How much soy is safe to eat?
For infants, we believe any soy is too much soy. Adults consuming soy should also exercise caution. As Dr Daniel Sheehan, Director of Reproductive Toxicology at the USA Department of Health and Human Services’ Research Center says "No dose is without risk; risk is a function of dose" in this letter to his head office opposing "health" claims for soy protein. Follow the following link to Dr Sheehans letter.
The observations from the Ishizuki Thyroid Clinic study indicate significant, goitrogenic effects in subjects fed 30 g soybeans per day. Based on the concentrations of isoflavones found in Japanese soybeans, 30 g of soybeans could contribute up to 23 mg total genistein and 10 mg of total daidzein. For a 70 kg adult this would equate to an intake of 0.33 mg/kg-body weight of genistein and 0.14 mg/kg-body weight of daidzein per day.
This amount of isoflavone consumption is approximately three times higher than the amount typically consumed in Japan, which is 0.08 to 0.13 mg/kg-body weight of total genistein per day for a 70 kg adult.
For infants fed soy-formulas, the exposure to isoflavones is greater than in any other population group. Infants less than 6 months of age who are solely fed soy formula have an intake of up to 5.4 mg/kg-body weight of genistein and 2.3 mg/kg-body weight basis of daidzein per day. Hence, soy formula fed infants are exposed to approximately 16 times greater levels of isoflavones than the subjects in the Ishizuki study.
The concentrations of isoflavones found in soy products can vary but studies from New Zealand indicate that a diet of 500g of soy milk plus 200g tofu per day would result in the consumption of up to 135 mg total genistein and 80 mg total daidzein. For a 70 kg adult this equates to an intake of 1.9 mg/kg-body weight of genistein and 1.1 mg/kg-body weight of daidzein per day. This degree of exposure to isoflavones is more than five times that of subjects in the investigation by Ishizuki.
Users of isoflavone supplements may consume up to 40 mg of genistein per day. For a 70 kg adult this is equivalent to 0.57 mg/kg-body weight basis of genistein per day which is about 1.7 times more than that found to have goitrogenic effects.
Therefore, soy formula fed infants, high soy consumers and users of isoflavone supplements might exhibit classic hypothyroid symptoms without recognizing a dietary connection. Unfortunately there is little data as what constitutes an appropriate level of soy intake, although it appears that some western consumers may now be eating far greater amounts of soy than that taken as part of a traditional Asian diet.
Soy users should be aware of the potency of just 30 mg soy isoflavones per day. Thyroid disorders (see above for discussion on the active dose in the Ishizuki Thyroid Clinic study) and other biological effects have been observed at dose around this level.
As an approximate guide 30 mg of soy isoflavones can be found in:
Soybeans and soyflours: 9 – 20g (0.3 – 0.7oz).
Soy mince: 12g (0.4oz).
Tofu: 50 – 110g (1.8 – 3.9oz).
Soy milks: 150 – 240g (5.3 – 8.5oz).
Miso: 35 – 45g (1.2 – 1.6oz).
Soybean sprouts: 80g (2.8oz).
What products contain soy?
As well as obvious soy foods like tofu, soy milk and miso, and other soy products such as isolated soy protein (ISP) and soy protein concentrate (SPC), very many processed foods contain soy, some examples are:
Hydrolyzed vegetable protein (HVP)
Textured vegetable protein (TVP)
Why isn’t this information readily available?
People deserve the right to know about what they are eating and what they are feeding their children. So why are government agencies so reluctant to share information with the public?
New Zealand environmental scientist and phytoestrogen researcher Dr Mike Fitzpatrick met with California DHS staff in June 1998 to express his concerns about soy, and particularly soy formulas. He received a written response from DHS toxicologist Dr Susan Loscutoff. Loscutoff stated:
"I agree that high levels of dietary exposures to isoflavones in infants fed soy-based formulas is cause for concern."
"I do not agree that parents have a right to know that soy-based formulas contain isoflavones and the kinds of toxicities isoflavones might demonstrate in infants, since parents would not know how to interpret the information."
This kind of response it quite typical of agencies fearing a severe backlash from the soy lobby should they alert the public to the potential health concerns of soy isoflavones.
What else can I do?
Write to your National or State Health Department representatives demanding information on the risks associated with the consumption of soy isoflavones, especially by infants, and the safety of isoflavone supplements/OTC-drugs.
Write to potentially sympathetic politicians (e.g. in the US Senator Barbara Boxer or Senator Fred Lautenberg), and express your concern about the presence of isoflavones in soy formulas. Ask for clarification regarding the safety of soy formulas and soy isoflavone supplements/OTC drugs.
Share this information with your health professionals and friends.
Be prepared for a ‘no evidence of harm’ response from government agencies. The facts, however, tell a different story.
Soy Free Manufactured Foods
The Manufactured Food Database (MFD) has been compiled by Nutrition Services, Auckland Hospital from information voluntarily supplied by New Zealand Food Manufacturers. This database provides information on which products have been declared Soy free.
It is not widely known that soy is one of the most allergenic foods in modern diets. It is reported in several research reports to contain at least 30 allergenic proteins.