"We are cautiously optimistic about these findings," said Wendy Demark-Wahnefried, Ph.D., senior author of the study. "The amount of flaxseed given to each mouse was 5 percent of its total food intake, which would be a very difficult amount for humans to eat, but it does signal that we are on the right track and need to continue research in this area."

According to Demark-Wahnefried, planned clinical trials must be completed before it can be concluded that dietary flaxseed is a useful protective against prostate cancer in humans.

Clinical studies by other researchers have suggested that dietary fiber reduces cancer risk, and omega-3 fatty acids also have shown a protective benefit against cancer. Flaxseed is the richest plant source of omega-3 fatty acids and is high in fiber. Also, flaxseed is a source of lignan, a specific family of fiber-related compounds that appear to play a role in influencing both estrogen and testosterone metabolism. Because testosterone may be important in the progression of prostate cancer, lignan could help inhibit the growth and development of the disease.

In the current study, 135 mice genetically engineered to develop prostate cancer were divided into a control group and an experimental group. The experimental group received a regular mouse diet, but 5 percent of the diet was in the form of flaxseed. Half of the mice in both groups were fed their respective diets for 20 weeks and the remainder for 30 weeks. At the 20- and 30-week end points, the mice were autopsied to check for tumor growth and metastatic progression.

"Tumors in the untreated control group were twice the size of tumors in the flaxseed group," said Xu Lin, M.D., lead author of the study. "The tumors were also less aggressive in the flaxseed group, and two of the mice in the flaxseed group did not develop prostate cancer at all. The rates of apoptosis were also higher in the flaxseed group. And while it was not statistically significant, the flaxseed group had fewer rates of the cancer spreading to other organs."

Although the results are promising, the researchers said they are not surprising. The study is the third in a series by the research group to show the benefits of flaxseed in reducing the growth and development of prostate cancer.

The first study, a pilot trial involving 25 men with prostate cancer, demonstrated that a low-fat diet supplemented with flaxseed was associated with slower tumor growth. This work was published in July 2001 in Urology. After 34 days, the men saw a drop in testosterone levels and a trend toward lower prostate specific antigen levels. The diet was tolerated well and encouraged the authors to further examine this dietary intervention.

The second study, published in the November-December 2001 issue of Anticancer Research, examined the effect of lignans on prostate cancer cell lines. This study showed that flaxseed-derived lignans inhibited the growth of three distinct human prostate cancer cell lines through both hormonally dependent and independent mechanisms.

"So far we have observed the suppression of prostate cancer in humans, mice and at the cellular level," said Lin. "It's not a fluke or a coincidence. It's an encouraging line of research."

Demark-Wahnefried adds, "Our results are encouraging. However, before we can truly state that flaxseed is beneficial in humans, larger well-controlled trials are needed. The National Cancer Institute has provided us with the support to conduct a randomized clinical trial in 160 men with prostate cancer that will examine whether a low-fat diet, flaxseed supplementation, or a combination of low-fat diet and flaxseed supplementation will be most effective in stopping prostate cancer cells from dividing. That trial is currently under way."