Highly refined soy products like those in unregulated dietary supplements and in soy isoflavone-boosted foods may increase the growth of estrogen-dependent breast cancer tumors.
University of Illinois researchers fed mice, with human breast cancer tumors implanted in them, a diet containing equal concentrations of genistein, one type of isoflavone, a family of plant estrogens found in soy.
But the substance came, in essence, in different packages. At one end of the spectrum was minimally processed soy flour, at the other a highly refined soy extract of 95 percent pure genistein.
In between were soy molasses and other soy products refined to varying degrees.
The mice on the soy flour diet showed no tumor growth. But the implanted tumors in the mice on all the other diets grew.
The nearly pure genistein prompted the greatest tumor growth. The amount of growth decreased proportionally with the amount of refining done to the soy products used in the study.
"If you look at the graph, it goes the exact opposite direction," said Clinton Allred, lead author of the study, which has appeared online and is to be published in the August edition of the journal Carcinogenesis.
Allred and UI Professor William Helferich said whole soy appears to contain biologically active components that offset the impact of genistein on tumor growth.
But refining soy can remove those components, upsetting the balance, they said.
"There's something present in the soy flour (that) greatly reduces the effect of genistein on tumor growth," said Helferich, a UI food science and human nutrition professor whose lab has done several studies looking at the effects of isoflavones in a variety of concentrations and mixtures.
"It's probably a collection of several things," Helferich added. "It's not just one thing. It's probably several that act additively and maybe synergistically. It's a complex story."
Allred, who worked with Helferich as a doctoral student at the UI, is now a Department of Defense breast cancer research program post-doctoral fellow at the University of Kentucky Medical School Department of Pharmacology.
The study, part of Allred's doctoral work, was funded by the National Institutes of Health and the U.S. Department of Agriculture. UI researchers Kimberly Allred (Clinton Allred's wife), Young Ju, Tracy Goeppinger and Daniel Doerge also contributed to the study.
Both Allred and Helferich said the mouse model used in the study is believed to translate well to humans and, in fact, is used in tests to gain clearance for new drugs, among them the breast cancer treatment tamoxifen. Helferich noted that the minimally processed soy flour used in the study approximates the soy found in Asian diets, where studies have identified lower cancer rates, including breast cancer, in part correlated to soy consumption.
Meanwhile, some of the refined soy products used in the UI study are akin to isoflavone-enhanced foods and dietary supplements increasingly populating American shelves.
Some low-carbohydrate foods, for example breads and cereals, use soy protein isolates with significant amounts of isoflavones – but without soy's other active components – as a no-carb source of protein.
"What we're consuming in the U.S. is not very similar to what they're consuming in Asia," Helferich said. "These products can actually be quite potent even though their safety has never been evaluated.
"The real take-home message is there's likely a difference consuming the supplement versus consuming the whole food," he said. "I agree with consuming a healthy diet and consuming soy as part of that diet. But the type of soy is critical."
Allred said the results of the study should be of particular concern to women with breast cancer, and probably those who have suffered from it in the past and are in remission.
"That's really the group of people we're trying to address here," he said.
Soy isoflavone products are often marketed to women over 50 as an alternative to hormone replacement therapy, as well as for potential health benefits, from lowering cholesterol to retaining calcium, which is good for the bones and may help mitigate osteoporosis.
But 75 percent of breast cancer cases are diagnosed in women over 50 and most of those cases are estrogen-dependent.
You can reach Greg Kline at (217) 351-5215 or via e-mail at firstname.lastname@example.org.