A University of Toronto study is providing reassuring evidence on consumption of soy foods during postmenopause.
In a systematic review and meta-analysis of 40 trials in over 3,000 participants, U of T researchers gathered results on the effects of soy isoflavones - estrogen-like compounds naturally found in plants - on biological outcomes related to risk of endometrial and other female-related cancers.
Their results, published in Advances in Nutrition , found that isoflavones had no effect on key markers of estrogen-related cancers, supporting the safety of soy as a food as well as potential therapy.
"The risk of cardiovascular disease increases substantially as women go through menopause, so soy can offer dual benefits during this particular phase of life," says the study's senior author Laura Chiavaroli, an assistant professor of nutritional sciences at U of T's Temerty Faculty of Medicine and affiliate scientist at the Li Ka Shing Knowledge Institute at St. Michael's Hospital, Unity Health Toronto.
Yet, Chiavaroli notes many people are hesitant to eat soy foods because they contain isoflavones, which have a similar structure to estrogen. In animal studies, large doses of isoflavones have been linked to a higher risk of cancer.
"Something we hear very often is that people have a lot of concern about consuming soy because there are so many conflicting messages out there," says Gabrielle Viscardi, a second-year PhD student in the department of nutritional sciences and the study's lead author.
This is despite the fact that several health advocacy groups, including the Canadian Cardiovascular Society, already recommend soy foods as part of a healthy diet. Health Canada and the U.S. Food and Drug Administration have both recognized soy - a high-quality and complete source of protein - as effective in reducing the risk of heart disease
Soy foods have also been shown to alleviate hot flashes associated with menopause, which affect many people and impact their quality of life.
The trials considered for the U of T study followed postmenopausal individuals from around the world who had consumed either soy isoflavones or a non-isoflavone control for at least three months.
The researchers concluded that consumption of soy isoflavones did not affect the four key estrogen-related markers, namely: thickness of uterus lining, vaginal maturation index (a measure of estrogen status) and levels of circulating estrogen and follicle-stimulating hormone.
Their findings support the idea that soy isoflavones behave differently from human estrogen, particularly when it comes to cancers that depend on estrogen to develop. "We have estrogen receptors throughout our bodies but, contrary to the hormone estrogen, isoflavones from soy don't bind to all the estrogen receptors equally," says Viscardi, who is also a registered dietitian. "That's why we see a beneficial effect on the cardiovascular system and no effect on the female reproductive system."
This difference in biological activity explains why soy isoflavones have been considered as a possible alternative to hormone replacement therapy (HRT), which is used to treat menopause symptoms by replacing the estrogen that the body stops producing during this period.
Chiavaroli notes some people don't want to take HRTs and are seeking alternative options.
Further, HRTs may also not be a good option for people at an increased risk of estrogen-sensitive cancers like breast cancer, as well as those with a history of heart disease and stroke. For these individuals, consuming soy foods as part of a balanced diet could help manage their menopausal symptoms while also reducing the risk of cardiovascular disease.
Chiavaroli says the study's findings also align with Health Canada's dietary guidelines that encourage people to choose plant-based proteins more often, a move that would also convey benefits for the environment.
"We hope our study will help people feel more comfortable including soy foods in their diet without being concerned that it's going to increase their risk of estrogen-dependent cancer," says Chiavaroli.
The study was funded by the United Soybean Board (U.S. Department of Agriculture) and the Canadian Institutes of Health Research through the Canada-wide Human Nutrition Trialists' Network.
