Thought to be one of the largest studies into the association between soy and endometrial cancer, the researchers from the Shanghai Cancer Institute and Vanderbilt University in the US report an inverse dose-response relation between intake of soya food and risk of the disease, which affects the lining of the womb.
They also found that the association was more pronounced among women with a higher body mass index or a higher waist:hip ratio.
Endometrial cancer is the fifth most common cancer among women worldwide - around 7,090 American women will die of the disease in 2004 - but incidence of the cancer varies more than 10-fold worldwide.
Given that Asian women have a lower incidence of endometrial cancer and eat more soya food than their Western counterparts, the authors hypothesized that dietary factors may play an important role in this international variation.
Soy isoflavones are thought to play a role in the development of hormone related cancers, including breast and endometrial cancer. These and other plant oestrogens have been shown to alter circulating concentrations of ovarian steroid hormones, prolong the menstrual cycle and alleviate symptoms of menopause, although evidence is not entirely consistent.
Researchers interviewed 832 women in Shanghai who were diagnosed with endometrial cancer between 1997 and 2001 and were between the ages of 30 and 69 years. A further 846 healthy women of the same age were randomly selected as a control group. Soya food intake over five years was measured and current body measurements were taken.
Women with the highest intake of soy protein were a third less likely to develop endometrial cancer than those with the lowest intake. Those with the highest isoflavone consumption had a 23 per cent lower risk than the lowest intake group.
The researchers added: "The indication that women with a high body mass index or waist:hip ratio may benefit more from increased soya food intake needs to be verified in future studies," said the authors.