A fat-laden diet and high calcium consumption are both well-known suspected risk factors for prostate cancer.
However, new findings from center researchers suggest that fat and calcium themselves may not cause prostate cancer, as previously thought, but instead may fuel its progression from localized to advanced disease.
While high intake of dietary fat and calcium is associated with an increased risk of clinically significant, advanced prostate cancer, it has no apparent impact on risk of early-stage disease, according to findings by Dr. Alan Kristal and colleagues in the Public Health Sciences Division.
Results of this population-based, case-control study were published in the August edition of Cancer Epidemiology, Biomarkers and Prevention, a journal of the American Association for Cancer Research. The National Cancer Institute and Fred Hutchinson funded the study.
"Our findings clearly show decreased risk for late-stage disease in men with diets that are low in fat and moderate in calcium, perhaps because these diets slow progression of prostate cancer into more aggressive disease," said Kristal of the Cancer Prevention Research Program.
"For men diagnosed with early-stage prostate cancer, this finding could be important because it suggests that moderating fat and calcium consumption may reduce the risk of cancer recurrence following treatment."
Previous studies have examined similar dietary factors on overall prostate-cancer risk, but few studies have focused on risk of early vs. advanced disease.
While there is no effective treatment for advanced prostate cancer, localized disease can be treated with prostatectomy or radiation therapy. While almost all men diagnosed with early-stage prostate cancer survive for more than five years, only 34 percent of men diagnosed after the disease has spread to distant organs are alive after five years, according to the American Cancer Society.
"As more men are diagnosed with early-stage disease due to the widespread use of the PSA (prostate-specific antigen) screening test, it becomes increasingly important to consider how dietary or lifestyle changes could decrease their risk of cancer recurrence," said Kristal, also an associate professor of epidemiology in the University of Washington School of Public Health and Community Medicine.
Half the risk
Kristal and colleagues found that men who ate lower-fat diets, with fat accounting for no more than 30 percent of their daily calorie intake, had half the risk of late-stage cancer than men who consumed more fat. However, there were no associations of fat intake with early-stage disease.
Saturated fats (found in meat and dairy fat) and monounsaturated fats (found in certain oils, such as olive and peanut) were associated with an increased risk of advanced prostate cancer. Polyunsaturated fats (found in certain oils, such as safflower and canola) were not. Consumption of omega-3 fatty acids (found in fatty fish such as salmon and mackerel) also did not have an impact on overall prostate-cancer risk, contrary to experimental studies in cell cultures that have suggested there may be a protective effect.
One reason low-fat diets could reduce the risk of prostate cancer is that they reduce blood levels of circulating male hormones such as testosterone. Growth of the prostate, and perhaps growth of prostate cancer, is fueled by male hormones.
The researchers also found the risk of advanced prostate cancer was 112 percent higher—more than double—among men who consumed the most calcium (more than 1,200 mg per day, equivalent to four or more glasses of milk) as compared to those who got the least (fewer than 500 mg). It didn't matter whether the calcium came from food or supplements.
"For regional/distant disease, there were consistent trends for increased, independent risks from both dietary and supplemental calcium," the researchers wrote.
The mechanism underlying the effect of calcium on prostate-cancer risk isn't clear, although relatively consistent findings from previous studies suggest that dairy products or calcium-supplement intake are associated with risk. Some scientists hypothesize that a high intake of calcium may suppress blood levels of the active form of vitamin D, a hormone that may protect against prostate cancer by preventing the development of cancerous cells.
The only dietary risk factor that appeared to carry equal weight among men with both early and advanced prostate cancer was the amount of total calories consumed, regardless of fat.
"Total energy intake was significantly associated with risk for both localized and advanced disease," Kristal said.
Men who ingested the most calories each day more than doubled their risk of localized prostate cancer (a 115 percent increased risk) and nearly doubled their risk of advanced prostate cancer (a 96 percent increased risk) compared to men who ate the fewest.
"Our interpretation of these results is that high energy intake increases prostate-cancer risk overall, while high dietary fat and calcium intakes increase the risk of more clinically significant, advanced stages of the disease," the researchers wrote. "These results are consistent with general dietary guidelines to moderate consumption of total energy and fat, and they motivate further research to consider the potential benefits and risks of high calcium intake."
Early stage benefit
One important implication of this research, Kristal said, is that men who have been diagnosed with early-stage prostate cancer may benefit from a diet low in fat and calories.
"While there is an increasingly popular message that fat intake is not important for health," he said, "there is no doubt that high-fat diets are associated with high calorie intake and obesity. A low-fat diet may well be important for the thousands of men who are diagnosed each year with early-stage prostate cancer."
Specific recommendations for calcium consumption are more complex, because calcium also helps prevent osteoporosis and colon cancer.
The U.S. Recommended Daily Intake of calcium for men age 50 and older is 1,200 mg. While the most well-known dietary sources of calcium include milk, dairy products, salmon and dark, leafy greens, men—particularly those taking calcium supplements—also should be aware of the added calcium content in fortified foods, from cereals to juices. A small bowl of fortified cereal, for example, can contain up to 1,000 mg of calcium.
"Men diagnosed with early-stage prostate cancer may wish to moderate their calcium intake, though the optimal level is simply not known," Kristal said. "Much more research is needed on factors that may prevent cancer recurrence in men treated for early-stage disease."