SOY CONSUMPTION DURING CHILDHOOD AND PREGNANCY

Soy is commonly consumed in Asian countries during pregnancy¹^-⁴ and childhood⁵^-⁹ as it is during other stages of life. However, a comprehensive technical review published in 2021 highlighted the need to better understand the effects of soy, and especially soybean isoflavones, in children and pregnant women as only limited research involving these 2 groups has been conducted.¹⁰ Research involving children is particularly important because young people may be especially sensitive to the influence of diet and because dietary habits established when young may track into adulthood. This sensitivity may also apply to pregnancy, with respect to both the mother and developing fetus. To help fill the research void, the Soy Nutrition Institute Global recently funded two 12-week intervention studies, one involving healthy children and the other involving children with fatty liver disease which will begin later in 2023.

A brief review of research relevant to children and pregnant women is presented below.

Children

Soy has been used for decades as a source of protein in U.S. government feeding programs targeting malnourished children throughout much of the developing world.¹¹^-¹³ The U.S. Agency for International Development (AID) outlines the importance of corn-soy blends in a comprehensive fact sheet for those who want to learn about these products and how to use them.¹⁴ The high quality of soy protein is one reason for its widespread use in these programs.¹⁵ Soy protein has also been shown to directly lower blood cholesterol in children as it has in adults; although relatively few studies in the former have been conducted.¹⁶^-²⁰ Arguably, the most intriguing proposed benefit of soy foods is that when consumed early in life, breast cancer risk is reduced. This hypothesis, which was proposed in 1995,²¹^,²² has gained support over the years from the results of animal²¹^,²³^,²⁴ and observational²⁵^-²⁸ studies.

The isoflavones in soy are thought to change cells in the developing breast in a manner that makes them permanently less likely to be transformed into cancer cells. More specifically, mechanisms for the protective effect of early isoflavone exposure include increasing cell differentiation,²⁹^,³⁰ BRCA1 gene expression,³¹ and estrogen receptor-β levels.³² However, although isoflavones are the soybean components thought to be responsible for this proposed benefit, they have also led to concerns about childhood soy consumption. Most concerns relate to the possibility that soy could advance the age of puberty onset, especially in girls. It is noteworthy that children may absorb isoflavones more efficiently than adults.³³^,³⁴

Over the last several decades, there has been a secular trend among girls to enter puberty, as judged by age of menses onset (AOM), earlier in life. However, this trend has occurred in soy food-consuming and non-consuming countries alike.³⁵^-⁴⁵ Furthermore, a retrospective observational study found no relationship between AOM and soy intake among U.S. Seventh-day Adventist girls, a high-soy-consuming population.⁴⁶ Clinical studies have also found no effects of soy on hormone levels including thyroid hormone (boys and girls),⁴⁷ estrogen (girls),⁴⁸^,⁴⁹ and testosterone (boys),⁴⁹ but these studies involved very small numbers of participants. A reasonable intake recommendation for children is up to 2 servings of soy daily; although age, body weight, and dietary habits may dictate what is best for any individual child.

Pregnancy: Maternal Effects

Recent evidence suggests that soy intake during pregnancy may reduce risk of gestational diabetes mellitus (GDM). Although the data are limited, all 3 prospective epidemiologic studies (2 from China⁵⁰^,⁵¹ and 1 from Japan)³ that examined this relationship found statistically significant protective effects even after adjustment for a variety of potentially confounding variables. In addition to the observational data, a small randomized controlled trial of Iranian women found that the addition of soy to the diet of women with GDM significantly improved glucose homeostasis parameters, triglycerides, and biomarkers of oxidative stress, as well as reduced the incidence of new-born hyperbilirubinemia and hospitalization.⁵² Also, consumption of a soy protein rich diet reduced the need for insulin therapy in Indian women with GDM while having no effect on maternal and neonatal thyroid function.⁵³

GDM is defined as the type of hyperglycemia diagnosed for the first time during pregnancy.⁵⁴ It is the most common metabolic disorder of pregnancy⁵⁵ and is associated with an increased risk of adverse perinatal outcomes⁵⁶ and possibly an increased risk of long-term ill-health outcomes in the mother⁵⁷ and her child(ren).⁵⁸

Pregnancy: In Utero Effects

Despite the common practice among Asian women of consuming soy while pregnant, concern has been raised that the resulting in utero isoflavone exposure could adversely impact the fetus.⁵⁹^,⁶⁰ It has been known for decades that maternal isoflavone exposure leads to quantifiable amounts of isoflavones in amniotic fluid.⁶¹ Nevertheless, concern arose after a British observational study reported that soy consumption was associated with an increased risk of hypospadias, a common genitourinary anomaly in which the opening of the urethra is on the underside of the penis.⁶² The authors speculated that isoflavones were responsible for this association. However, in this study, the consumption of dried peas, beans, lentils, and chickpeas was associated with a 7-fold increased risk of hypospadias, despite non-soy legumes containing negligible amounts of isoflavones.⁶³^,⁶⁴

More importantly, subsequently published observational research shows maternal soy consumption is not associated with an increased risk of hypospadias.⁶⁵^,⁶⁶ In fact, in a large Japanese study involving 41,578 mothers who delivered singleton live male births, compared with mothers in the reference group (genistein intake 11th-89th percentiles), those in the low intake group (≤10th percentile) had an elevated risk of their sons having hypospadias. Neither adverse nor beneficial effects of genistein, which is the primary soybean isoflavone, on hypospadias were observed in the high intake group (≥90^th percentile). The authors concluded that low maternal isoflavone intake in early pregnancy was associated with an elevated risk of hypospadias. Low natto and tofu intake were each associated with about a two-fold increased risk.

Finally, although only conjecture, there is a school of thought that maintains isoflavones are unlikely to exert in utero effects because of the much larger amounts of endogenously produced estrogen to which the fetus is exposed.⁶⁷^,⁶⁸ With respect to intake, there is no reason to differentiate soy food consumption recommendations between pregnant and non-pregnant women.

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