"Vitamin D decreases inflammation in tissues, and inflammation is a driver of cancer," explained Bruce Hollis, the study's lead researcher and a professor of paediatrics, biochemistry and molecular biology at the Medical University of South Carolina in Charleston.
For the study, researchers randomly assigned 37 men who elected to have their prostate removed to receive either 4,000 international units (IU) of vitamin D or an inactive placebo daily for 60 days before their operation.
When the prostate gland was examined after their surgery, researchers found that many who received vitamin D had improvements in their prostate tumours, while the tumours in the placebo group remained the same or got worse.
"In more than 60 percent of those taking it, vitamin D actually made the cancer better," said Hollis.
Hollis reported that in some cases the tumour shrank and in others the cancer went away. However, the study was small, and results from a larger trial aren't expected for several years, he added.
Watch and wait period
Doctors often recommend a "watch and wait" period for men with low-grade, or less aggressive, prostate tumours. But many patients and their families aren't comfortable waiting and opt to have surgery before it's deemed medically necessary. These findings suggest that taking vitamin D might help reduce the need for such radical treatment.
But Dr. Anthony D'Amico, chief of radiation oncology at Brigham and Women's Hospital in Boston, said that this study was too small to reach any definitive conclusion about the value of vitamin D in fighting prostate cancer.
"It's premature to make any conclusions," he said. The findings also need to be replicated in a much larger number of patients, D'Amico said.
D'Amico stressed that men should not start taking vitamin D supplements in the hope of of slowing or curing prostate cancer.
Vitamin D, known as the "sunshine vitamin", is produced by the body when it's exposed to sun. It's also found in fortified dairy products and fatty fish.
The study results were scheduled for presentation at the annual meeting of the American Chemical Society in Denver. Data and conclusions presented at meetings are usually considered preliminary until published in a peer-reviewed medical journal.
Image: Food sources of vitamin D from Shutterstock