New findings contrast with those of epidemiologic studies suggesting a higher risk of prostate cancer
Flaxseed slows the growth of prostate cancer in men awaiting surgery, a new controlled study has found. The finding contrasts with epidemiologic evidence suggesting that alpha-linoleic acid (ALA), a fatty acid prevalent in flaxseed and other foods, is associated with an elevated risk for the disease.
“Our previous studies in animals and in humans indicated that flaxseed supplementation slowed tumor growth, but the participants in those studies had taken flaxseed in conjunction with a low-fat diet,” said Wendy Demark-Wahnefried, PhD, professor of nursing and surgery at Duke University Medical Center in Durham, N.C. “For this study, we demonstrated that it is flaxseed that offers the protective benefit.”
She and her colleagues randomly assigned 161 patients awaiting prostatectomy to four groups: one consumed 30 g/day of flaxseed for an average of 30 days; one followed a low-fat diet (20% of calories per day or less); one took the flaxseed and followed the low-fat diet; and the fourth took neither (controls).
Analyzing the tumors after surgery, the researchers determined the median proliferation rate (MIB-1 volume) of cancer cells by determining the ratio of the number of cells that were actively dividing to the number of cells that were not. The scores were 1.66 for the flaxseed group; 1.5 for the flaxseed and diet group; 2.56 for the low fat diet-alone group; and 3.23 for the controls. The findings indicated a cancer cell growth rate 40% to 50% slower among those who ate flaxseed. Reduced-fat diets did not appear to have an effect.
“We were a little concerned when associations between ALA and prostate cancer started coming out, several years ago,” Dr. Demark-Wahnefried says. “In fact we purposely performed interim analyses to assure that no harm was befalling men as-signed to our flaxseed arms even though our two pilot studies, our animal study, and our cell-culture study showed exceptionally protective effects. In these interim and final analyses, we did not see any evidence that flaxseed was harming the men; we only saw protective effects.”
Funded by the National Institutes of Health (NIH), the study also involved investigators at the University of Michigan in Ann Arbor and the University of North Carolina at Chapel Hill. Findings were presented in Chicago at the annual meeting of the American Society
of Clinical Oncology.
Dr. Demark-Wahnefried is not ready to endorse flaxseed supplements, however. “This is just the first study,” she says. “We will need to replicate these results before we start to make recommendations.” Future studies will focus on patients with recurrent prostate cancer as well as on dose levels and flaxseed’s effects over longer periods.
About seven years ago, Charles E. Meyers Jr., MD, former chief of the medicine and pharmacology branches of the NIH, published an article in his online newsletter, the Prostate Forum, linking ALA—an omega-3 fatty acid abundant in flax-seed oil—to prostate cancer (www.prostateforum.com/flaxseedoil.pdf). Dr. Meyers’s article focused specif-ically on ALA. In a survey of six epidemiologic studies, he found that five of them showed an association between ALA and an increased prostate cancer risk.
The sixth showed no effect, either positive or negative. “Thus, six out of six stud-ies found no benefit to increased alpha-linolenic acid, while five found potential harm.” he wrote. “I should also note that our laboratory, as well as others, found that ALA is one of the most powerful growth stimuli for human prostate cancer cells,” he adds.
Although flaxseed itself is rich in ALA, it was not the ALA source in the epidemiologic studies that Dr. Meyers cited, Dr. Demark-Wahnefried pointed out. The ALA in those projects came from animal and dairy products or from salad dressing and mayonnaise. Flaxseed, however, is rich in lignans, an anti-angiogenic substance found in plants that may cut off the blood supply to a tumor, thereby stunting its growth. Flaxseed is also a rich source of vitamin E, which previous studies have associated with protective effects against prostate cancer.
Writing in Seminars in Preventive and Alternative Medicine (2006;2:205-207), Dr. Demark-Wahnefried noted that the research to date suggests that some nutrients in flaxseed may be linked to risk whereas others may be protective. “Indeed, it is highly unlikely that a single nutrient or one specific functional food is either all bad or all good. Like most things in life, shades of gray prevail, and we must look to well-controlled studies to guide our understanding and our recommendations.”