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A diet rich in complex carbohydrates and lower in protein and fat is associated with a 60 percent to 70 percent reduced risk of prostate cancer, said Adriana Vidal, a co-author of two of the studies and an assistant professor at Duke University School of Medicine in Durham, N.C.
"Good carbs, high-quality carbs, and high fiber are definitely protective against prostate cancer," Vidal said.
The two other studies found that:
- Drinking lots of milk could increase a man's risk of advanced prostate cancer.
- Men suffering from two or more health problems linked to metabolic syndrome also have an increased risk of aggressive prostate cancer.
Metabolic syndrome is a group of risk factors that increase a person's risk of heart disease, diabetes and stroke. They include obesity, high blood pressure, elevated blood sugar levels, elevated levels of triglycerides (blood fats) and reduced levels of "good" HDL cholesterol.
"When men have two metabolic syndrome components, their risk of high-grade prostate cancer goes up almost 35 percent," Vidal said. "With three to four components, their risk goes to almost 94 percent increased."
These studies shed more light on connections between diet, lifestyle and prostate cancer that up to now have been "tenuous," said Dr. Durado Brooks, director of prostate and colorectal cancers for the American Cancer Society.
"We don't have as good evidence regarding a link between diet and prostate cancer as we do with colorectal cancer or breast cancer, and there has been some conflicting data in previous studies," Brooks said.
The first study focused on a group of 430 veterans at the VA Hospital in Durham, N.C., including 156 men with confirmed prostate cancer. Researchers had the men fill out questionnaires to track the amount of carbohydrates, protein and fat in their daily diets.
The researchers found that when men received more of their energy from carbohydrates rather than protein or fat, their risk of prostate cancer declined. High fiber intake also appeared to reduce prostate cancer risk.
Additionally, they found that foods like simple carbohydrates that cause blood sugar to spike appear to increase prostate cancer risk in black men.
That finding, along with the results of the metabolic syndrome study, seem to indicate there could be an as-yet-unknown connection between blood sugar levels and male hormones like testosterone that increase prostate cancer risk, Vidal said.
In the second study, doctors reviewed the consumption of dairy products among nearly 3,000 people, including almost 1,900 men with either localized or advanced prostate cancer.
The investigators found that drinking milk was associated with advanced prostate cancer. However, total dairy consumption was not related to prostate cancer risk, nor were consumption of yogurt, ice cream and cheese.
The analysis also found that men with low overall calcium intake were at greater risk of prostate cancer when they ate more dairy products, compared with men with average or high levels of calcium in their diet.
The findings suggest that although calcium intake likely contributes to an increased risk of prostate cancer, "additional components in dairy may contribute to prostate cancer development," the authors concluded.
The final study focused on the effects of metabolic syndrome on a man's chances of prostate cancer, with researchers reviewing data gathered for almost 6,500 men in an unrelated clinical trial.
Researchers found that men with multiple metabolic syndrome risk factors had a progressively increased risk of prostate cancer.
"The more metabolic syndrome components, the more risk for high-grade prostate cancer," Vidal said.
The findings are in keeping with previous studies linking one of those risk factors, obesity, to a higher risk of aggressive prostate cancer, Brooks said.
"The question is whether because of their obesity these men are less likely to have their cancer identified and biopsied at an earlier stage," he said. "These researchers feel there's more than just delayed diagnosis, that there's something about these risk factors that contributes to prostate cancer."
Findings from these studies were scheduled for presentation Tuesday at the American Urological Association's annual meeting in Orlando, Fla. Results from studies presented at meetings are generally considered preliminary until they've been published in a peer-reviewed journal.
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SOURCES: Adriana Vidal, Ph.D., assistant professor, Duke University School of Medicine, Durham, N.C.; Durado Brooks, M.D., M.P.H., director of prostate and colorectal cancers for the American Cancer Society; May 20, 2014, presentations, American Urological Association annual meeting, Orlando, Fla.