The researchers said dietary fat and fiber can affect hormone levels and may influence cancer progression. Flaxseed is high in fiber and is the richest source of plant-based, omega-3 fatty acids. Studies suggest that dietary fiber reduces cancer risk, and omega-3 fatty acids also have shown a protective benefit against cancer. Flaxseed is also a rich source of lignan, a specific family of fiber-related compounds that appear to play a key role in influencing both estrogen and androgen metabolism.
"We thought flaxseed would be the perfect food for prostate cancer patients," said lead author Wendy Demark-Wahnefried, associate research professor in the department of surgery at Duke. "It's full of omega-3 fatty acids, fiber and lignan. Testosterone may be important in the progression of prostate cancer, and lignan in the flaxseed binds testosterone, so we thought the flaxseed might suppress the growth of prostate cancer cells. By pairing a low-fat diet with the flaxseed supplement, we also thought we could maximize the effect of the omega-3 fatty acids, since studies in animals show that the kind of fat we eat may be important for cancer progression."
The pilot study involved 25 patients with prostate cancer who were awaiting prostatectomy (surgical removal of the prostate). Baseline levels of prostate-specific antigen (PSA), testosterone, free androgen index and total serum cholesterol were determined at the beginning of the study. The tumors of those on the diet were then matched with 25 historic cases, equal in age, race, PSA level at diagnosis and biopsy Gleason sum (a scoring system used to grade prostate tumors) to compare tumor progression and biomarkers after the dietary intervention.
The men were on the low-fat, flaxseed-supplemented diet for an average of 34 days. Finely ground flaxseed was used in the study because, in its natural form, flaxseed is a pointy, tough seed that can puncture the intestines when consumed in the amounts used in this study (three rounded tablespoons a day). The ground flaxseed in the study was vacuum-packed (ground flaxseed can quickly go rancid) and had added emulsifiers for ease of mixing. The men were instructed to sprinkle the flaxseed on their cereal or mix it into juices, yogurt or applesauce. Researchers reported good compliance with the diet and said it was tolerated well.
At the end of the study, the researchers observed that the men on the diet had significant decreases in cholesterol, and both total and free testosterone. While there was a decrease in testosterone levels, they noted that none of the participants in the study suffered decreased libido or sexual dysfunction. There was a trend toward a decrease in PSA levels in men with early-stage prostate cancer (Gleason sums of six or less), but in men with advanced prostate cancer (Gleason sums of more than six) PSA levels continued to rise.
"It's not surprising that a diet therapy that was only taken for an average of 34 days had little effect on men with aggressive disease," Demark-Wahnefried said. "But what we did see was that for the men on the diet, their tumor cells did not divide as quickly and there was a greater rate of apoptosis (tumor cell death) in this group."
With such a short-term dietary intervention, the researchers said they did not expect to see a difference in tumor biology between the diet-treated patients and the control patients, but were encouraged by the lower proliferation rates and significantly higher rates of apoptotic cell death. However, they said the results should be interpreted with caution, stressing that randomized controlled clinical trials are needed to confirm the results of the pilot study. Research on mice models is currently under way, and preliminary results support the findings in humans.
Demark-Wahnefried said it is still unknown if the low fat diet or the flaxseed--or a combination of the two--is the active component in the tumor reductions, adding more studies examining these elements independently are needed.