Men who have a diet rich in soya products, beans and sunflower seeds run a much lower risk of contracting prostate cancer. New findings from Karolinska Institutet show that foods rich in phytoestrogens - plant-produced oestrogens - protect against the most common form of cancer in the western world.
Some 10,000 men develop prostate cancer in Sweden each year. Just why prostate cancer is so common is still something of a mystery, but age, ethnicity and genes are usually considered risk factors. However, several studies also show that diet also has an important part to play. Recently, scientists have become interested in the protective effect of oestrogen-like compounds in plants, or phytoestrogens. In countries like China and Japan, people ingest more of these compounds than in the west, and the incidence of prostate cancer is lower. Phytoestrogens are usually divided into two groups: lignans and isoflavones, the former being found mainly in linseed, rye, berries and vegetables, the latter in soya beans.
Scientists at Karolinska Institutet’s Department of Medical Epidemiology and Biostatistics have studied the correlation between prostate cancer and phytoestrogen in a large population-based case-control study. Involving 1,499 patients between the ages of 35 and 79 with recently diagnosed prostate cancer, it is the largest study of its kind on a western population. 1,130 healthy controls were identified through a process of matching by age and place of residence. All participants were asked to complete a questionnaire about their dietary habits and to take a blood test. A smaller group (209 cases and 214 controls) had the amount of the phytoestrogen enterolactone in their blood measured.
The study shows that men who had a high intake of phytoestrogen-rich food, such as beans (which offered the greatest protection), soya products, linseed, sunflower seeds, berries and peanuts, ran a 26 per cent lower risk of developing prostate cancer. We also found that men with very low blood levels of enterolactone had a greater chance of contracting the disease.
The intake of lignans matches closely that of other western countries, the richest sources being linseed, rye bread, wheat bread, and berries. The intake was low, which can be expected as soya products are not so common in Sweden. In China, where soya beans are much more popular, one study revealed a thousand-fold higher isoflavanoid intake than in our study.
The protective effect might derive directly from the phytoestrogens alone or in combination with other substances found in the same type of food. We do not, however, recommend the intake of artificial phytoestrogen supplements, as the tablets have not been clinically tested; they also contain high concentrations and can have adverse side-effects.
The results of the study shed light on the intake of phytoestrogens in Sweden. This can be of guidance in studies of other cancer forms that the compounds can be thought to affect. The study also brings us closer to preventative measures and future treatments. Using the genetic information gathered, we will go on to study if there is an interaction between the intake of phytoestrogens from the diet and variations of certain genes in the population that influence the risk of contracting prostate cancer.