FRIDAY, Sept. 14, 2007 (HealthDay News) -- Men who've been adding vitamin E or the tomato nutrient lycopene to their diets to cut their risk of prostate cancer may need to think again.
According to a new study, neither carotenoids (such as lycopene), retinol, nor tocopherols (forms of vitamin E) appear to reduce the odds of prostate malignancy -- findings that are in line with two other recent publications.
"Our overall findings are null," said lead researcher Timothy Key, deputy director of the Cancer Research UK Epidemiology Unit at the University of Oxford, U.K.
"This large study does not support the hypothesis that consuming large amounts of these nutrients will reduce prostate cancer," he added. "That is disappointing, but that is the overall message."
The findings are published in the September issue of the American Journal of Clinical Nutrition.
His team examined the effect of the blood levels of 10 micronutrients on the risk of developing prostate cancer for almost 2,000 males from eight European countries.
The research, which the authors call "the largest prospective study to date of plasma carotenoids, retinol, tocopherols, and prostate cancer risk," was part of the EPIC (European Prospective Investigation into Cancer and Nutrition) study, which includes more than half a million men and women.
The authors did find evidence to suggest that, once a cancer forms, high levels of lycopene (or of carotenoids in general, including lycopene) may reduce by about 60 percent the risk of the tumor progressing to an advanced-stage prostate cancer. Carotenoids appeared to have no effect on the rate of localized, earlier-stage disease, however.
According to Dr. Peter Scardino, head of the Prostate Cancer Program at Memorial Sloan-Kettering Cancer Center in New York City, prostate cancer is the most common cancer in men in the developed world. A Western male, he said, has about a 42 percent risk of developing cancerous cells in his prostate over his lifetime, a 16 percent risk of being diagnosed with the disease, and about a three percent risk of dying as a result. In other words, nearly one-quarter of Western males have a subclinical form of prostate cancer, which will never progress to more advanced disease.
Stopping progression is crucial. According to Scardino, for those whose disease does progress, the risk of death is much higher -- nearly 50 percent.
"I think it's an important study," Scardino said. That lycopene and bulk carotenoids reduced the risk of progressing to advanced disease without impacting the risk of developing prostate cancer overall, he said, "suggests maybe these micronutrients are not as important in [stopping] carcinogenesis as they are in [slowing] progression of a very small early tumor to one that becomes invasive and larger and develops the ability to metastasize."
"The study provides supportive evidence that lycopene and the carotenoids may have an effect on delaying the progression of prostate cancer, so, from that point of view, it is an interesting study," Scardino added.
But Alan Kristal, associate head of the Cancer Prevention Program at the Fred Hutchinson Cancer Research Center in Seattle, remained more skeptical. Though he called the study "well-executed," Kristal noted, for instance, that the authors were unable to control for prostate-specific antigen (PSA) testing among the men. These blood tests often detect clinically irrelevant tumors, he explained.
"You can never do an observational study of prostate cancer without rigorously controlling for whether or not the person got PSA screening," Kristal said. "The more times you take the test, the more likely you are to get the disease."
He also noted that the finding for lycopene contradicts a report published in May in Cancer Epidemiology Biomarkers and Prevention. That study did account for PSA testing, and it found no effect of lycopene whatsoever on prostate cancer risk -- including the risk of advanced disease.
"To my mind, that study is definitive," said Kristal. "It's a big study, extremely well executed, properly analyzed, and not biased by PSA screening."
A review of lycopene's effect on cancer by the U.S. Food and Drug Administration, published in July in the Journal of the National Cancer Institute, likewise found "no credible evidence to support an association between lycopene intake and a reduced risk of prostate, lung, colorectal, gastric, breast, ovarian, endometrial, or pancreatic cancer and very limited evidence to support an association between tomato consumption and reduced risks of prostate, ovarian, gastric, and pancreatic cancers," according to that study's authors.
So, with tomatoes, ketchup and pizza sauce crossed off the list of prostate-protecting foods, Key and others continue the search. Kristal, for instance, is on the executive committee of a randomized trial examining the effects of selenium and/or vitamin E on prostate cancer risk in 35,000 men. Results are expected in 2012, he said.
Said Key, "I am optimistic we will find something. This paper is an important piece of work, but it doesn't look like this is the answer."
For more on vitamins and cancer, visit the American Cancer Society.