Eating cooked tomatoes was particularly beneficial, according to the study, which showed that regularly eating tomato sauce, ketchup and other tomato-based foods lowered the prostate cancer risk by as much as 36 percent.
Dr. Edward Giovannucci of Brigham and Women's Hospital and the Harvard School of Public Health, the first author of the study, said it supported earlier research involving foods such as tomatoes that were high in lycopene, a powerful antioxidant.
"These most recent findings add support to the notion that a diet rich in tomatoes and lycopene-containing foods, as well as other fruits and vegetables, may reduce the risk of prostate cancer," Giovannucci said.
A report on the study appears in the Journal of the National Cancer Institute.
Researchers analyzed the food choices and prostate cancer histories of more than 47,000 men and found that those who ate at least two meals a week containing tomato products lowered their risk of prostate cancer by 24 percent to 36 percent.
Giovannucci said that lycopene is thought to protect against cancer by absorbing oxygen free-radicals, which are chemicals created during metabolism that can damage the genetic structure of cells.
The finding is based on data from the Health Professional Follow-Up Study, a project that followed the health history and dietary habits of 47,000 men, aged 40 to 75, from 1986 to 1998. During that period, 2,481 of the men developed prostate cancer.
Dietary questionnaires in the study included such food items as tomatoes, tomato sauce, tomato juice, pizza, watermelon and pink grapefruit, along with salsa, ketchup and other tomato-based condiments.
When the data was adjusted for the effects of other life style factors, the researchers found that tomatoes, especially if they had been cooked, were beneficial against prostate cancer.
"Spaghetti sauce was the most popular" and also seemed to give the most protection, said Giovannucci. He said that cooking raw tomatoes, as is done to make spaghetti sauce, may break down cell walls of the fruit and allow the body to absorb more lycopene.
Jo Ann Carson, a clinical nutritionist at the University of Texas, Southwest Medical Center in Dallas, said the study "is an example that what we eat can affect our risk of cancer."
The study also supports the idea that foods rich in antioxidants, rather than vitamin pills, provide the most cancer protection, she said.
"Eating the whole foods seems to give a beneficial combination that would be lacking in supplements," Carson said.