SEATTLE – Aug. 29, 2002 -- A low-fat, high-fiber diet rich in fruits and vegetables may not affect men's risk of prostate cancer after all, according to one of the first studies to actually test the theory with an experiment.

The researchers led by Dr. Moshe Shike of the Memorial Sloan Kettering Cancer Center in New York are quick to add that the results of their study published in the Sept. 1, Journal of Clinical Oncology do not necessarily mean diet has no affect on prostate cancer.

"The failure of the intervention to alter the distribution of PSA levels cannot be interpreted as definitive evidence that the diet has no role in prostate cancer prevention," the authors wrote. "The PSA is a surrogate marker, and it is possible that the study diet could reduce the occurrence and growth of prostate cancer without affecting the serum PSA."

Over the past decade, a number of observational studies have identified a growing list of dietary components that apparently increased or reduced the risk of prostate cancer. Starting with dietary fat and red meat as factors increasing risk, these studies have added calcium to the list of culprits, while identifying lycopene, selenium, soy and vitamin E as elements that protect against prostate cancer.

Most of these studies, however, relied on men remembering what they ate over their lifetimes. The current study is one of the first to compare results of two groups of men consuming different diets.

The study involved 689 men who were randomly assigned to an intensive educational counseling program aimed at convincing them to adopt a diet low in fat and high in fiber, fruits, and vegetables. The other 661 men were randomly selected to receive a standard brochure about the benefits of such a diet. The men's prostate specific antigen (PSA) levels were measured at the start of the study and annually thereafter for four years. Newly diagnosed prostate cancers were recorded.

After four years, there was a significant difference in the eating habits of the two groups. The intense counseling group reduced the daily percentage of calories from fat from 35 percent at the start of the study to 24 percent after four years. The comparison group's daily consumption of fat remained about the same, hovering around 35 percent of daily calories throughout the trial.

The counseling group also doubled their fruits and vegetable intake, consuming 6.4 servings a day after four years compared to 3.8 servings a day at the start of the study. Men in the comparison group consumed 3.9 servings of vegetables a day after four years compared to 3.6 servings a day at the start of the study.

Despite these substantial dietary changes, the researchers saw no significant difference in the PSA levels of the two groups over the four years. Neither was there a significant difference in the number of men diagnosed with prostate cancer during the trial, with 22 men in the counseling group and 19 men in the comparison group being newly diagnosed.

While the results were surprising, the researchers caution against reading too much into it. They cited the relatively small number of men involved and the four-year duration of the trial as likely reasons so little difference was seen. They also note that it is possible that diet, while affecting the prostate, my not affect the PSA.

"The fact that the intervention duration, four years, is relatively short in the context of the lifetime of the participant and the length of the process of carcinogenesis," they wrote. "It is thus possible that a longer intervention beginning earlier in life, or indeed a longer follow-up of the patients in this study, would ultimately lead to an observable beneficial impact. Epidemiological studies that examine the role of the diet on cancer incidence usually report on diets that have been consumed throughout the lifetimes of the subjects."

In an accompanying editorial, Dr. Alan Kristal of the Fred Hutchinson Cancer Research Center in Seattle said that the results are not incompatible with the theory that diet affects prostate cancer risk.

"Two consequences of markedly reduced fat consumption are decreased intake of vitamin E and lower absorption of lycopene, and both of these antioxidants have been associated with reduced prostate cancer risk," Kristal wrote. "It may well be that dietary prevention of prostate cancer is not addressed by a low-fat and high-fiber food pattern, whose positive and negative consequences offset each other and provide no overall benefit."

Kristal also noted that many of the observational studies showing the greatest effect of diet on prostate cancer involved advanced disease, and it might be that diet has little effect on the early stages of the disease.

Despite the findings, Kristal said the study demonstrates the value of randomized clinical trials to test whether dietary change can reduce cancer risk.

"These trials are difficult and expensive, and their results are often not consistent with hypotheses that were based on observational studies. Nevertheless, the unanticipated results of clinical trials have been scientifically challenging and have often led to a new understanding of diet and cancer risk," he concluded.

SOURCE: Journal of Clinical Oncology, Vol 20, Issue 17 (September), 2002: 3592-3598

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