Link to study

<sup>References:
1. Yang WS, Va P, Wong MY, et al. Soy intake is associated with lower lung cancer risk: results from a meta-analysis of epidemiologic studies. Am J Clin Nutr. 2011 Nov. 9 [Epub ahead of print]</sup>

Katy recently joined WhiteWave Foods after spending more than 25 years at Kraft Foods. At Kraft she provided leadership to global business partners for the development of nutrition strategies as well as guiding the development of nutrition and health positions and policies. During her career she has led the applied nutrition, nutrition strategy and nutrition research groups, and done basic research, product development and R&D strategy. She has also worked as a clinical dietitian and in private practice. She earned her bachelor’s degree in food and nutrition from the University of Vermont and her masters in human nutrition from New York Medical College. Katy is a registered dietitian and the co-author of a U.S. patent.

Over the past several years scientists have examined the effect of soyfoods and isoflavone supplements on indicators or markers of breast cancer risk. The results of these studies show that unlike hormone therapy, neither isoflavones nor soyfoods are harmful.¹ Furthermore, studies conducted in China and the United States, have found that breast cancer patients who regularly consume soy have an improved prognosis, both in terms of recurrence and survival, in comparison to those who consume soy infrequently.^2,3,4

Despite the encouraging results, the existing evidence is not sufficient to provide you with a definitive answer to your question. Having said this, many researchers in the field, after considering the most recent study findings, have now concluded that whole soyfoods can be part of the diet of women with a history of breast cancer.

<sup>References
1. Messina MJ, Wood CE. Soy isoflavones, estrogen therapy, and breast cancer risk: analysis and commentary. Nutr J 2008; 3; 7; 17.
2. Guha N 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: 395-405.
3. Shu XO et al. Soyfood intake and breast cancer survival. JAMA 2009; 302(22): 2437-2443.
4. Xiao S et al. Presented at the 102nd Annual Meeting of the American Association for Cancer Research, Orlando, FL, April 2-6, 2011.</sup>

The impact of soyfoods in thyroid function has been studied for almost 80 years. Much of the interest in the soy and thyroid connection is because soyfoods contain high amounts of isoflavones, a group of naturally-occurring plant chemicals. Isoflavones are being studied for their health benefits in a wide range of areas. Nevertheless, some types of research suggest isoflavones may adversely affect thyroid function in certain individuals.

Extensive clinical (human) research shows conclusively that in healthy individuals with normal thyroid function, neither soyfoods nor isoflavone supplements impact thyroid function.¹ This research includes studies that have been conducted for as long as three years and involved consumption of large amounts of isoflavones.² It is also clear that in hypothyroid patients taking synthetic thyroid hormone, soyfoods are not contraindicated.

What is less clear, however, is the impact of soyfoods on thyroid function in people with Hashimoto’s disease or subclinical hypothyroidism; the latter refers to people with normal thyroid hormone levels, but who are at risk of developing overt hypothyroidism. This research question has yet to be definitively addressed but it is conceivable that in sensitive individuals, soyfoods could adversely impact the thyroid. If that is the case, then one should take thyroxine several hours after (or before) ingesting soy foods.

Therefore, until more data are available, the best advice for people with Hashimoto’s is to work closely with an endocrinologist. Perhaps small amounts of soy can be gradually added to the diet, especially in the case of normal thyroid function tests while not receiving thyroxine. If a person is receiving thryroxine, the strategy noted above may allow consumption of soy foods. However, until definitive research specifically addresses this condition, it is best to take a cautious approach.

<sup>References
1. Messina M. Redmon G. effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thryoid 2006; 16: 249-58.
2. Bitto A et al. Genistein aglycone does not affect thyroid function: results form a three-year, randomized, double-blind, placebo-controlled trial. J Clin Endocrinol. Metab 2010; 95: 3067-72.</sup>

<sup>References
1. Badger T et al. The health implications of soy infant formula. Am J Clin Nutr 2009; 89(suppl):1668S–72S.
2. Bhatia J, Greer F, and the Committee on Nutrition. Use of Soy Protein-Based Formulas in Infant Feeding. Pediatrics. 2008; 121:1062-1068.</sup>

There is also intriguing but still speculative data indicating that soy may be useful for men with prostate cancer. Several studies have found that isoflavones, naturally-occurring chemicals in soybeans, slow the rise in prostate specific antigen (PSA) levels in prostate cancer patients who have been unsuccessfully treated for their disease^4,5,6,7 although not all studies show this to be the case.⁸ PSA is the most common clinical test for the detection of prostate cancer and is a measure by which treatment efficacy can be assessed. In addition, recent research showed that in prostate cancer patients, the main soybean isoflavone, genistein, inhibited the activity of an enzyme found in the prostate that enables cancer cells to spread to other tissues.⁹ Genistein also enhances the ability of radiation^10,11 and chemotherapeutic agents¹² to kill prostate cancer cells in test tubes. Finally, very preliminary, just-published research found that isoflavones reduced the side effects associated with radiation treatment for prostate cancer. These side effects include urinary leakage, cramping, and erectile dysfunction.¹³

