Are soy foods safe?

Posted on Wed, 2 Mar 11

Does soy affect your thyroid, raise cancer risk, influence your hormone levels and are fermented soy foods really a healthier option? Find the answers here.

Q: Will soy cause hypothyroidism?

There have been concerns about possible harmful effects of soy on thyroid function for decades. Soy isoflavones have been shown to interfere with thyroid function in some but not all experimental studies. Similarly, results from human studies are not entirely clear.

In a review of 14 clinical studies it was found, with one exception, that soy foods or soy isoflavone supplements did not significantly affect thyroid function particularly if consumed with adequate iodine, a mineral thought to prevent any possible detrimental effects (1). However, these studies included only people with healthy thyroid function. Those with existing thyroid disease may need to be more cautious.

Perhaps the only study to date assessing the effects of soy on thyroid function in people with existing thyroid disease (subclinical hypothyroidism) found that consumption of soy protein containing 16 mg of isoflavones for 8 weeks resulted in worsening of thyroid function, despite sufficient iodine intake (2). In this study soy protein containing 2 mg of isoflavones did not affect thyoid health.

A: It seems unlikely soy presents a problem for people with healthy thyroid function. However, soy should be avoided in those with hypothyroidism.

 

Q: Does soy affect endometrial or ovarian cancer risk?

Soy foods have properties that appear to protect against gynaecological cancers such as endometrial or ovarian cancers. A review of the totality of evidence found that across multiple studies people with a higher soy food intake had a reduced risk for endometrial cancer and ovarian cancer, when compared with lower soy intake (3).

The existing studies examining the relationship between soy and endometrial or ovarian cancers are mostly epidemiological studies which cannot prove that soy protects against cancer. Although further research is needed the evidence to date suggests a reduced cancer risk in women who eat soy.

A: Higher soy food intake appears to protect against endometrial and ovarian cancer risk.

Q: Can soy lower testosterone in men?

Because soy foods contain estrogenic substances there have been concerns about the hormonal effects of soy foods in men. To ascertain whether or not soy food affect testosterone levels some 40 clinical studies of soy protein and soy (or red clover) isoflavones and their effect on testosterone levels in men were analysed (4).

 Soy protein or isoflavone intake had no significant effect on testosterone levels or other relevant hormonal markers (sex hormone-binding globulin [SHBG], free testosterone or free androgen index [FAI]).

A: Soy protein and soy isoflavones appear to have no effect on testosterone levels.

 

Q: Will soy isoflavones cause hormonal imbalance in women?

Soy isoflavones act as a weak oestrogens so concerns have been raised about their possible hormonal effects in women.  A review of 47 studies looked at the impact of soy in both pre and post menopausal women (5).

In premenopausal women soy reduced follicle stimulating hormone (FSH) and luteinising hormone (LH) levels but did not affect other hormones such as estrogens and progesterone. The study authors note that it is unclear whether this modest hormonal effect of soy is detrimental or beneficial. The decrease in FSH and LH may, they point out, explain why soy appears to protect against breast cancer.

In postmenopausal women soy was associated with a small non-significant increase in total circulating estradiol but did not affect other hormone levels including other oestrogens, LH or FSH. Collectively these findings argue against a physiologically important estrogenic effect of soy in post menopausal women.

A: Soy appears to have modest effects on hormone balance in women that are unlikely to be of major significance.

 

Q: Does soy increase breast cancer risk?

Breast cancer is strongly associated with oestrogens and because of their anti-estrogenic effects soy foods have been associated with reduced cancer risk. Indeed, a number of studies have found an association between soy food consumption and reduced risk of breast cancer (6-8).

In addition, three recent epidemiological studies in breast cancer survivors found that soy foods do not increase risk of breast cancer reoccurrence or interfere with the breast cancer drug tamoxifen (9-11). On the contrary these studies suggest a possible benefit of soy foods including a reduced risk of cancer recurrence, reduced risk of death and increased efficacy of tamoxifen.

A: Soy foods appear to offer modest protection against breast cancer and breast cancer recurrence in breast cancer survivors.

 

Q: Are fermented soy foods healthier?

There is a popular argument that fermented soy foods are healthier than non-fermented soy foods. Both fermented (e.g. tempeh, miso) and non-fermented (e.g. tofu, soy beans, soy milk) are consumed traditionally in Asian cultures.

There is little evidence to suggest that fermented foods are healthier than non-fermented soy foods. On the contrary fermented soy foods may contain higher levels of sodium and nitrates and lower levels of nutrients than non-fermented soy foods. Accordingly, a review of 37 observational studies found that non-fermented soy foods protected against gastric cancer, whereas fermented soy foods increased cancer risk (12).

A: Non-fermented soy foods may be healthier than fermented soy foods.

 

 

References:

1. Messina M, Redmond G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid. 2006 Mar;16(3):249-58.

2. Sathyapalan T, Manuchehri AM, Thatcher NJ, Rigby AS, Chapman T, Kilpatrick ES, Atkin SL. The Effect of Soy Phytoestrogen Supplementation on Thyroid Status and Cardiovascular Risk Markers in Patients with Subclinical Hypothyroidism: A Randomized, Double-Blind, Crossover Study. J Clin Endocrinol Metab. 2011 Feb 16.

3. Myung SK, Ju W, Choi HJ, Kim SC; Korean Meta-Analysis (KORMA) Study Group. Soy intake and risk of endocrine-related gynaecological cancer: a meta-analysis. BJOG. 2009 Dec;116(13):1697-705.

4. Hamilton-Reeves JM, Vazquez G, Duval SJ, Phipps WR, Kurzer MS, Messina MJ. Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Fertil Steril. 2010 Aug;94(3):997-1007.

5. Hamilton-Reeves JM, Vazquez G, Duval SJ, Phipps WR, Kurzer MS, Messina MJ. Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Fertil Steril. 2010 Aug;94(3):997-1007.

6. Trock BJ, Hilakivi-Clarke L, Clarke R. Metaanalysis of soy intake and breast cancer risk. J Natl Cancer Inst. 2006;98(7):459-471.

7. Wu AH, Yu MC, Tseng CC, Pike MC. Epidemiology of soy exposures and breast cancer risk. Br J Cancer. 2008;98(1):9-14.

8. Lee SA, Shu XO, Li H, et al. Adolescent and adult soy food intake and breast cancer risk: results from the Shanghai Women’s Health Study. Am J Clin Nutr. 2009;89(6):1920-1926.

9. Shu XO, Zheng Y, Cai H, et al. Soy food intake and breast cancer survival. JAMA 2009;302(22):2437-43.

10. Guha N, Kwan ML, Quesenberry CP, Jr., et al. Soy isoflavones and risk of cancer recurrence in a cohort of breast cancer survivors: the Life After Cancer Epidemiology study. Breast Cancer Res Treat 2009;118(2):395-405.

11. Caan BJ, Natarajan L, Parker BA, Gold EB, Thomson CA, Newman VA, Rock CL, Pu M, Al-Delaimy WK, Pierce JP. Soy Food Consumption and Breast Cancer Prognosis. Cancer Epidemiol Biomarkers Prev. 2011 Feb 25. [Epub ahead of print]

12. Kim J, Kang M, Lee JS, Inoue M, Sasazuki S, Tsugane S. Fermented and non-fermented soy food consumption and gastric cancer in Japanese and Korean populations: a meta-analysis of observational studies. Cancer Sci. 2011 Jan;102(1):231-44.