Flaxseed slowed the growth of prostate tumours in men, while ginseng helped relieve the fatigue that cancer patients often feel, US researchers reported on Saturday in two of the first scientifically rigorous looks at alternative medicine.

The studies reflect doctors' efforts to explore the risks and benefits of foods and supplements that are routinely taken by their patients with little scientific proof that they help.

"Patients are taking these compounds but we need to know if they are doing any good or any harm," said Dr Bruce Cheson of Georgetown University Hospital in Washington, who presented the findings to a meeting of the American Society of Clinical Oncology.

In the flaxseed study, researchers at Duke University Medical Center in North Carolina and colleagues evaluated the seed's role as a food supplement in 161 men who were scheduled to undergo surgery for prostate cancer.

"The growth rate was decreased in the men who got flaxseed," said Dr Nancy Davidson, an oncologist at Johns Hopkins University in Baltimore who is president-elect of ASCO. "I think this is fascinating."

Flaxseed is rich in omega-3 fatty acids and lignans, a fiber found on the seed coat.

"We were looking at flaxseed because of its unique nutrient profile," said Wendy Demark-Wahnefried, a researcher in Duke’s School of Nursing. In the study, half of the men added 30 grams of flaxseed daily to their diets for about 30 days. And half of the flaxseed group also went on a low-fat diet.

After the surgery, the researchers looked at the men's tumour cells to see how quickly the cancer had multiplied.

The cancer cells in both the flaxseed groups grew about 30 to 40% slower than the control group.

But Demark-Wahnefried is not ready to prescribe flaxseed. "It's a healthy food. It has a lot of vitamins and a lot of fiber. But we can not definitively say at this point you should take flaxseed because it is protective against prostate cancer," she said, adding that flaxseed now needs to be studied to see if it can prevent prostate cancer.