A new prospective study investigated whether the protective effect of soy is due to soy isoflavones alone or to their combination with other dietary factors in Singaporean Chinese. Population studies in Asian populations consistently show that soy is associated with reduced risk of breast cancer. The study compared a "meat/dim sum" dietary pattern characterized by meat, starch, and dim sum foods and a "vegetable/fruit/soy" pattern characterized by cruciferous vegetables, fruit, and tofu. The study included 34,028 women recruited during the period 1993 to 1998. As of year-end 2005, 629 new breast cancer cases had been diagnosed among study participants.

Postmenopausal women in the highest quartile of intake of the vegetable/fruit/soy dietary pattern were found to have a 30% lower risk of breast cancer than women in the lowest quartile of consumption. The association was even stronger after five years. However, a greater intake of the meat/dim sum dietary pattern was not found to be associated with increased risk of breast cancer. One of the study authors speculated that this unexpected finding could be explained by differences in the way meat is prepared since stir-fried meat does not contain as many carcinogenic heterocyclic amines (HCAs) as charred or barbequed meat. The authors conclude that the findings support the hypothesis that a diet characterized by vegetables, fruit, and soy has an early-acting protective effect on development of breast cancer.

Implications of the study results

By this time, it is fairly well known that a diet high in vegetables and fruits, and low in meat and starchy foods, reduces the risk of cancer although not all studies are in agreement. The participants in this study also ate a great deal more of soy than the typical U.S. consumer. Numerous studies have found soy consumption to be protective, especially when consumed during childhood. However, while tofu and soybeans may be beneficial, U.S. residents tend to get most of their soy in the form of soybean oil and soy protein isolate, both of which might actually promote breast cancer. Nor do we recommend the consumption of the soy isoflavones, genistein and daidzein, which might not be safe for breast cancer patients or those at high risk of breast cancer or recurrence.

It is also true that not all vegetables and fruits are equally beneficial in protecting against breast cancer and a few may actually promote it. Eating bananas is not as effective as consuming raspberries and kale is much better for you than lima beans. There is also some evidence that the components of some foods act synergistically in reducing breast cancer risk. We recommend eating a wide variety of the foods from our recommended list and limit or avoid those on our avoid list. Please also see our article on how to optimize your breast cancer diet for information on what to eat during all stages of treatment and recovery.