Author: Mark Messina, PhD, MS
Soyfoods, long important in Asian diets, have been the subject of rigorous research related to their potential health benefits. Traditional soyfoods such as tofu, miso, tempeh, and soymilk are rich in isoflavones. These plant estrogens differ in important ways from the hormone estrogen but their presence in soyfoods has raised concern about the potential adverse effects of these foods. However, these concerns about based almost exclusively on the results of animal studies. In contrast, research in humans confirms that soyfoods are safe.
Breast cancer: Research shows that isoflavones do not adversely affect breast tissue or raise risk for breast cancer. The safety of soy consumption for women who have had breast cancer has been confirmed by leading health organizations including the American Cancer Society, the American Institute for Cancer Research, the Canadian Cancer Society and the European Food Safety Authority among others.
Thyroid function: Neither soyfood consumption nor isoflavone supplements (often used in research) have been found to affect thyroid hormones or thyroid function in healthy people. Although soyfoods can inhibit the absorption of thyroid medication, this is true of food in general. For this reason, thyroid medication should always be taken on an empty stomach.
Testosterone: A recent statistical analysis of 41 clinical studies found that neither soyfood consumption nor isoflavone supplements affect testosterone levels in men. Also, clinical studies show no effects on sperm concentrations or breast tissue in men.
Cognitive function: In 2020, a statistical analysis of 16 clinical studies (1386 participants, mean age, 60 years) found soy isoflavones improved overall cognitive function and memory.
Puberty onset: Puberty is occurring at younger ages throughout the world. The reasons for this trend are not known, but earlier puberty is occurring in countries that consume soy as well as those that do not. Soy consumption has been shown not to affect hormone levels in children, or to affect the age at which girls being menstruating.
Fetal development: Soyfoods are commonly consumed during pregnancy among women in Asia which suggests that isoflavones do not impact the developing fetus. Concentrations of naturally occurring estrogen are much higher in the womb than levels of isoflavones in women who consume soy. Isoflavones are also not as potent as estrogen. {Kuiper, 1998 #3208
Allergy: While soy is one of the eight foods responsible for 90% of food allergies among Americans, only around 3 of every 1,000 adults are allergic to soy. While allergies are more common in children, 70% outgrow their soy allergy by age 10.
Mineral absorption: All beans and whole grains contain compounds that can interfere with absorption of calcium, iron, and zinc. Nonetheless, calcium is absorbed as well from fortified soymilk and from tofu as from cow’s milk. Iron absorption from soy may be better than previously believed because much of the iron in soy occurs in a form that is resistant to inhibitors or iron absorption.
How Much and What Kinds of Soy Should You Eat
Studies aimed at determining both benefits and risks of soy consumption show that the amount of soy protein and isoflavones in as much as four servings of soyfoods per day does not have adverse effects. Both fermented soyfoods such as tempeh and miso, and unfermented foods such as tofu and soymilk are associated with health benefits. Throughout Asia, unfermented soyfoods are more commonly consumed. The exception is Japan, where about half of soyfoods consumed are fermented.
Although higher amounts of soy have not been shown to be harmful, capping intake at 4 servings per day is one way to make sure your diet has plenty of variety, a key factor in healthy eating.
Please find a more detailed article here including references:
Concerns about the Safety of Soyfoods without Scientific Foundation (PDF)
About the Author:
Dr. Mark Messina is an adjunct associate professor at Loma Linda University and the Executive Director of the Soy Nutrition Institute. He has been studying the health effects of soy for more than 30 years and has published more than 60 scientific papers and given more than 500 presentations on soyfoods to health professionals.