Soy may be the most controversial plant based protein, constantly competing for first place with gluten. Depending on who you ask, soy is either regarded as a heart healthy, plant based source of protein with healthy fats and minerals that everyone needs, or a carcinogen that leads to infertility. Discussion surrounding soy can become heated and complex, so I decided to investigate the connection between soy intake and breast cancer and fertility.
Why is Soy Associated With Estrogen
The theories that soy may affect fertility and breast cancer risk base themselves off of the high phytoestrogen content of soy. Phytoestrogens are natural, plant based molecules that exhibit similar properties to estrogen. Soy contains a high amount of a specific type of phytoestrogen called isoflavones. Phytoestrogens are very similar in shape to the hormone estrogen which gives them the ability to bind to estrogen receptors, and prevent estrogen from functioning at that receptor. The potential of this interference of natural estrogen signaling raises the question of whether or not consuming high quantities of soy may be beneficial or harmful for our health.
Note: If a phytoestrogen molecule blocks an estrogen receptor, it does NOT act in the same way estrogen does. Eating phytoestrogens does not essentially increase the estrogen content in the body. This is a common misconception.
Breast Cancer Risk and Soy
I have heard from dozens of female clients over the years that they had decided to cut back on their soy intake for fear that it would increase their chances of developing breast cancer. Some of them, in fact, had been advised by their doctors to cut back on their soy intake for the same reason. One client in particular was actually told by her oncologist that she probably had developed breast cancer because she ate too much tofu. Although I am a proponent of always following your doctor’s advice, this advice may have been misguided. High soy intake has actually been associated with decreasing breast cancer risk, along with as osteoporosis, some cardiovascular disease, and endometrial cancer. There is little to no evidence that soy causes breast cancer.
Ovarian Function/Female Fertility and Soy
One can imagine that interference with estrogen signaling could alter ovulation, and the potential effect that soy could have on estrogen signaling has been a recent hot topic for women of reproductive age. Finding a link between female fertility and soy has proven to be slightly more difficult than with breast cancer. There are a few human research studies that have tried to investigate the effects of soy on ovarian function, and so far, the results have remained to be inconclusive. The data is weak and conflicting, but at this time, a moderate soy intake does not appear to be associated with decreased fertility or impaired ovarian function. Soyfoods appear to increase the length of the menstrual cycle in some women, and although ovulation is not prevented, it is simply delayed by one day. It is hypothesized that this increase in menstrual cycle by one day is a contributing factor to the decreased risk of breast cancer associated with soy intake.
Interestingly, one research study showed a very strong correlation between soy intake and menopausal symptoms. The study showed that soy isoflavones may significantly decrease the frequency of hot flashes in menopausal women.
Male Fertility and Soy
Similar to the female fertility studies, there is not a lot of conclusive evidence supporting a link between soy intake and decreased male fertility. One study found that very modest soy consumption was associated with lower sperm concentration, however, the actual sperm count did not decrease and fertility appeared unaffected.
There have also been data from two research studies floating around that soy intake can cause male feminization and a drop in testosterone. In both studies, the subjects were required to eat nine times the typical daily intake of soy and neither showed any actual increase in blood estrogen or a significant drop in testosterone. Needless to say, it is very unlikely that soy causes feminization in men.
How Much Soy Is Recommended to Eat
There are no formal recommendations for soy intake, however, the body of research available showing positive health effects suggests that the benefits are associated with an intake of around 1-2 servings per day. This is equivalent to a cup of tofu or a cup of soy milk. There are several reasons as to why someone would want to eat soy. It is a relatively affordable plant based protein, also high in omega 3s, calcium, and iron. Ideally, if you choose to incorporate soy into your diet, it does not replace other healthy foods, but instead becomes slowly integrated into an overall healthy diet. Obviously, if you are allergic to soy, it is not recommended as part of your diet.
Take-Aways
Modest soy intake is associated with a decrease in breast cancer risk.
The research looking at the effects of soy on fertility is still pretty weak. More research should be done to make any definitive recommendations.
If you are concerned about the effects that soy could have on your health, ask a registered dietitian in your area to give you more individualistic advice.
If you want more information on how soy is related to breast cancer and fertility, you can refer to the research studies below.
Resources:
Chavarro, Jorge E. et al. “Soy Food and Isoflavone Intake in Relation to Semen Quality Parameters among Men from an Infertility Clinic.” Human Reproduction (Oxford, England) 23.11 (2008): 2584–2590. PMC. Web. 15 May 2018.
Jefferson, Wendy N. “Adult Ovarian Function Can Be Affected by High Levels of Soy.” The Journal of Nutrition 140.12 (2010): 2322S–2325S. PMC. Web. 15 May 2018.
Messina, Mark. “Soy and Health Update: Evaluation of the Clinical and Epidemiologic Literature.” Nutrients 8.12 (2016): 754. PMC. Web. 15 May 2018.