Taking vitamin C or E does not reduce the risk of prostate cancers - or other forms of the disease, two large US studies suggest.
Both trials were set up following some evidence that taking supplements might have a positive effect.
But one study of 35,533 men, and a second of 15,000 doctors, found no evidence that cancer rates were any lower in those taking supplements.
Both studies feature in the Journal of the American Medical Association.
Supplements don't substitute for a healthy diet and some studies have shown that they may actually increase the risk of cancer
Dr Jodie Moffat
Cancer Research UK
A number of trials had suggested that taking vitamins could cut the risk of certain cancers by boosting levels of beneficial antioxidants which work to minimise damage in the tissues, but the results were mixed.
The latest studies set out to come up with more definitive results, by involving large numbers of volunteers.
In the first study, researchers from University of Texas and the Cleveland Clinic Lerner College of Medicine gave healthy men either the trace mineral selenium, vitamin E, both or a dummy pill.
The team intended to monitor all the participants for at least seven years but the trial was stopped early because the results were so disappointing.
The researchers found there were no statistically significant differences in the numbers of men who developed prostate cancer in the four groups.
In all cases the proportion of men diagnosed with prostate cancer over a five-year period was 4% to 5%.
In the second study, researchers at Boston's Brigham and Women's Hospital tested the impact of regular vitamin E and C supplements on cancer rates among 14, 641 male doctors.
Over eight years, taking vitamin E had no impact at all on rates of either prostate cancer, or cancer in general. Vitamin C had no significant effect.
No substitute
Dr Jodie Moffat, of the charity Cancer Research UK, said: "There are a lot of studies looking at whether vitamin and mineral supplements can reduce the risk of cancer but many of them, like this one, don't support a link.
PROSTATE CANCER
Prostate cancer is the most common cancer diagnosed in men in the UK
Every year in the UK 35,000 men are diagnosed
One man dies every hour of prostate cancer in the UK
African Caribbean men are three times more likely to develop prostate cancer than white men
"This new research means it is even less likely than we previously thought that supplements can protect against prostate cancer.
"Supplements don't substitute for a healthy diet and some studies have shown that they may actually increase the risk of cancer."
She added that eating a diet high in fruit and vegetables was still the best way to get the required vitamins and minerals.
John Neate, of The Prostate Cancer Charity, described the findings as "disappointing".
"Diet does seem important in the development of prostate cancer and we recommend reducing the amount of saturated fat eaten, keeping weight under control, and increasing the intake of fruit and vegetables," he said.
Dr Pamela Mason, scientific advisor to the Health Supplements Information Service, said all three nutrients were essential for human health.
But she added: "Vitamins and trace elements are not intended to be used like drugs. They are intended for health maintenance and for making up dietary gaps in the population."
Research published earlier this year suggested Vitamin C supplements may substantially reduce the benefit from a wide range of anti-cancer drugs.
