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Soy Side Effects, Interactions and Warnings



  • Soy consumption has been suggested to exert effects in premenopausal women, such as increased menstrual cycle length and sex hormone-binding globulin levels and decreased estrogen levels. One cross-sectional study showed serum estrogens to be inversely associated with soy intake. Seven soy intervention studies controlled for phase of menstrual cycle. These studies provided 32-200 mg/d of isoflavones and generally showed decreased midcycle plasma gonadotropins and trends toward increased menstrual cycle length and decreased blood concentrations of estradiol, progesterone and sex hormone-binding globulin.
    - J Nutr 2002 Mar;132(3):570S-573S -- Hormonal effects of soy in premenopausal women and men. -- Kurzer MS.

  • Soy isoflavones can function as estrogen agonists, antagonists or selective estrogen receptor modulators, depending on the conditions.
    -J Nutr 2002 Mar;132(3):559S-565S -- The health consequences of early soy consumption. -- Badger TM, Ronis MJ, Hakkak R, Rowlands JC, Korourian S.

  • Decreased serum estradiol concentration associated with high dietary intake of soy products in premenopausal Japanese women.
    - Nutr Cancer 1997;29(3):228-33 -- Decreased serum estradiol concentration associated with high dietary intake of soy products in premenopausal Japanese women. -- Nagata C, Kabuto M, Kurisu Y, Shimizu H.

  • Various wheat and soy protein sources, including the soy protein isolates used to make infant formulas, have been related to juvenile or insulin-dependent diabetes mellitus (IDDM), a common chronic disease of childhood.
    - Can J Physiol Pharmacol 1997 Apr;75(4):241-54 -- Adverse reactions to food constituents: allergy, intolerance, and autoimmunity. -- Kitts D, Yuan Y, Joneja J, Scott F, Szilagyi A, Amiot J, Zarkadas M.

  • Soy protein formulas are often poorly tolerated by infants with chronic nonspecific or postinfectious diarrhea syndrome.
    - Am J Dis Child 1987 Oct;141(10):1069-71 -- Chronic diarrhea and soy formulas. Inhibition of diarrhea by lactose. -- Donovan GK, Torres-Pinedo R.

  • Soy and soy products may have an effect on calcium absorption.
    - Am J Clin Nutr 1991 Mar;53(3):745-7 -- Soybean phytate content: effect on calcium absorption. -- Heaney RP, Weaver CM, Fitzsimmons ML.

  • Soy may also have an effect on zinc absorption.
    - Am J Clin Nutr 1984 Nov;40(5):1064-70 -- The effect of individual components of soy formula and cows' milk formula on zinc bioavailability. -- Lonnerdal B, Cederblad A, Davidsson L, Sandstrom B.

  • Use caution if you have thyroid problems or conditions. Iodine deficiency greatly increases soy antithyroid effects. The possibility that widely consumed soy products may cause harm in the human population via either or both estrogenic and goitrogenic activities is of concern.
    - Environ Health Perspect 2002 Jun;110 Suppl 3:349-53 -- Goitrogenic and estrogenic activity of soy isoflavones. -- Doerge DR, Sheehan DM. -- Division of Biochemical Toxicology, National Center for Toxicological Research, Jefferson, Arkansas, USA.

  • There is a possibility that dietary soy raises the serum homocysteine level because isoflavones, which are weak estrogens contained in soybeans, may exert antiestrogenic effects in a high estrogen environment, such as in premenopausal women.
    - J Nutr 2003 Mar;133(3):797-800 -- Soy product intake is inversely associated with serum homocysteine level in premenopausal Japanese women. -- Nagata C, Shimizu H, Takami R, Hayashi M, Takeda N, Yasuda K.

  • Soy contains genistein and daidzein, two phytoestrogens, which work through the estrogen receptor and cause alterations in serum lipids, bone metabolism, and possibly cognition.
    - Maturitas 2003 Mar 14;44 Suppl 1:S21-9 -- Soy isoflavones: hope or hype? -- Fitzpatrick LA.

  • Soy protein food-drug interactions with warfarin may be more common than the literature suggests, and further studies are needed to determine the exact mechanism. Healthcare professionals should be alerted to the potential implications of this food-drug interaction.
    - Ann Pharmacother 2002 Dec;36(12):1893-6 -- Effect of soy milk on warfarin efficacy. -- Cambria-Kiely JA.

  • Caution is warranted for postmenopausal women consuming dietary genistein while on tamoxifen therapy for E-responsive breast cancer.
    - Cancer Res 2002 May 1;62(9):2474-7 -- Dietary genistein negates the inhibitory effect of tamoxifen on growth of estrogen-dependent human breast cancer (MCF-7) cells implanted in athymic mice. -- Ju YH, Doerge DR, Allred KF, Allred CD, Helferich WG.

  • Administration of levothyroxine concurrently with a soy protein dietary supplement results in decreased absorption of levothyroxine and the need for higher oral doses of levothyroxine to attain therapeutic serum thyroid hormone levels.
    - Endocr Pract 2001 May-Jun;7(3):193-4 -- Use of soy protein supplement and resultant need for increased dose of levothyroxine. -- Bell DS, Ovalle F.

  • Results show that low dose simvastatin may enhance the hypocholesterolemic effect of soy protein compared to animal proteins in the rabbit.
    - J Am Coll Nutr 1997 Apr;16(2):166-74 -- Simvastatin further enhances the hypocholesterolemic effect of soy protein in rabbits. -- Giroux I, Lavigne C, Moorjani S, Jacques H.




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  • last update: February 2014



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