Soy formula associated with higher risk of fibroids in women.
D'Aloisio, AA, DD Baird, LA DeRoo and DP Sandler. 2009. Association of intrauterine and early life exposures with diagnosis of uterine leiomyomata by age 35 in the sister study. Environmental Health Perspectives doi:10.1289/ehp.0901423.
Women who were fed soy-based infant formula as babies are 25 percent more likely to develop uterine fibroids than those who were breastfed or given milk-based formula. Hormones guide the development of these noncancerous tumors in the uterus that can cause pelvic pain, heavy bleeding and reproductive problems. Fibroids affect about a quarter of all women and are the leading cause of hysterectomy. This is the first study to examine whether exposure to soy estrogens early in life is associated with fibroids development later in life.
Soy-based foods such as tofu, soy beans and soy milk contain naturally occurring estrogen-like substances called isoflavones.
Soy foods have been part of Asian adults diets for centuries. In adults, isoflavones have been shown to reduce risk of coronary heart disease and breast, endometrial and prostate cancers.
However, feeding infants soy-based formula is a relatively recent development – just decades at most. The effects of early life exposure to these estrogenic compounds through soy formula are largely unknown.
The American Academy of Pediatrics recommends soy-based infant formula only in a small number of situations. Yet, soy formula accounts for approximately 20 percent of the U.S. formula market. Many parents believe that soy-based formula is healthier than milk-based formula, even though there is no proof it prevents colic or allergies.
With the popularity of soy formula, the U.S. pattern is opposite to the traditional Asian diet. Although Asian children and adults tend to eat a diet high in soy, their infants are traditionally breastfed. In contrast, millions of children in the United States have a high intake of soy in early infancy, but low soy intake for the rest of their lives.
Infants fed soy-based formula receive particularly high levels of the compounds since soy is their main source of nutrition, and they drink a lot relative to their body size. Isoflavone levels in infant blood are 10 times higher than Japanese adults who eat soy. Their levels are 200 times higher than infants who are fed breast milk or cow milk formula.
There is increasing concern that this early life exposure to isoflavones, in both boys and girls, may be particularly important to the developing reproductive and endocrine systems. Most studies have found no dangers of soy formula on children’s growth and development. However, very few studies have examined the association of later reproductive outcomes with soy formula intake.
The researchers used a subset of data from a large research effort called the Sister Study. Approximately 50,000 white women living throughout the United States participated in the extensive four-year Sister Study that examined environmental risk factors for cancer and other reproductive health outcomes.
This study was limited to 20,000 Caucasian women between 35 and 59 years of age who participated in the Sister Study. The researchers looked only at development of early fibroids – that is, fibroids diagnosed before age 35 – because the disease becomes very common in older women and is often undiagnosed.
Women were asked whether they had ever been diagnosed with fibroids and their age at diagnosis. They were also asked about a variety of behaviors during infancy and early childhood, including whether they had been fed soy formula.
Women were encouraged to telephone their mothers to learn more information about their early life exposures. Because of the difficulty remembering such long-ago behaviors, women could answer whether they were definitely, probably, probably not, or definitely not fed soy formula.
Women who reported that they were fed soy formula as infants were 25 percent more likely to have been diagnosed with fibroids by age 35. The relationship was even stronger in women who could say that they had definitely been given formula containing soy.
Other factors associated with increased risk of fibroids included low socioeconomic status in childhood, being born at least one month early and having a mother with diabetes.
As shown in other studies, women whose mothers took diethylstilbestrol (DES) during pregnancy were also at increased risk of fibroids. In this study, DES exposure was associated with a 40 percent increase in fibroids risk.
DES is a synthetic estrogen given to pregnant women before the 1970s to prevent miscarriages. Although it didn't prevent the loss of pregenancies, DES has been linked to reproductive cancers in the daughters of those who took it while pregant.
This is the first study in people to show an association between drinking soy formula as an infant girl and developing uterine fibroids later in life.
Since it is a first of its kind study, the results need to be replicated in other studies to confirm the link.
Prior animal and human studies find soy can affect reproduction in various ways. A small study reports that women who were fed soy formula as infants experienced longer and more painful menstruations – a symptom of uterine fibroids. Laboratory animals fed soy isoflavones early in life can develop fibroids, reproductive cancers and infertility.
A sobering finding is that soy formula has similar – although weaker – association with fibroids as does DES. This may not be surprising since both soy and DES have estrogen activity. The hormones estrogen and progesterone regulate fibroid development.
Fibroids are non-cancerous tumors found in the muscle of the uterus that can cause heavy, painful menstruation and pain during intercourse. About one in every 4 women has detectable fibroids during her adult life – even more women have asymptomatic fibroids without ever realizing it. African-American women have higher rates of fibroids than other ethnic groups.
In general, additional research is needed on the long term effects of early life exposure to estrogens, including soy formula, on reproductive health.
DES Update. Centers for Disease Control and Prevention.
Uterine fibroids. Medline Plus. US National Library of Medicine and National Institutes of Health.