The news that tomatoes could prevent prostate cancer sounded too good to be true, and apparently it was.
Lycopene, found mainly in tomatoes and tomato products, had little impact on prostate cancer risk in a new study from the National Cancer Institute and Seattle's Fred Hutchinson Cancer Research Center.
Early research suggesting a protective role for lycopene spurred great commercial and public interest in the antioxidant in the late 1990s.
But subsequent studies have been either contradictory or inconclusive, Fred Hutchinson assistant professor and researcher Ulrike Peters, PhD, MPH, says.
The new research, led by Peters, is one of the largest and most rigorously designed trials ever to examine the issue. And the lycopene findings were unequivocal.
"It would be great if it were true. [Eating tomatoes and tomato products] would be a cheap and easy way to lower prostate cancer risk, and it would be a great public health message," Peters says. "Unfortunately, it's not that easy."
Just as the similar nutrient beta-carotene makes carrots orange, lycopene is responsible for the bright red color of tomatoes. The two compounds are among the pigments synthesized by plants and are known as carotenoids.
The latest study involved 28,000 men between the ages of 55 and 74 participating in a larger, nationwide cancer screening trial. Blood samples were taken from all study participants at enrollment, and blood levels of lycopene, beta-carotene, and other carotenoids were measured.
The men also completed questionnaires assessing their diet, lifestyle, and overall health.
During up to eight years of follow-up, 1,320 cases of prostate cancer were diagnosed among the men in the study.
There was no significant difference in blood lycopene levels among the men who developed prostate cancer during the follow-up and those who did not.
However, beta-carotene was associated with an increased risk of aggressive prostate cancer.
The researchers found that men with the highest blood beta-carotene levels had a much higher risk of developing aggressive prostate cancers.
Peters calls the finding surprising.
"We are not sure if this was a real effect, or one that was due to chance," she says. "We do know [from other studies] that very high doses of beta-carotene seem to increase lung cancer risk in smokers."
The results do not support the use of lycopene, beta-carotene, or other carotenoids in prostate cancer prevention, the researchers write.
Focus on Diet, Not Pills
So what can be said about the influence of diet and lifestyle on prostate cancer risk? Not much, says Peters.
There is growing evidence that obesity increases a man's risk for the disease and some suggestion that a healthy diet may be protective. But neither association has been proven.
American Cancer Society nutritional epidemiologist Marji McCullough, PhD, says it is increasingly clear that cancer prevention efforts should focus more on healthy diets and less on single nutrients like lycopene and beta-carotene.
"Single nutrients don't always give you the big picture," she says. "We saw that a few years ago in the beta-carotene studies."
Researchers thought taking beta-carotene would help prevent lung cancer in smokers, but they ended up finding more cancers in the smokers who took the antioxidant supplements.
McCullough says the message from those studies and this one is that until more is known, individual carotenoids should not be taken in high doses in supplement form.
"I would recommend that people try to get their carotenoids and other nutrients from the foods they eat," she says. "That ensures that they are getting the nutrient in its natural form along with the other nutrients in those foods."
Reviewed by Brunilda Nazario, MD