It's a small study that tracked men whose tumors weren't aggressive. Still, the research, published in the September issue of The Journal of Urology, promises to increase interest in whether diet might really help battle cancer.
The study was led by heart-health guru Dr. Dean Ornish, and used his famously strict regimen, where people become vegetarians, limit dietary fat to 10 percent of total calories, exercise regularly and learn stress-management techniques such as yoga.
Ornish's studies show that regimen can help heart disease, but why try it on prostate cancer? There is some evidence that diets high in fat increase the risk of prostate cancer, and that certain foods — such as broccoli, or the nutrient lycopene from cooked tomato products — are protective.
So Ornish and fellow researchers at the University of California, San Francisco, recruited 93 men who had decided against treatment for early-stage prostate cancer, a route known as "watchful waiting."
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.
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.
Also, six of the non-dieters had undergone cancer treatment in that year after all, because their disease was progressing. None of the dieters were treated.
Other cellular tests suggested the diet wasn't just affecting PSA production, Ornish said.
"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," cautioned Dr. Durado Brooks of the American Cancer Society. But, "this definitely should open the door to more research."
"This report undoubtedly will excite the aficionados and devotees of lifestyle changes for cancer but it should also give pause to the skeptics," wrote Dr. Paul Lange of the University of Washington in an accompanying editorial.
Indeed, it comes just months after another study suggested low-fat diets might help women avoid a recurrence of breast cancer.
Ornish stressed that his study, partly government-funded, doesn't mean men should opt for diet over conventional therapy.
But these men weren't getting conventional treatment anyway, allowing a clearer test of dietary effects, he explained. The diet may help men undergoing therapy, too, he added.
"I always find it amusing" that people call the diet hard, Ornish said. "Compared to having your prostate removed? ... The only side effects are you feel better and it helps prevent heart disease."
More than 230,000 U.S. men are expected to be diagnosed with prostate cancer this year, and 29,500 will die, the cancer society estimates.