Vegetable Intake and Prostate Cancer Mortality

Richman EL, Kenfield SA, Chavarro JE, et al. Fat intake after diagnosis and risk of lethal prostate cancer and all-cause mortality. JAMA Intern Med. 2013;173(14):1318-1326.

A prospective study of men with non-metastatic prostate cancer, comparing dietary intake of fat by type and lethality of the disease.

The participants in this study were part of the “The Health Professionals Follow-up Study” cohort that was initiated in 1986 among 51,529 male health professionals aged 40 to 75 years. Prostate-specific antigen (PSA) screening practices were added in 1994. Dietary data were collected via food frequency questionnaire at baseline and every 4 years thereafter. To be included in this study, men had to be free of cancer at baseline and have a diagnosis of non-metastatic prostate cancer between 1986 and 2010. This narrowed the larger cohort down to the 4,577 men in this current study.

Diet: Food frequency questionnaires (FFQ) were completed at baseline and every four years of the study. The men reported their usual intake of approximately 130 foods and beverages during the prior year. They reported fried food consumption, types of cooking fat, and visible fat on meat consumed. Consumption of fat was calculated including saturated fats, polyunsaturated fats, monosaturated fats, trans fat, and whether fats were animal- or vegetable-derived.

 

Death: The primary outcome tracked was lethal prostate cancer, defined as a distant metastases or death due to prostate cancer, and all-cause mortality. Death was attributed to prostate cancer only if cancer metastases were present and no more plausible cause of death was mentioned in medical records or death certificate.

During the 8.4-year course of this study, 1,064 of the participants died, 315 of them from lethal prostate cancer. The difference that consumption of dietary fats made on the outcome measures is summarized below:

 

 

 

Replacing 10% of energy intake from carbohydrates with vegetable fat was associated with a 29% lower risk of lethal prostate cancer (hazard ratio [HR]: 0.71; 95% CI: 0.51–0.98; P=0.04) and 26% lower all-cause mortality (HR: 0.74; 95% CI: 0.61–0.88; P=0.001).

 

No other fats were associated with lethal prostate cancer. Saturated and trans fats after diagnosis (replacing 5% and 1% of energy from carbohydrate, respectively) were associated with higher all-cause mortality ([HR: 1.30; 95% CI: 1.05–1.60; P=0.02] and [HR: 1.25; 95% CI: 1.05–1.49; P=0.01], respectively).

Among men with non-metastatic prostate cancer, replacing carbohydrates and animal fat with vegetable fat may reduce the risk of both prostate cancer and all-cause mortality.

 

Nearly 2.5 million men in the United States are living with prostate cancer. We know little about what effect food choices have on these men. This study suggests that diet may significantly affect prostate cancer progression and overall mortality. Moderate changes, particularly reduction of animal fat and carbohydrates and replacement with vegetable fat, might have a significant impact.

 

The impact of specific foods high in vegetable fat were analyzed. The authors report several were “suggestively associated” with lower risk; that is, while the data did not reach statistical significance, it was close. These almost associations are worth mention not to repeat to patients (they were not significant) but for our contemplation as they hint at a problem with this study. Eating an additional single serving of salad dressing per day (1 Tbsp) after diagnosis was “suggestively associated” with a 29% lower risk of lethal prostate cancer (HR: 0.71; 95% CI: 0.50–1.00) and a 13% lower risk of death (HR: 0.87; 95% CI: 0.72–1.05). A 1-ounce increase in daily nut consumption was “suggestively associated” with an 18% lower risk of lethal prostate cancer (HR: 0.82; 95% CI: 0.67–1.01) and an 11% lower risk of death (HR: 0.89; 95% CI: 0.79–0.99).

 

Does the salad dressing really deserve credit, or was it simply a measure of salad and vegetable consumption? These findings may not be as simple as they sound. The beneficial effects associated with vegetable fat intake may be from other components of the food sources of the vegetable fats or other foods paired with them. While the study authors attempted to adjust for all known dietary factors (ie, calcium, vitamin E, lycopene, vitamin D, choline, phosphorous, zinc) they cannot rule out some confounding factor.

 

Seed oils and nuts provided most of the vegetable fats in this study population, and we know that these foods have multiple effects, including increasing plasma antioxidants and reducing insulin, LDL cholesterol, and various inflammatory markers.¹ As an example the authors cite the research on flax seed and prostate cancer. Are the associations seen in this study the result of the oil consumed, or some other phytonutrient components found in the food from which the oil was derived?

 

We may not know the answer to these questions, yet these results are still worth paying heed to. Based on these results it is still prudent for men with prostate cancer to consume less animal fat and fewer carbohydrates, and to replace the “lost calories” with calories from vegetable fats. It appears that even moderate changes may have significant impact. 

 

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