess. "We have attributed the largest risk to dietary factors," they wrote. "It must be emphasized the figure chosen is highly speculative and chiefly refers to dietary factors which are not yet reliably identified."

However, even by 1981 two possible preventive factors, beta-carotene and other precursors of Vitamin A, and dietary fiber were already identified. It was a satisfying overall package. Beta-carotene was an antioxidant, and there was some evidence that antioxidants might have a role in preventing the No. 1 cancer killer, lung cancer. Fiber seemed to have a preventive role in the No. 2 cancer--colorectal cancer-and eating more fiber usually meant a lower fat diet, and both animal and vegetable fats were also suspects in increasing cancer risk.

Finally, these theories nicely fit the dietary recommendations that had already been made. While the scientific literature grew to include literally hundreds of studies examining these issue from some narrow perspective, the scientists at the National Cancer Institute understood that convincing scientific evidence was going to be achieved with only one scientific technique: the randomized clinical trial.

In a clinical trial intervention study, the only meaningful difference between a treatment group and an untreated control group is the chemical or food item under study. The best studies are double blind, with neither investigator nor volunteer knowing whether they are receiving the active treatment, or a placebo.

Beta-carotene was the first to be studied in such trials, but researchers used a dietary supplement pill rather than foods. The pill had a known quantity of the chemical under study, could be given in a double blind trial using a placebo, and did not involve the experimental complexities of trying to modify diet in a consistent fashion for years on end.

Enthusiasts believed that beta-carotene had wondrous powers-that it might be an antioxidant, boost the immune system, and even inhibit the formation of cancerous cells. This evidence came from bench biochemistry, experiments with cells in petri dishes, and examinations of large populations of people where many, many factors could be involved. Now this idea was being tested in a manner that might provide definitive scientific evidence of the benefit to ordinary people.

As a National Academy of Sciences panel recently noted, not one major trial of beta-carotene produced any evidence of a beneficial effect on cancer, and one study suggested a possible harmful effect. Whether beta-carotene was studied in low risk patients (22,000 practicing physicians), or among high risk asbestos workers and heavy smokers, no benefit was seen in studies of 8 to 12 years duration.

Fiber, fruits, vegetables-the heart of the "5 a day program" --were the next to be systematically studied in clinical trials. One central problem in studying cancer is that despite it being the second ranked cause of death, cancer is quite rare in any group of healthy people and therefore requires studying literally tens of thousands for many years to acquire a few dozen cases of the specific cancer of interest.

However, colon cancer begins as benign polyps and adenomas that only later become malignant. By leaving out all the thousands of people who were not at great risk anyway, and focusing on people who already developed the earliest precursors, the investigators could learn much more about fiber in trials of only a few thousand patients.

Still the problem of modifying diet remained. In one study participants were given one of two identical-looking breakfast cereals from Kellogg that contained either 13 grams of additional fiber or just 2 grams. The second study attempted and achieved a broader dietary modification. Through training and counseling sessions, the intervention group was induced to increase fiber by 75 percent, boost their number of servings of fruits and vegetables by two thirds; and to reduce the fat in their diet. The other group was not counseled and food intake remained largely unchanged.

In both studies the intervention was foods rather than the purified chemical ingredient. In both studies real dietary changes were maintained for three to four years. Nevertheless, no effect whatever was found on the recurrence of polyps or adenomas in either study. Fiber, fruits and vegetables worked no better in preventing cancer than had beta-carotene

One difficult feature of widely publicized but premature health advice is that the enthusiastic but now embarrassed proponents almost invariably do not accept the clinical trials or other definitive scientific evidence. It would be possible to fill an entire book with arguments, criticism and scientific data from proponents who will claim either that the evidence still proves the diet-cancer link, or allege fatal flaws in the five clinical trials just described. As in the case of other failed dietary advice, proponents will claim the clinical trials were faulty, not their thinking.

It is also possible--if not likely--that among the avalanche of alibis and explanations are some findings that are both valid and support some kind of link between cancer and some aspect of the diet. What proponents continue to lack is scientific evidence from properly structured diet interventions that show the diet changes recommended actually result in the prevention of cancer. That the dietary advice has become so familiar through repeated advertising and promotion does not improve its validity one iota.

The National Cancer Institute is to be commended for designing and funding large clinical trials that have returned convincing answers on these significant scientific issues. But the science base needs to be greatly strengthened for a wide array of dietary recommendations with more intervention studies capable of focusing on the totality of that complex biological mechanism we call a human being.