CHICAGO - Flaxseed slowed the growth of prostate tumors 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.

A seasonal worker holds ginseng rhizomes reaped on a field in Graefendorf, eastern Germany in May 2007. [Agencies]

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 they help.

Americans spend between $36 billion and $47 billion a year on complementary and alternative therapies, according to the National Center for Health Statistics.

"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 led a panel on alternative therapies at 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, who led the study.

Half of the men in the study added 30 grams of flaxseed daily to their diets for about 30 days. Half of the flaxseed group also went on a low-fat diet.

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

The cancer cells in both the flaxseed groups grew about 30 to 40 percent 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 needed to be studied to see if it can prevent prostate cancer.

In the ginseng trial, Debra Barton of the Mayo Clinic in Rochester, Minnesota, and colleagues tested three different doses of the herb on patients with a variety of cancers who were expected to live at least six months.

Twenty-five percent of patients taking a 1,000-mg dose and 27 percent of patients taking a 2,000-mg dose said their fatigue symptoms were "moderately better" or "much better."

Only 10 percent of those taking a 750-mg dose reported an improvement, which was about the same as the placebo group.

Patients in the trial took Wisconsin ginseng from a single crop that was tested for uniform potency. It was powdered and given in a capsule form.

"I wouldn't have predicted this, I have to admit," Davidson said in an interview. "We might want to test this on a large scale."

The flaxseed study was funded by the National Institutes of Health and the ginseng study was supported by US Public Health Service grants.