April/May 2022 Issue
Kids, Teens, and Soyfoods
By Densie Webb, PhD, RD
Vol. 24, No. 4, P. 34
Today’s Dietitian explores the latest research on their safety, health benefits, and recommendations for daily consumption.
As more Americans adhere to recommendations to shift toward a more plant-based diet, consumption of soyfoods has grown. According to a survey conducted by The Vegetarian Resource Group, 54% of respondents said they sometimes or always ate vegan or vegetarian, 25% said they sometimes or always ate vegan, and 6% said they were either completely vegetarian or vegan.1 Greater interest among Americans to incorporate more plant foods in their diet has led to more introductions of soy-based products made with whole soybeans or other soy protein options, such as soy protein isolate, soy protein concentrate, soy flour, and textured vegetable protein used in the production of foods designed to mimic meat and dairy.
The increased consumption of soyfoods is a result of soy’s proposed health benefits, such as protection against coronary heart disease, breast and prostate cancers, osteoporosis, and alleviation of menopausal symptoms.2 While soy is an excellent source of high-quality protein, it’s also a source of isoflavones (ie, plant-based estrogens) and nonsteroidal phytoestrogen compounds (genistin, daidzin, and glycitin), naturally present in soyfoods.3
These phytoestrogen compounds have been shown to have estrogenlike properties under experimental conditions, but these properties have been the source of concern, especially when incorporated into the diets of children and adolescents.
The controversy regarding possible hormonal effects of isoflavones intensified in the late 1990s. The primary concern has been that soy isoflavones may affect blood levels of estrogen and/or testosterone in children and adolescents, impacting the onset of puberty, especially in girls. Early onset of menstruation in girls has been associated with an increased risk of breast and ovarian cancers later in life. It’s true that isoflavones often are classified as endocrine disruptors, based primarily on animal studies, but a recent technical review of human studies concluded that the evidence doesn’t support classifying isoflavones as endocrine disruptors in humans.4
“I would say the biggest concern is that soy may feminize boys and that it may interfere with development in girls and boys,” says Mark Messina, PhD, executive director of the Soy Nutrition Institute. Underlying this concern, he says, is equating phytoestrogens with estrogen. But, he says, while estrogens and phytoestrogens do have some properties in common, they are very different molecules.
“The evidence indicates that this concern [about negatively affecting levels of sex hormones and sexual development] is not justified,” Messina says, emphasizing that studies involving the general populations of non-Asian countries can’t provide real insight into the health effects of soy because intake is generally low. Soyfoods have been an integral part of the diet in many Asian countries for centuries. The average intake among older adults in Japan, for example, ranges from 25 to 50 mg per day of isoflavones, whereas in the United States and Europe the average per capita intake is less than 3 mg per day.5-8
Research of Concern
Despite questions raised about the possibility of soy isoflavones affecting hormones and the onset of puberty, there has been little investigation on the impact of soy consumption on reproductive hormones and age of puberty in children. A Japanese cross-sectional study found that in later stages of childhood, an increase of androgens in girls and a decrease in estrogens in boys were associated with intake of dietary phytoestrogens.9 While it’s the only study of its kind, no adjustments were made for lifestyle factors or intake of dietary sources of phytoestrogens other than soy.
Two Korean studies initially raised concern about isoflavone intake and early puberty, but Messina says both suffered from design limitations.10,11 Amy Reed, MS, RD, CSP, LD, a dietitian at Cincinnati Children’s Hospital Medical Center and a spokesperson for the Academy of Nutrition and Dietetics, says, “There are limited studies that show changes in hormones, but there’s a question as to whether those changes are even significant.”