Overall, there are encouraging data suggesting that soyfoods can be helpful to prostate cancer patients. However, none of the studies in this area are definitive. Therefore, no conclusions can be made at this time. Nevertheless, because soyfoods are very nutritious, they certainly can be part of an overall healthy diet for all men.

<sup>References
1. American Cancer Society. Cancer Facts and Figures 2010. Atlanta, GA, 2010.
2. Pisani P, Bray F, Parkin DM. Estimates of the world-wide prevalence of cancer for 25 sites in the adult population. Int. J. Cancer. 2002, 97, 72-81.
3. Yan L, Spitznagel EL. Soy consumption and prostate cancer risk in men: a revisit of a meta-analysis. Am. J. Clin. Nutr. 2009, 89, 1155-63.
4. Messina M, Kucuk O, Lampe JW. An overview of the health effects of isoflavones with an emphasis on prostate cancer risk and prostate-specific antigen levels. J. AOAC Int. 2006, 89, 1121-34.
5. Pendleton JM, Tan WW, Anai S, Chang M, Hou W, Shiverick KT, Rosser CJ. Phase II Trial of Isoflavone in prostate specific antigen recurrent prostate cancer after previous local therapy. BMC Cancer. 2008, 8, 132.
6. Kwan W, Duncan G, Van Patten C, Liu M, Lim J. A phase II trial of a soy beverage for subjects without clinical disease with rising prostate-specific antigen after radical radiation for prostate cancer. Nutr. Cancer. 2010, 62, 198-207.
7. Ide H, Tokiwa S, Sakamaki K, Nishio K, Isotani S, Muto S, Hama T, Masuda H, Horie S. Combined inhibitory effects of soy isoflavones and curcumin on the production of prostate-specific antigen. Prostate. 2010, 70, 1127-33.
8. Devere White RW, Tsodikov A, Stapp EC, Soares SE, Fujii H, Hackman RM. Effects of a high dose, aglycone-rich soy extract on prostate-specific antigen and serum isoflavone concentrations in men with localized prostate cancer. Nutr. Cancer. 2010, 62, 1036-43.
9. Xu L, Ding Y, Catalona WJ, Yang XJ, Anderson WF, Jovanovic B, Wellman K, Killmer J, Huang X, et al. MEK4 function, genistein treatment, and invasion of human prostate cancer cells. J. Natl. Cancer Inst. 2009, 101, 1141-55.
10. Raffoul JJ, Wang Y, Kucuk O, Forman JD, Sarkar FH, Hillman GG. Genistein inhibits radiation-induced activation of NF-kappaB in prostate cancer cells promoting apoptosis and G2/M cell cycle arrest. BMC Cancer. 2006, 6, 107.
11. Raffoul JJ, Banerjee S, Che M, Knoll ZE, Doerge DR, Abrams J, Kucuk O, Sarkar FH, Hillman GG. Soy isoflavones enhance radiotherapy in a metastatic prostate cancer model. Int. J. Cancer. 2007, 120, 2491-8.
12. Li Y, Ahmed F, Ali S, Philip PA, Kucuk O, Sarkar FH. Inactivation of Nuclear Factor {kappa}B by Soy Isoflavone Genistein Contributes to Increased Apoptosis Induced by Chemotherapeutic Agents in Human Cancer Cells. Cancer Res. 2005, 65, 6934-42.
13. Ahmad IU, Forman JD, Sarkar FH, Hillman GG, Heath E, Vaishampayan U, Cher ML, Andic F, Rossi PJ, et al. Soy isoflavones in conjunction with radiation therapy in patients with prostate cancer. Nutr. Cancer. 2010, 62, 996-1000.</sup>

A recently published case report described a 19-year old male who developed a loss of libido, erectile dysfunction and low levels of biologically active testosterone. ¹ These conditions were attributed to the dietary habits of the subject. More specifically, the male in question indicated he was a vegan who consumed large amounts of a variety of soy products. The authors of this report estimated the isoflavone intake from soy was 360 milligrams per day. After one year of consuming a non-vegan diet devoid of soy products the conditions resolved. As a result, the authors concluded that soy caused the drop in testosterone, loss of libido and erectile dysfunction.

