A DIET rich in soya may reduce the risk of developing hormone-related cancers of the prostate and breast, according to doctors in Britain and Hong Kong. The incidence of both diseases is far lower in Asia and the Far East - where soya beans form a major part of the diet - than in Europe and the United States. A report in General Practitioner says that soya is a rich source of phyto-oestrogen, a vegetable hormone that is thought to play a significant role in regulating hormones produced by the pituitary gland.