Research Showing No Effect
Soy consumption among Seventh-day Adventists in the United States generally is much higher than that of the general US population, but research shows it has no effect on the onset of puberty in girls and boys. A US cross-sectional study looked at Seventh-day Adventist girls with relatively high intakes of soy and found that intake was unrelated to the age of onset of menstruation.12 A cross-sectional study of Seventh-day Adventist boys found that development of facial hair in those aged 12 to 18 wasn’t associated with soy intake, and even among boys who consumed higher levels of soy, puberty onset was later than is typical for US boys.13
Messina emphasizes two small intervention pilot studies: one conducted in the United States and one in Israel. Neither shows an effect of soy intake on levels of sex hormones excreted in the urine or on the onset of puberty. The Israeli study included 12 girls, aged 5 to 11, given 16 or 48 mg of isoflavones per day.14 The US study includes girls aged 8 to 14 consuming 30 mg of isoflavones per day for eight weeks.15
It’s important to note, he says, that the onset of early puberty is now occurring in countries where soy isn’t a traditional part of the diet, as well as in those where soy is consumed regularly. There are several factors other than soy consumption that may contribute to the trend of early onset of menstruation, including increased rates of overweight and obesity among children and exposure to xenoestrogens in plastics, pesticides, chemicals, and water systems. Xenoestrogens are “foreign” estrogens, substances close enough in molecular structure to estrogen, causing them to bind to estrogen receptor sites and act like estrogens.
Research Showing Benefits
While concern regarding the potential effects of isoflavones in soy has been voiced, there’s also research suggesting potential benefits of soy consumption early in life. Both animal and human research suggest an association between soy consumption early in life and a decreased risk of breast cancer later in life for women. Four population studies evaluated the effect of adolescent soy intake on the risk of developing breast cancer.16-19 Two were conducted in China and two in the United States. Three of the four found that adolescent soy intake was associated with a statistically significant reduction in breast cancer risk. A fifth study from Canada also found an association, but the results weren’t statistically significant.
While the mechanism behind the proposed reduction in breast cancer risk isn’t completely understood, it has been suggested that exposure to isoflavones early in life may cause increased cell differentiation, making breast cells less likely to transform into cancer cells, an effect similar to that of early pregnancy.20
And while evidence suggests that soy protein lowers blood cholesterol in children as it does in adults, it doesn’t appear that isoflavones are involved.21-23
While soy has been studied extensively in the United States and abroad, Messina says that population studies involving general populations of non-Asian countries can’t provide insights into the health effects of high intakes of soy because intakes generally are too low. One exception is the Seventh-day Adventist population in the United States, which includes a large number of vegetarians and vegans. Researchers usually highlight the inability to generalize any findings to populations other than those being studied.
The American Academy of Pediatrics hasn’t issued an official recommendation for soy consumption for children and adolescents. However, Messina suggests 5 to 10 g of soy protein per day is reasonable. Each gram of soy protein in traditional soyfoods is equivalent to approximately 3.5 mg isoflavones.5 Five to 12 g soy protein is equal to 1 cup soymilk, 31/2 oz tofu, 1 oz soynuts, or 1/2 cup edamame. But Messina says his suggestion isn’t meant to imply that consuming more soy protein per day is contraindicated. In fact, the FDA has stated that 25 g of soy protein per day, when part of a diet low in saturated fat and cholesterol, can lower blood cholesterol and help reduce the risk of developing heart disease.24 In addition, a recent review concluded that the optimal amount of soy varies according to the age of the child or adolescent, but what that amount is hasn’t been identified.
“Sometimes I see that parents have a fear of soy and feel like they can’t give it to their children at all,” Reed says. “If a child is allergic to soy, then, of course, they cannot have any soy, but, for the most part, families need to focus on feeding their kids a variety of foods and not be hyperfocused on a particular food that must be avoided.”
Soy has been identified as one of eight major food allergens by the Food Allergen Labeling and Consumer Protection Act, but only about 0.2% of children in the United States are thought to suffer from a soy allergy.25 Reed emphasizes that the 2020–2025 Dietary Guidelines for Americans places nuts, seeds, and soy products into the same group and recommends 2 to 5 oz per week for those aged 2 to 8, 3 to 5 oz for those aged 9 to 13, and 4 to 6 oz for those aged 18 to 24.
“For children and adolescents, I think including soy in their diets can help increase variety and introduce the concept that meals do not always have to include meat,” Reed says.
— Densie Webb, PhD, RD, is a freelance writer, editor, and industry consultant based in Austin, Texas.
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