Based on pre-clinical studies demonstrating that decreasing dietary fat and decreasing the omega-6:omega-3 fatty acid ratio inhibits carcinogenesis and prostate cancer progression (1
) we conducted a prospective randomized Phase II trial to evaluate biomarkers associated with prostate cancer development and progression. The rationale for the primary endpoint of this trial (between group change in serum IGF-1 levels) was based on prior pre-clinical studies suggesting decreased dietary fat and omega-6 fatty acid intake decreased prostate cancer development and progression through a reduction in serum IGF-1 levels (4
) In the present trial we found no significant change in serum IGF-1, IGFBP-1 or insulin levels. Several other dietary intervention trials incorporating dietary fat reduction also demonstrated no change in serum IGF-1 or IGFBP-1 levels (33
). In prior clinical trials, decreased IGF-1 and increased IGFBP-1 levels were typically found in the setting of significant weight loss or reduction in serum insulin (24
). Subjects in the present study did not have significant weight loss or reduction in serum insulin which might explain the lack of effect on IGF-1 and IGFBP-1 levels.
While we found no effect on the primary outcome, we did find significant effects in some secondary analyses. Malignant epithelium proliferation, as measured by Ki67 immunostaining, was significantly reduced in the low-fat/fish oil group versus the western diet group. Ki67 immunostaining has been shown to independently predict recurrence after radical prostatectomy and prostate cancer specific survival (41
). The mechanism through which the dietary intervention affected malignant epithelium proliferation is unknown. The fact that there were no changes in the serum IGF axis parameters and no change in tissue COX-2 and PGE-2 levels suggests the intervention targeted other pathways involved in proliferation. The finding that the low-fat/fish oil intervention reduced serum-stimulated proliferation of 22RV1 cells in an ex-vivo bioassay suggests alterations in serum growth factors may be responsible for the reduced proliferation seen in the tissue. Potential targets affected by the low-fat/fish oil intervention include eicosanoid synthesis pathways and expression of inflammatory cytokines (26
). Given the well-established association of Ki67 and prostate cancer progression, and the impact of the low-fat/fish oil intervention on Ki67, future trials are warranted evaluating whether altering dietary fat and the dietary omega-6:omega-3 ratio favorably alters proliferation and other clinical prostate cancer endpoints.
A novel finding in this trial was that reducing dietary fat and the omega-6:omega-3 fatty acid ratio resulted in significant changes in the fatty acid levels in benign and malignant prostate tissue membranes. Subjects in the low-fat/fish oil group had lower omega-6 levels and higher omega-3 levels in the prostate tissue membranes relative to the western diet group. The ratio of omega-6:omega-3 fatty acid levels in cell membranes is believed to play an important role in signaling pathways leading to prostate cancer development and progression (1
). In prior xenograft studies, decreasing the dietary omega-6:omega-3 fatty acid ratio resulted in a reduction in the omega-6:omega-3 fatty acid ratio in the xenograft membranes and decreased COX-2 and PGE-2 levels (2
). PGE2 is known to increase prostate cancer proliferation, invasiveness, and angiogenesis (26
). Whereas altering the dietary omega-6:omega-3 fatty acid ratio in our trial led to alteration in the fatty acid composition of benign and malignant prostate tissue membranes, there was no significant difference in COX-2 or PGE-2 levels in prostate tissue and no difference in urinary PGEM levels, a stable metabolite of PGE-2 (27
). In a 3-month trial evaluating 3 grams of fish oil per day (without modifying dietary fat) in men on expectant management, Chan et al. found no change in COX-2 expression in prostate tissue (48
). Berquin et al. previously demonstrated that a high omega-3 diet delayed the development and progression of prostate cancer in a prostate-specfic PTEN-knockout mouse model (1
). In their model they saw increased apoptosis in the omega-3 fed group, possibly through effects on Bad phosphorylation. In the present trial altering the omega-6:omega-3 fatty acid ratio in humans did not impact on apoptosis in malignant tissue.
One shortcoming of the present trial is the short duration of the intervention. In our experience, patients that elect to undergo radical prostatectomy generally desire their surgery within 1–2 months and it is not feasible to enroll large numbers of patients for a longer duration. A potential criticism of this trial is that dietary fat reduction alone or intake of fish oil capsules without dietary fat reduction was not tested. As such, we cannot discern whether either treatment alone would have affected proliferation or whether the combination of dietary fat and fish oil supplementation is required. Demark-Wahnefried et al. previously found no effect of dietary fat reduction on proliferation (Ki67) in men undergoing radical prostatectomy, suggesting that dietary fat reduction alone does not alter proliferation (34
). However, the low-fat intervention in that study was targeted to 20% Kcal fat whereas in our trial the low-fat diet provided 15% Kcal from fat, so it remains possible that a more stringent reduction of dietary fat (as was the case in the present trial) may have led to reduced proliferation. It is unknown if consumption of the fish oil capsules without changing the dietary fat content would have affected proliferation. We elected to combine the two interventions (dietary fat reduction and fish oil supplementation) based on preclinical trials that demonstrated decreased development and progression of prostate cancer associated with reducing the ratio of omega-6:omega-3 fatty acids and reducing dietary fat intake (2
). We would not have been able to achieve an omega-6:omega-3 ratio of 2:1 without reducing fat intake since lowering dietary fat intake reduces omega-6 fatty acid intake and, when combined with the omega-3 fish oil capsules, allows for a significant reduction in the omega-6:omega-3 ratio. Another potential criticism of the present trial is that there was a difference in carbohydrate intake between the groups with 45% of energy from carbohydrates (15 grams fiber/day) in the western diet group vs. 70% of energy from carbohydrates (39 grams fiber/day) in the low-fat fish oil group, and potentially the alteration in carbohydrate and/or fiber intake may have been responsible for the change in prostate cancer proliferation. The low-fat/fish oil group also had a reduction in serum cholesterol levels relative to the western diet group, and cholesterol (through a number of mechanisms) may potentially affect prostate cancer growth (50
). In addition, there was a trend for increased weight loss in the low-fat/fish oil group relative to the western group and this may potentially effect proliferation. It is likely that multiple factors play a role in nutritional effects on tumor biology including nutrient-nutrient interactions, gene-nutrient interactions, and host susceptibility factors (11
). The findings in the present trial, that modulation of dietary fat with fish oil intake modified benign and malignant prostate tissue fatty acid levels and affected prostate cancer proliferation, suggests that the dietary intervention has the potential to affect important aspects of tumor biology related to progression. These results are to be considered hypothesis generating, and further prospective randomized trials are warranted to evaluate alteration of quantity and quality of dietary fat on tumor biology and carcinogenesis.
In summary, 4–6 weeks of neoadjuvant dietary fat reduction with fish oil supplementation did not affect serum IGF-1 levels. In secondary analyses, we found this intervention resulted in a decrease in omega-6:omega-3 fatty acid ratios in benign and malignant prostate tissue and a decrease in malignant epithelial cell proliferation as measured by Ki67 immunostaining. Validation of these results with proliferation as the primary outcome will support the performance of long-term dietary intervention trials with clinical progression endpoints.