The small study tracked 93 men with non-aggressive tumors who had decided against treatment for early-stage prostate cancer.
Led by heart-health guru Dr. Dean Ornish, participants became vegetarians, and limited dietary fat to 10 percent of total calories, among other lifestyle changes.
Half were randomly assigned to the Ornish diet and lifestyle regimen; the others weren't asked to vary their usual routines. The researchers sent participants' blood samples to Memorial Sloan-Kettering Cancer Center in New York to measure PSA, or prostate specific antigen, a marker used to track prostate cancer growth, says The Associated Press
After one year, PSA levels had decreased 4 percent in the diet group - unusual for untreated patients - while PSA levels rose by 6 percent in the control group. That difference wasn't big but it's statistically significant, and the researchers plan to continue tracking the men to see if it really signals better health.
Six of the non-dieters ended up receiving cancer treatment in that year, because their disease was progressing. None of the dieters were treated.
Other cellular tests suggested the diet wasn't just affecting PSA production, Ornish says.
"It's hard to get too excited about these results because you took a population of men who, frankly, are likely to do well no matter what," says Dr. Durado Brooks of the American Cancer Society. But, "this definitely should open the door to more research."
The study will be published in the September issue of The Journal of Urology.
