Eating More Veggies Did Not Slow Prostate Cancer Progression

Liam Davenport

January 14, 2020

Switching to a vegetable-rich diet did not reduce the rate of clinical progression of prostate cancer, concludes the first randomized controlled trial to test the efficacy of a dietary intervention in this disease.

This was the Men's Eating and Living (MEAL) study, conducted in more than 430 men, half of whom received a dietary intervention.

As reported when the study was first presented in 2018 at the annual meeting of the American Urological Association (AUA), the results showed that following a vegetable-rich diet did not have a significant effect on clinical progression at 2 years in comparison with standard active surveillance.

The final results, which were published online January 14 in JAMA, show that the intervention, in which patients substantially increased their vegetable intake, also had no effect on time to progression.

The results "fail to support prevailing assertions in evidence-based clinical guidelines and the popular media that diets high in micronutrient-enriched vegetables improve cancer-specific outcomes among prostate cancer survivors," the authors comment.

Enthusiasm for Diet-Based Cancer Interventions

The researchers, led by J. Kellogg Parsons, MD, MHS, UC San Diego Moores Comprehensive Cancer Center, La Jolla, California, comment that "enthusiasm for diet-based cancer interventions remains high, driven by assumptions of causality made from epidemiological data."

Several prostate cancer clinical guidelines propose that cancer progression and mortality are decreased among patients who follow a vegetable-enriched diet and that following such a diet could not only reduce the need for treatment and improve quality of life but also reduce healthcare costs.

The researchers point that these recommendations are drawn from "expert opinion, epidemiological studies, and small preclinical experiments" and that there is a lack of data from randomized controlled trials.

So they set out to conduct a randomized trial, the phase 3 MEAL study.

It involved men aged 50–80 years with biopsy-proven prostate adenocarcinoma, stage ≤cT2a, whose serum prostate-specific antigen (PSA) level was <10 ng/mL.

The participants were recruited from 91 urology and medical oncology clinics across the United States and were randomly assigned either to a group that received a telephone-based counseling behavioral intervention or to a control group.

The intervention, which was adapted from social cognitive theory, involved assigning participants to a counselor who encouraged the consumption of at least seven daily vegetable-fruit servings, including at least two servings each of cruciferous vegetables, such as cauliflower, cabbage, or broccoli, and tomatoes.

Participants in the intervention group were called six times during the first month, then four times over 2 months to consolidate the new diet, then four times over 4 months to focus on relapse prevention, and then eight times over 16 months for ongoing monitoring.

In contrast, individuals in the control group simply received printed material from the Prostate Cancer Foundation encouraging a vegetable-rich diet.

Results Show No Difference

The trial involved 443 prostate cancer survivors. The mean age of the participants was 64 years, and the mean PSA level of 4.9 ng/mL.

At 12 months, patients in the intervention group reported significant increases in daily total vegetable servings in comparison with control patients (mean change, 2.43 vs 0.45; P < .001).

These differences persisted for 24 months. The mean change for daily total vegetable servings was 2.01 in the intervention group, vs 0.37 for control patients (P < .001). The differences were seen with respect to individual vegetable groups (ie, cruciferous servings and total carotenoids).

However, there was no significant difference between the two groups in the number of patients who experienced disease progression (124 vs 121 in control group) nor in the time to treatment progression (adjusted hazard ratio, 0.97; P = .84).

On Kaplan-Meier analysis, the percentage of patients who were progression free at 24 months was 43.5% in the intervention group and 41.4% in the control group (mean difference, 2.1%).

"The behavioral intervention in this study produced robust, sustained increases in carotenoid, cruciferous-rich, and leafy green vegetable intake for 2 years, but did not significantly reduce the risk of clinical progression compared with control in patients with early-stage prostate cancer on active surveillance," the authors conclude.

However, the study "may have been underpowered to identify a clinical important difference" in cancer progression between the groups, they add.

When this study was reported by Medscape Medical News in 2018 after it was presented at the annual AUA meeting, several readers commented on the negative findings and raised a number of questions.

For instance, two readers wondered what other foods intervention patients were allowed to eat (processed food, meat, soy, nuts, etc) and whether that could have confounded the results.

Another asked, with the average daily serving of vegetables increasing from three to six half cups, "is it any wonder that no difference between both groups was observed?"

This view was echoed by another reader, who wrote: “Epidemiology shows there is no additional benefit on mortality beyond five servings of vegetables….the main problem I see here is the small difference between groups (4 servings of vegetables in control group vs 5-7 on intervention).”

The authors of the study maintain that the difference in diet was real. "The magnitudes of the dietary changes, which persisted through 24 months of follow-up, were substantial and suggest a clinically meaningful effect," they write.

"Patients assigned to the intervention increased their intake of fruits and vegetables to a statistically significant degree, and significantly more than control patients did," noted senior coauthor James Marshall, PhD, distinguished professor with the Department of Cancer Prevention and Population Sciences at Roswell Park. "These findings were supported by significant changes in the blood carotenoid levels of patients," he added in a statement.

"Nonetheless, these data fail to support prevailing assertions in clinical guidelines and the popular media that diets high in micronutrient-rich vegetables improve cancer-specific outcomes among prostate cancer survivors," Marshall added.

"The most common question I receive from men on active surveillance is, 'Can I decrease the chances that I will need treatment for prostate cancer by changing my diet?' We now have good evidence that a diet rich in fruits and vegetables and light on red meat is not likely to impact need for treatment," commented coauthor James Mohler, MD, professor of oncology with Roswell Park's Department of Urology. "But this study does not provide justification for eating anything you want, either. The overall health benefits of a diet that's relatively low in fat and rich in fruits, vegetables, and healthy grains are well-established," he said in a statement.

The study was supported by the National Cancer Institute of the National Institutes of Health; the Department of Defense; the Prostate Cancer Foundation; and the William Hamilton Fund. Parsons reported receiving grants from the National Cancer Institute and personal fees from Endocare and INSIGHTEC outside the submitted work and owning stock in Pfizer and Johnson & Johnson.

JAMA Neurology. Published online January 14, 2020. Abstract

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