Newswise — Colon cancer is the third most common form of cancer in the United States. In 2001, 46,200 men and 52,000 women were diagnosed with new cases of colon cancer. For the first time, a University of Missouri-Columbia researcher has shown that estrogen may protect against the disease. In addition, the researcher also found that soy protein may help minimize the number and size of tumors that do occur. "This study suggests that colon cancer may be a hormone-responsive cancer which may provide new ways to treat and or prevent this disease," said Ruth MacDonald, professor of food science. "In addition, we discovered that soy protein could have a very positive effect on the number and size of tumors that do occur." In her study, which was published in the January 2004 edition of the Journal of Nutrition, MacDonald fed female mice five different diets, and then followed their progress for a year. The five diets were designed to compare the effects of specific ingredients. Diet one was made with milk protein, and diet two contained soy protein. Both diets were lacking any kind of estrogen. The other three diets contained soy protein with the addition of an estrogen component. Diet three contained soy protein and genistein, an estrogen-like compound found in soy. Diet four contained Novasoy, a commercial product containing a mixture of soy-derived compounds including genistein, and diet five contained estrone, a naturally occurring human estrogen. Somewhat to her surprise, MacDonald found that while all the soy/estrogen diets gave some protection, the diet containing estrone was the most effective in preventing colon cancer. This is the first time such a finding has been documented. The dose of estrone the mice received was similar to levels used in hormone replacement therapy. The researcher also discovered that those mice that ate soy protein and did develop colon cancer had fewer and smaller tumors than those mice that did not eat soy protein. "This data goes against the silver-bullet theory and tells us that it is more beneficial to eat the food and not the supplement," MacDonald said. "We know that soy protein may be helpful in the prevention of heart disease, but this work suggests it may also be beneficial in the prevention and control of colon cancer. The good news is that there are many ways to add soy to your diet now and we know of no harmful side-effects to eating soy protein." MacDonald, who is a faculty member in the MU Center for Phytonutrient and Phytochemical Studies (www.phyto-research.org), is continuing her study to determine how the compounds work to provide protection of colon cancer. The American Institute for Cancer Research funded the study.