Cancer Prevention Diet
In Fact, Only a Guess
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 on Trial
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 and Fruit to the Test
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