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Eating more veg didn’t affect prostate cancer outcomes

Telephone intervention linked to rise in men’s vegetable consumption but not to drop in risk of progression

Louise Prime

Thursday, 16 January 2020

Men with early-stage prostate cancer who increased their consumption of vegetables did not have a lower risk of cancer progression than men who were not encouraged to eat more vegetables, a small US study has found. The authors of the study*, published in JAMA, said that although it did not support an intervention to increase vegetable intake, it might have been underpowered to detect a clinically important beneficial effect. They also recommended that prostate cancer nutrition guidelines should rely more on definitive randomised controlled trials (RCTs), rather than observational studies.

A US research team noted that guidelines based on expert opinion, preclinical studies, and observational data endorse vegetable-enriched diets to improve outcomes for prostate cancer survivors – but there have been no data from RCTs focused on actionable clinical end points. So, they conducted an RCT to investigate whether or not a behavioural intervention promoting vegetable consumption would affect cancer progression in men with early-stage prostate cancer on active surveillance.

They recruited 478 men aged 50-80 years, who had been diagnosed with stage cT2a or less prostate adenocarcinoma within 24 months of baseline with prostate biopsy. They randomised eligible men to either a control group (217 men), who were given a publication from the Prostate Cancer Foundation encouraging consumption of a vegetable-rich diet; or to telephone-based counselling intervention (MEAL intervention – 226 men) whereby trained counsellors encouraged consumption of at least 7 daily vegetable/fruit servings, including at least two servings each of cruciferous vegetables and tomatoes, in 22 phone calls spread over 23 months.

After 24 months’ follow up, the researchers compared outcomes between the groups. Their primary outcome was time to progression, with progression defined as: PSA level of at least 10ng/ml; PSA doubling time of less than three years; or upgrading (defined as increase in tumour volume or grade) on follow-up prostate biopsy.

At 12-month follow-up, intervention participants reported significant increases compared with controls in daily total vegetable servings (mean change 2.43 vs 0.45), cruciferous servings (mean change 43.10g/d vs 6.44 g/d), and total carotenoids (mean change 13,839.31µg/d vs 2,030.79µg/d. By the time of the 24-month follow-up, there were still significant differences between the intervention and control groups for total vegetable servings (mean change 2.01 vs 0.37), cruciferous servings (mean change 0.50 vs 0.01), and total carotenoids (9,687.87µg/d vs 324.73µg/d).

Despite this, during follow up there were a similar number of ‘progression events’ (124 in the intervention group and 121 in the control group); and there were no significant differences in time to progression. The 24-month Kaplan-Meier progression-free percentages were not statistically significantly different either, at 43.5% and 41.4% for the intervention and control groups respectively.

The study authors commented that enthusiasm for diet-based cancer interventions has remained high, despite lack of RCT-derived evidence, driven largely by “assumptions of causality made from epidemiological data” with inherent limitations such as small effect sizes and substantial confounding. They said: “The overdependence of prostate cancer nutrition guidelines on observational studies with uncertain clinical validity suggests a need to shift nutritional research toward definitive RCTs.”

They concluded: “Among men with early-stage prostate cancer managed with active surveillance, a behavioural intervention to increase vegetable consumption did not significantly reduce the risk of prostate cancer progression. The findings do not support use of this intervention to decrease prostate cancer progression in this population, although the study may have been underpowered to identify a clinically important difference.”

* Parsons JK, Zahrieh D, Mohler JL, et al. Effect of a behavioral intervention to increase vegetable consumption on cancer progression among men with early-stage prostate cancer: the MEAL randomized clinical trial. JAMA. 2020; 323(2): 140–148. doi:10.1001/jama.2019.20207

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