Vegan for Life - Jack Norris [84]
High-fiber foods, including soyfoods, can reduce absorption of the synthetic thyroid hormones used by people with hypothyroidism. This is why synthetic thyroid drugs should be consumed on an empty stomach. The key to balancing intake with medication is to be consistent in consumption—that is, eat roughly the same amount of soyfoods every day—which is also important for the goitrogenic vegetables.
Finally, approximately 10 percent of older people have subclinical hypothyroidism—a condition that lies somewhere between having a healthy thyroid and hypothyroidism. As a precaution, people with this condition should have their thyroid function monitored if they decide to change their intake of soyfoods because this is an area that hasn’t been well-researched.
Reproductive Health and Feminization
Stories that make their way around the Internet about the negative impact on testosterone and feminizing effects of isoflavones are not supported by the research. A comprehensive analysis published in 2010 showed that neither soyfoods nor isoflavones affect testosterone levels.57 Similarly, there is no evidence that soyfoods or isoflavones, when consumed in amounts even greatly exceeding typical Japanese intake, have any effect on estrogen levels in men.58
In a study of soy intake and sperm characteristics, researchers found that sperm counts in men with higher soy intakes did not differ from those who ate no soy. Men who ate more soy had lower sperm concentrations, but this was partly due to a higher semen volume.59
In contrast, clinical studies show no effects of isoflavones on sperm or semen, even when isoflavone intakes are ten times greater than what Japanese men typically consume.60 In fact, as a result of their findings, one research team suggested that isoflavones may be a treatment for low sperm concentration.61
Soyfoods have been a part of Asian diets for centuries and there has never been any indication that they affect reproduction in these populations. Current research on this issue supports what history has long shown.
HOW MUCH AND WHAT KIND OF SOY TO EAT
It’s always smart to build a diet on a foundation of whole, unprocessed foods and to include a variety of foods in every meal. Where do soyfoods fit in? We can look to traditional Asian diets for guidance, staying mindful that a couple of common beliefs about Asian soyfood consumption—that soy is consumed mostly as fermented foods and used only as a condiment—are both wrong.
Fermented foods, such as miso, were the first soyfoods consumed in Asian countries, but that in no way means that people in Asia consume mostly fermented soyfoods. Nonfermented foods like tofu have been a part of Asian diets for at least 1,000 years and continue to play a significant role in these cultures. In China, soymilk and tofu make up the bulk of the soyfoods in diets. In Japan, about half the soy intake is from the fermented foods miso and natto and the other half comes from unfermented foods like tofu.62
A number of vegan foods, including veggie burgers and other meat analogs, are made from isolated soy protein or soy protein concentrate. They have been the target of some criticism because they are highly processed, but there is actually nothing unsafe about them. In fact, most of the studies on the protein quality of soy—which looked at the ability of soy to support protein balance in humans—used isolated soy proteins.63
Surveys