Scans of the prostate gland
Taking a soy extract may help slow the progress of prostate cancer, researchers claim.
US researchers found the extract reduced prostate-specific antigen (PSA) levels in men with untreated cancer, in some cases by almost two thirds.
PSA is a protein produced by the cells of the prostate gland.
PSA levels rise if the prostate gland enlarged due to cancer.
We too find it difficult to get the message over to older men that the PSA test is not foolproof
Chris Hiley, The Prostate Cancer Charity
The researchers say their study suggests genistein could help men at risk of developing prostate cancer, but say more studies need to be carried out.
A second study of 300 American men found many were unaware that the test for PSA levels is not "foolproof".
In the soy study, researchers from the University of California Davis Cancer Center studied 62 men known to have prostate cancer and elevated PSA levels.
They were given 5 grams of a dietary supplement containing genistein every day for six months.
Genistein is an isoflavone, a plant-based chemical that mimics the effects of oestrogen in the body.
Sixteen of the men had untreated prostate cancer, they were in the so-called "watchful waiting" group - where the cancer is slow-growing and causing no symptoms.
Three had to stop the therapy because they suffered from diarrhoea, but eight saw their PSA level fall between 3 and 61%.
The remaining five (38%) saw their PSA levels rise, but the researchers say this is a far smaller proportion than in the remaining 46 men who had been treated for prostate cancer, 98% of whom saw a rise.
The research was presented to the American Urological Association meeting in Chicago.
Professor Ralph deVere White, who led the study, said: "It must be interpreted cautiously because the numbers of men enrolled are small.
But he said: "Patients on watchful waiting may do better due to grade of disease or distribution and concentration of genistein within the prostate."
Risks 'outweigh benefits'
In the second study, 270 men were asked their opinion on the PSA test.
University of Texas-Houston researchers found 60% did not realise there were possible downsides to the test.
Ninety per cent believed the PSA test had cut deaths from prostate cancer - which the researchers said was not yet proven.
The test has been used to detect cancer since the early 1990s, but some experts say the risks of follow-up tests and the side effects of treatments could outweigh the benefits.
If men whose cancer is developing slowly, especially if they are elderly or in poor overall health, there are fears surgery or other treatments such as radiation could be too risky.
Dr Evelyn Chan, who led the research, said: "The easy sound-bite is that screening saves lives. There is really a lot more to it.
"The PSA test is not the end-all of diagnosing prostate cancer.
"The test can lead you down a path where you have to make a series of difficult decisions."
The research is published in the American Journal of Public Health.
'Not a complete answer'
Chris Hiley, of the UK's Prostate Cancer Charity, told BBC News Online: "The study into the effects of genistein is too small to make recommendations to men at present."
But she said it did indicate further research was needed.
Ms Hiley said she was not surprised men assume PSA screening saved lives.
"It seems 'obvious' that it does, but the evidence that this is actually happening is not the least obvious.
"The PSA test is a complicated issue and we too find it difficult to get the message over to older men that the PSA test is not foolproof.
"The fact that it is the best thing we have for indicating a possible risk of prostate cancer doesn't mean that we can present it as if it is the complete answer."