Low-fat, high-fiber diet found to have no effect on PSA levels in men after 4 years
For more than a decade, evidence has been mounting that a diet low in fat and high in fruits, vegetables, and fiber might be protective against prostate cancer. But a recent study published in the September 2002 Journal of Clinical Oncology found that this “healthful” diet has no effect on reducing the levels of prostate-specific antigen (PSA), a marker for prostate cancer. The diet also had no effect on the number of new cases of prostate cancer over the 4-year period.
Past studies have suggested that diet plays a role in the start and progression of prostate cancer, and links have been found between these dietary factors and reducing or increasing the risk of developing prostate cancer. In this study, researchers sought to more vigorously test the theory that reducing dietary fat and increasing consumption of fruits, vegetables, and fiber would affect the levels of prostate-specific antigen (PSA) in the blood.
About the Study
Researchers at the Memorial Sloan-Kettering Cancer Center in New York and the National Cancer Institute studied 1169 men (average age 61) over four years who participated in the Polyp Prevention Trial (PPT), a multicenter trial designed to evaluate the impact of diet on the recurrence of colorectal adenomas (growths that may become cancerous). Men in the PPT were eligible for this recent study of prostate cancer if they had no previous prostate cancer diagnosis at the study’s start and had undergone more than two PSA blood tests over the four years.
For the original study, these men had already been randomly assigned to receive either intensive dietary counseling (intervention group) or the standard brochure on a healthful diet (control group). The intervention included more than 50 hours of individual and group counseling for a low fat (20% of total calories), high fiber (18g/1000 calories), and high fruit/vegetable consumption (5 to 8 servings/day). Blood samples were collected at the beginning of the study and then once a year for the next four years. For this recent analysis, the blood samples were analyzed for PSA levels. The men were also asked to complete a food record, food frequency questionnaire (FFQ), and a health/lifestyle questionnaire that asked about medical history, hospitalization, and disease diagnosis. These forms were filled out at the start of the study and then again at each annual visit when the blood was drawn.
The researchers compared the PSA levels and number of new prostate cancer cases in the intensive counseling group with the healthful diet brochure group.
The dietary intervention had no impact on PSA blood levels. The dietary intervention also did not appear to reduce the number of new cases of prostate cancer over the 4-year study period.
However, there were some promising findings related to the success of the intensive nutrition counseling in significantly improving diet, weight, and total blood cholesterol. For instance, by the end of the study the men in the intervention group had an average of 23.7% of their total calories from fat compared to 34.3% in the control group. The intervention group also ate significantly more fruits and vegetables, 6.4 servings/day compared to 3.9 in the control group. Men who received the intensive nutrition counseling also saw a significant decrease in total blood cholesterol in the first year and lost a significant amount of weight—4.5 pounds on average—compared to those who didn’t receive the intensive nutrition counseling. This is even more startling considering there was no difference in the total intake of calories between the two groups.
Adjustments were made for other potential prostate cancer risk factors such as age, diet, race, BMI, education, smoking status, alcohol intake, and family history, but this did not affect the results. The researchers did perform a subset analysis that looked only at the men considered to be at “high-risk” for getting prostate cancer (all men whose baseline PSA was greater than 4 ng/mL). No effect of diet was seen in this high-risk group either.
Although these results suggest that a diet low in fat and high in fiber, fruits, and vegetables does not affect PSA levels, there are a number of limitations to the study. First, the study was limited in its power to detect prostate cancer incidence. The intervention duration of 4 years is relatively short, especially compared to the decades it may take to develop a cancer. We also don’t know what role other foods and supplements may play in protecting against prostate cancer. Additionally, there are always limitations with dietary assessments. It is hard to know if they reflect actual intake and whether they were filled out correctly. In this case, the authors made special efforts to limit the errors of underreporting intake and interviewer bias by having trained nutritionists review the FFQ with each participant. The complete food records also helped to gauge the accuracy of reporting actual consumption.
How Does This Affect You?
So, does this mean that there is no hope in preventing prostate cancer with a healthful diet? Not necessarily. Just because the PSA levels didn’t change doesn’t mean that diet has no role in prostate cancer. Keep in mind that PSA levels can be elevated for reasons other than prostate cancer and that some prostate cancers will never grow enough to cause problems before a man dies of another cause. Scientists believe that many cancers actually start years, if not decades, before they are ever diagnosed. This means it is still possible that the process of prostate cancer development may have begun long before this four-year study, in which case it would be too late to prevent it. It’s still possible, however, that a diet low in fat and high in fiber, fruits, and vegetables may slow the growth of prostate cancer.
Additionally, other studies have also shown that specific dietary components (like the antioxidant lycopene, which is found in tomato products) may play a role in protecting against prostate cancer. And don’t forget all the other benefits of eating a diet low in fat and high in fiber, fruits, and vegetables such as: weight control, heart health, and the prevention of other cancers.
Shike M, Latkany L, Riedel E, et al. Lack of effect of low-fat, high-fruit, -vegetable, and -fiber diet on serum prostate-specific antigen of men without prostate cancer: Results from a randomized trial. Journal of Clinical Oncology. 2002;20:3592-3598.
Last reviewed Sep 5, 2002 by Richard Glickman-Simon, MD
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