Men diagnosed with early stage prostate cancer should try to reduce the levels of fat and calcium in their diet in order to delay the onset of a more serious form of the disease. This is the recommendation of scientists from the Fred Hutchinson Cancer Research Center in the US, whose latest research is published in this month's Cancer Epidemiology, Biomarkers and Prevention.
Fat intake has long been linked to increased health risks, including cancer, but the new research by Alan Kristal and colleagues suggests that fat and calcium intake are also linked to the progression of cancer.
"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 a more aggressive disease. 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," said Kristal.
This study looked at the associations of total calorie, fat, calcium and vitamin D intake on prostate-cancer risk among 1,200 Seattle-area men aged 40 to 64. Half of the men had been recently diagnosed with prostate cancer and the other half were healthy, randomly selected Puget Sound-area residents who served as a comparison group. The men were interviewed about their dietary habits three to five years prior to diagnosis (or an equivalent time frame among the control group). They also completed a detailed dietary questionnaire that asked how much and how often they consumed certain foods and dietary supplements.
While previous studies have examined similar dietary factors on overall prostate-cancer risk, few studies have looked specifically at risk comparing early with advanced disease, Kristal claimed.
There is no effective treatment for advanced prostate cancer, but the early stages of the disease can be treated via a prostatectomy or radiation therapy. While almost all men diagnosed with early-stage prostate cancer survive for more than five years, only 34 per cent of men diagnosed after the disease has spread to distant organs are alive after five years, according to the American Cancer Society.
"As more and 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 at the University of Washington School of Public Health and Community Medicine.
Kristal and his colleagues found that men who ate lower-fat diets, with fat accounting for no more than 30 per cent 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 that low-fat diets could reduce the risk of prostate cancer is because they reduce blood levels of circulating male hormones such as testosterone which fuel growth of the prostate and possibly of prostate cancer.
The researchers also found the risk of advanced prostate cancer was 112 per cent higher 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). "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 is not clear, Kristal said, although there are relatively consistent findings from previous studies suggesting that dairy products or calcium-supplement intake are associated with risk - a salutary warning about taking excessive amounts of a mineral which is widely perceived to be good for the body, in particular for fighting osteoporosis and colon cancer.
One hypothesis is 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 localised and advanced disease," Kristal said.
Men who consumed the most calories each day more than doubled their risk of localised prostate cancer (a 115 per cent increased risk) and nearly doubled their risk of advanced prostate cancer (a 96 per cent 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."
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."
The potential risks of taking calcium also have to be weighed against its known benefits, Kristal said. The US Recommended Daily Intake of calcium for men over age 50 is 1,200 mg, but it is often difficult to gauge exact intake levels because the mineral is used in such a wide range of fortified foods.
"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."