Increased vegetable consumption fails to reduce risk for prostate cancer progression
The value of a healthy diet in disease prevention and overall health has been well-established in the scientific literature and among the general public.
Guidelines recommend diets rich in fruits and vegetables for patients with cancer, including men with prostate cancer. However, until recently, the effect of diet on prostate cancer progression had not been studied extensively.
“There wasn’t as much research in terms of men changing their diets to ward off the advance of prostate cancer,” James Marshall, PhD, distinguished professor in the department of cancer prevention and population sciences at Roswell Park Comprehensive Cancer Center, said in an interview with Healio. “There was evidence that men who ate a diet low in fat and high in fruits and vegetables had a lower risk [for] developing prostate cancer, but that was observational. There was no experimental evidence.”
Marshall, along with James Mohler, MD, professor of oncology in the department of urology at Roswell Park, J. Kellogg Parsons, MD, MHS, professor of urology at University of California San Diego School of Medicine and Moores Cancer Center, and colleagues conducted a randomized clinical trial to investigate the impact of increased produce consumption on prostate cancer progression.
“John P. Pierce, PhD, professor emeritus of cancer prevention at University of California San Diego School of Medicine, developed an intervention and we thought, ‘let’s test it,’” Mohler told Healio.
The Men’s Eating and Living (MEAL) study included 478 men aged 50 to 80 years (mean age, 64 years) who had been diagnosed with early-stage prostate cancer. Researchers randomly assigned the men to the telephone counseling intervention, which promoted consumption of at least seven daily vegetable-fruit servings (n = 237), or a control group that received printed information about diet and prostate cancer (n = 241).
Researchers hypothesized that the behavioral intervention would reduce incidence of prostate cancer progression.
The findings were not what they expected.
“We thought that there would be a good chance that this would work,” Mohler said. “It turned out it didn’t. It was surprising.”
Marshall and Mohler spoke with Healio about the study’s findings, how they will influence their recommendations, and the value of an overall healthy lifestyle.
Question: What prompted you to undertake this study?
Mohler: This diet hypothesis originated in research in the early 1990s that revealed prostate cancer is less common in Asia. When Asian men moved to the United States, their rate of prostate cancer diagnosis appeared the same as someone born in the U.S. by the second generation. People began to wonder if diet was responsible for this.
Some studies — including one in the 1980s that I was involved in — have looked at fish oil. Some other observational studies have tried to connect what people eat to the aggressiveness of prostate cancer. We’re doing a large study in North Carolina and Louisiana. However, we haven’t found any significant differences associated with these factors. When men ask me what they should do, I have them take genistein; there is reasonable data for that. Genistein is how you convert to an Asian diet without eating a pound of tofu every day.
Marshall: The only other studies that have been done are the Harvard Channing Lab studies. They enrolled a cohort of health professionals and looked at increased intake of lycopene and lower intake of some fatty acids. These studies found associations between these changes and decreased risk for prostate cancer. However, this is all observational data; it’s not an experiment.
Q: What did the study entail?
Mohler: We approached 478 men who already opted to enter a program of active surveillance for low-risk prostate cancer about participating in our study. Their average age was 64 years and almost all of their cancers had been detected by PSA. The average PSA level was only 4.9 ng/mL, so this was a very favorable group of men. They were randomly assigned to get a handout describing a vegetable-rich diet that is recommended by many organizations, including the NCI, for patients with cancer, or receive the handout and a telephone-delivered enhancement to encourage them to change their diets. That involved an average of 20 telephone calls.
We followed the men very carefully, and all of them agreed to undergo a biopsy at the end of 2 years. We had three criteria for determining whether their cancer showed evidence of growth that might convert them from active surveillance to treatment: a PSA level increase of 10 ng/mL or more, a PSA doubling time of less than 3 years, or any evidence of cancer growth on the biopsy performed at the end of the study.
Q: How did you expect fruits and vegetables to slow the progression of prostate cancer?
Marshall: We didn’t know. We used carotenoids as a blood marker, but it was just a marker. A whole raft of nutrients could be pertinent. Our goal was to see whether jacking up the intake of fruits and vegetables made a difference, and the answer was no.
The probability that a man will die of prostate cancer is about 3%, which means they have a high likelihood of dying of something else. You need patients to understand that they face a whole host of hazards; prostate cancer is not the only one. Healthy diet is associated with lower risk for heart disease, obesity, diabetes and more. We know that from many other studies.
Q: What did you find?
Mohler: Among these men, the same number met the criteria for prostate cancer progression in the MEAL group as in the control group. Restricting that to the most objective measure, progression on biopsy, we also observed equal distribution between the control group and the intervention group. The logical question then became, “Did the men in the intervention group really change their diets?” This diet initially was used for women with breast cancer in a pilot study, and the men in the intervention changed their diet more than the women with breast cancer. The carotenoids, measured in the blood, documented adherence to the diet. These men did really well, just like in the pilot study.
The most important thing is that when the men would come into the clinic, they would say, “I cannot tell you how grateful I am for being allowed to be on this study, because I feel so much better.”
Q: Will you still recommend a diet rich in produce to your patients?
Marshall: Yes, but we’re not going to recommend it to decrease your chance of requiring treatment for prostate cancer. We’re going to recommend it because you’re not going to die of the cancer anyway, and this will make you healthier.
Mohler: For years, I’ve been advising my patients [with prostate cancer] to do four things: get more rest, get more exercise, eat a healthier diet — including more fruits and vegetables and less red meat — and reduce your anxiety and stress levels. This study indicates to me that I should put the fruits and vegetables at the bottom of that.
Q: Can any lifestyle changes slow the progression of prostate cancer?
Marshall: Nothing has been proven so far. People are studying exercise, yoga, all sorts of things. We would encourage them to study those with the same precision and objective measures that were used in this study. We don’t want to tell people to do things simply because we believe it’s better. We want to prove it’s better.
Mohler: This diet may very well reduce risk for being diagnosed with prostate cancer; we don’t want to confuse people. Also, if you have advanced prostate cancer, improving your diet might help. Additionally, a healthier diet might make men feel better while receiving treatment. – by Jennifer Byrne
For more information:
James Marshall, PhD, can be reached at Roswell Park Comprehensive Cancer Center, Elm and Carlton streets, Buffalo, NY 14263; email: email@example.com.
James Mohler, MD, can be reached at Roswell Park Comprehensive Cancer Center, Elm and Carlton streets, Buffalo, NY 14263; email: firstname.lastname@example.org.
Disclosures: Marshall and Mohler report no relevant financial disclosures.