Currently 13.7 million people in the United States are cancer survivors. The number is expected to rise to 18 million by the year 2022. Of these, 2.5 million are men who have had prostate cancer.
A growing body of research is looking at dietary factors that lower risk for developing this type of cancer. Less is known, however, about how diet affects outcome in men who have already had the disease.
To address this, Harvard University researchers in combination with investigators from University of California, San Francisco, evaluated the prognosis of 4577 men with non-metastatic prostate cancer who were enrolled in the Health Professionals Follow-up Study.
The men were followed for about 8 ½ years during which time 315 developed lethal prostate cancer (defined as metastases or death due to prostate cancer) and 1064 subjects died from all causes. The primary causes of death were cardiovascular disease (31.2%), prostate cancer (21.3%), and other cancers (20.6%).
The men had completed food frequency questionnaires that included 130 foods and beverages. The questionnaires were repeated over the course of the study.
When the researchers analyzed the relationship of dietary intake to risk of developing lethal prostate cancer, they found those who consumed more vegetable fat after their cancer diagnosis had a markedly lower risk. Replacing 10% of calories from carbohydrates with vegetable fat was associated with a 29% lower risk. Replacing saturated fat with vegetable fat was almost as protective.
Men who consumed more vegetable fat after diagnosis also had a lower risk of death from other causes. Replacing 10% of calories from carbohydrates with vegetable fat was associated with a 26% lower risk of death from any cause. In addition, a 5% increase in saturated fat was associated with a 30% higher risk of death.
When the investigators evaluated whether specific foods were primarily responsible for the protective effects of vegetable fat, they found that an increase in oil-based dressing intake of one serving (1 tablespoon) per day after diagnosis was associated with a 29% lower risk of lethal prostate cancer and a 13% lower risk of death. An increase in nut intake of one serving (1 oz) per day after diagnosis was also protective, but to a lesser extent than oil-based dressing.
The results of this study have important implications for soyfoods and soybean oil. First, soybean oil is the major source of vegetable fat in the United States and is a major component of oil-based dressing. Soybean oil is high in polyunsaturated fat; this study strongly suggests that polyunsaturated fat is protective against lethal prostate cancer. Full-fat soyfoods such as tofu, soynuts and soymilk made from whole soybeans are also high in vegetable fat.
Furthermore, other evidence has found that soyfoods may be protective against the development of prostate cancer and may help to stop its progression. Interestingly, there is also evidence that soyfoods improve the prognosis of breast and lung cancer patients. Although definitive proof is lacking, there is now intriguing research to support a protective role for soybean oil and soyfoods in the diets of people who have had cancer.
JAMA Intern Med. Published online June 10, 2013. doi:10.1001/jamainternmed.2013.6536