To put this case report into proper perspective, it is important to recognize that 360 milligrams isoflavones is the amount provided by approximately 14 to 20 servings of soyfoods. Obviously, this individual was consuming an excessive and inappropriate amount of soy. Older Japanese adults consume on average about two servings per day. ²

Soy experts generally recommend consuming two to four servings of soy daily to take advantage of the nutritional attributes of these foods and to derive their proposed health benefits. ³ Exceeding four servings represents an unbalanced diet—one that places too much emphasis on a single food—and that doesn’t adhere to the important dietetic principles of variety and moderation.

A wealth of clinical data (>30 studies) show that an intake of between 50 and 150 mg per day of isoflavones from soyfoods or supplements doesn’t lower testosterone levels. ⁴ Furthermore, the evidence shows that similar amounts of isoflavones do not affect estrogen levels in men. ⁵ It is possible that the excessive amount of isoflavones caused the problems experienced by this young man, although his unbalanced diet in and of itself may have adversely affected the synthesis of testosterone.

Consuming excessive amounts of any single food or nutrient can cause problems. In many cases, the safe upper limit for a nutrient is only 3 to 4 times the recommended intake. The lesson to be learned from the problems developed by the male described in the case report is not that soyfoods are harmful, but that consuming an inappropriate and unbalanced diet can be.

<sup>References
1. Siepmann T, Roofeh J, Kiefer FW, Edelson DG. Hypogonadism and erectile dysfunction associated with soy product consumption. Nutrition. 2011.
2. Messina M, Nagata C, Wu AH. Estimated Asian adult soy protein and isoflavone intakes. Nutr. Cancer. 2006, 55, 1-12.
3. Messina M. Investigating the optimal soy protein and isoflavone intakes for women: a perspective. Womens Health (Lond Engl). 2008, 4, 337-56.
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, 94, 997-1007.
5. Messina M. Soybean isoflavone exposure does not have feminizing effects on men: a critical examination of the clinical evidence. Fertil. Steril. 2010, 93, 2095-104.</sup>

A second concern is that soyfoods may interfere with the absorption of synthetic thyroid hormone, which is taken by hypothyroid patients. Many foods, herbs and drugs are thought to interfere with the absorption of thyroid hormone. However, even if soy is one of these, there is no need for hypothyroid patients to avoid soyfoods. The key is to maintain consistency in medication administration;³ that is, to be consistent in taking the medication in either a fasted or fed state and with or without soyfoods.

There is also concern that soy may be harmful to subclinical hypothyroid patients. Individuals with subclinical hypothyroidism have normal levels of the two main thyroid hormones, but elevated levels of a hormone that controls the synthesis of thyroid hormones. Over time, anywhere from 2 to 5 percent of patients with this condition will progress to overt hypothyroidism per year and require medication.⁴ Only one small study has examined the impact of soy on thyroid function in subclinical hypothyroid patients. This preliminary study found that about 10% of the patients converted to overt hypothyroidism in response to soy protein containing isoflavones.⁵ However, in the group overall, there were dramatic reductions in inflammation, insulin sensitivity and blood pressure. Thus, in 90% of the subclinical hypothyroid patients, soy actually reduced risk factors for cardiovascular disease and diabetes.

In conclusion, even the consumption of large amounts of soy for several years does not impact thyroid function in healthy people. Further, there is no reason for hypothyroid patients to avoid soyfoods. Although a small percentage of subclinical hypothyroid patients may be adversely affected by soy, in the vast majority of these patients, soy appears to be extremely beneficial.

<sup>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, 16, 249-58.
2. Bitto A, Polito F, Atteritano M, Altavilla D, Mazzaferro S, Marini H, Adamo EB, D’Anna R, Granese R, et al. Genistein aglycone does not affect thyroid function: results from a three-year, randomized, double-blind, placebo-controlled trial. J. Clin. Endocrinol. Metab. 2010, 95, 3067-72.
3. Zeitler P, Solberg P. Food and levothyroxine administration in infants and children. J. Pediatr. 2010, 157, 13-14 e1.
4. Huber G, Staub JJ, Meier C, Mitrache C, Guglielmetti M, Huber P, Braverman LE. Prospective study of the spontaneous course of subclinical hypothyroidism: prognostic value of thyrotropin, thyroid reserve, and thyroid antibodies. J. Clin. Endocrinol. Metab. 2002, 87, 3221-6.
5. 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.</sup>

Link to study

^{References:
1.Jenkins DJ et al. Effect of a Dietary Portfolio of Cholesterol-Lowering Foods Given at 2 Levels of Intensity of Dietary
Advice on Serum Lipids in Hyperlipidemia Epidemiology of soy exposures and breast cancer risk. JAMA 2011; 306(8): 831-839.}