Diet for Prostate Cancer Survivors

Reference

Wilson KM, Mucci LA, Drake BF, et al. Meat, fish, poultry, and egg intake at diagnosis and risk of prostate cancer progression. Cancer Prev Res (Phila). 2016;9(12):933-941.

Study Objective

To examine the association between recurrence of prostate cancer and consumption of total red meat, processed and unprocessed red meat, poultry, fish, and eggs

Participants

The study included 971 men treated with radical prostatectomy for prostate cancer and clinical stage T1 (81%) or T2 (19%) disease (mean prostate specific antigen [PSA]=5.9 ng/mL); men were participants in the Washington University Genetics Study, a cohort of men with biopsy-diagnosed prostate cancer treated in St. Louis between 2003 and 2010.

Outcome Measures

Details on diagnosis, initial treatment, and follow-up visits were collected from medical records. After diagnosis and before treatment, men completed a questionnaire with demographic, smoking, and health information along with a food frequency questionnaire (FFQ). Logistic regression was used to study the association between diet and high-grade or advanced stage disease in all 971 participants. Only participants with pathological stage T3N0M0 (n=940) were included in the recurrence analysis cohort. Median follow-up was 3.0 years (range: 1 month to 7 years, 8 months).

Study Medication and Dosage

The FFQ assessed frequency of consumption of 137 individual food items, 77 with questions on usual portion size and frequency, over the year prior to diagnosis. Additional questions asked about cooking methods, including frequency of fried-food consumption and doneness preferences for meats.

Key Findings

Consumption of well-done and very well-done red meat was associated with advanced disease at diagnosis (odds ratio [OR] top vs bottom quartile: 1.74; 95% confidence interval [CI]: 1.05-2.90; P for trend=0.01). Less-cooked meats and overall meat intake was not associated with advanced disease. There was no association with other dietary parameters.

Total red meat intake was associated with high-grade (Gleason 4+3) prostate cancer (hazard ratio [HR]: 1.66; CI: 0.93-2.97; P for trend=0.05) at time of diagnosis. No other dietary parameters were associated with high-grade prostate cancer at diagnosis.

Of the 940 men with pathological stage T3N0M0 or lower disease, 94 had recurrences of prostate cancer (10%) as determined by a rising PSA (n=79), starting a new treatment (n=12), or evidence of metastasis (n=3). Comparing quintiles, intakes of red meat, fish, poultry, or eggs were not associated with recurrence.

Using a substitution model, the authors calculated that replacing 30 grams per day of total red meat with 30 grams of poultry or fish would lead to a significantly lower risk of recurrence (HR: 0.79; 95% CI: 0.66-0.94). This calculation was true for unprocessed red meat (HR: 0.76; 95% CI: 0.63-0.92) but not for processed red meat (HR: 1.05; 95% CI: 0.67-1.64). No other dietary categories or substitutions reached statistical significance when comparing upper quartile of consumption vs lower quartile.

Neither total red meat nor egg intake was significantly associated with risk of disease.

Practice Implications

While we are eager for any data that can inform men diagnosed with prostate cancer about which lifestyle choices will slow progression of their disease, this study adds little to our knowledge base.

The authors state, "Our findings support advising men with prostate cancer to replace red meat and eggs in the diet with poultry or fish. This is associated with reduced risk of recurrence independent of stage and grade at diagnosis and is consistent with previous findings on diet and prostate cancer survivorship.”

The authors of this study imply associations between dietary components and disease risk, but these associations did not reach statistical significance. They enlisted several almost-plausible word choices to make these implications, such as the following:

• “We found a suggestive positive association with progression after radical prostatectomy for egg consumption.”
• “Total red meat intake was marginally associated with risk of high grade disease.”
• “We did see a suggestion of a positive association for fried poultry and an inverse association for non-fried poultry and recurrence.”
• “This study raises the possibility that substitution of poultry or fish for red meat and eggs could decrease progression in men surgically treated for prostate cancer.”

Suggestive associations, marginal associations, possibilities, and other turns of phrase do not equal a statistically significant association.

The authors also resorted to creative comparisons. In an attempt to find high intake of eggs associated with disease, they compared the top decile of consumption vs the bottom quartile and were so able to report an OR of 1.98 with a 95% CI of 1.08 to 3.63, and a P-value for trend of 0.08. Note the P-value for the trend is greater than 0.05. A similar statistical manipulation was employed to imply that very high poultry intake was associated with progression (HR top decile vs bottom quartile: 0.19; 95% CI: 0.06-0.63; P for trend=0.02).

Suggestive associations, marginal associations, possibilities, and other turns of phrase do not equal a statistically significant association.

This is not how comparisons are typically done in these studies. Equivalent segments of a data set should be used for comparison; for example, top quartile vs lower quartile. All of these examples describe trends that did not reach statistical significance and should have been reported as “no association” or “no significant association.”

While sticklers for accurate reporting prefer authors to report only findings that are statistically significant, there may be more generous readers who would like to be informed of strong trends that border on statistical significance. Even so, they expect the choice in language to make it clear that such trends were not significant.

This imprecise writing confuses the average reader. The study concludes with information we already suspected:

“In conclusion, our findings support advising men with prostate cancer to replace red meat and eggs in the diet with poultry or fish. This is associated with reduced risk of recurrence independent of stage and grade at diagnosis and is consistent with previous findings on diet and prostate cancer survivorship.”

What should we now tell men with prostate cancer regarding diet and lifestyle?

Perhaps the most informative study to date is Kenfield et al’s 2015 paper on lifestyle score. Using data from the Health Professionals Follow-up Study (HPFS; N=42,701), Kenfield and colleagues developed a “lifestyle score” to rank traits that affect prostate cancer. They applied this scoring method to the HPFS cohort and to the 20,324 men in the Physicians’ Health Study (PHS). One point was given for each of these traits:

1. Not currently smoking or quit 10 or more years ago
2. Body mass index under 30 kg/m²
3. High vigorous activity
4. High intake of tomatoes and fatty fish
5. Low intake of processed meat

Men with 5 to 6 vs 0 to 1 points had a 68% decreased risk of lethal prostate cancer (HR: 0.32; CI: 0.19-0.52) in the HPFS and a nonsignificant 38% decreased risk (HR: 0.62; 95% CI: 0.30-1.26) in the PHS. For dietary factors only, men with 3 vs 0 points had a 46% decreased risk (HR: 0.54; 95% CI: 0.30-0.96) in the HPFS and a nonsignificant 30% decreased risk (HR: 0.70; 95% CI: 0.40-1.23) in the PHS.¹

Based on the Kenfield study, the current study, a 2010 study by Richman,² and a 2013 study by Kenfield and Richman together,³ we can compile this list of recommendations:

• Do not smoke.
• Maintain a BMI under 30 kg/m².
• Exercise vigorously.
• Eat a lot of tomatoes and fatty fish.
• Limit processed meat consumption.
• Avoid well-done and very well-done meat.
• Replace red meat in the diet with poultry or fish.
• Avoid poultry skin and eggs.
• Replace some carbohydrates and animal fat with vegetable fat.