Soy doesn’t have the best reputation in the western world. Whilst foods like tofu and edamame are embraced in many Asian cultures. It seems to be quite a demonized food in countries like Australia and the United States.
First things first, I work with predominantly plant-based and vegan people. Mostly assisting them with optimizing their diet for athletic performance and muscle gain.
I am a huge advocate for soy as a predominant source of protein in this situation. Soy has what we would consider a ‘complete’ amino acid profile. Products made from soy are usually very efficient sources of plant-based protein in that they have a good chunk of protein per 100 calories.
Soy also has the best leucine content out of all plant-based protein sources. Leucine is an amino acid well known for its role in triggering muscle protein synthesis.
So soy is basically the best option that vegan and plant-based exercisers and athletes have to reach their protein targets, build muscle, and recover well.
Even for plant-based people who are not athletes, soy can be a great way to meet protein requirements for health. It also comes in many forms and can be utilized in so many different types of dishes.
From tofu in a stir fry to textured vegetable protein (TVP) in a shepherd’s pie. Even just snacking on some salted edamame pods.
I think it is also a great addition to the diets of non-vegans to increase plant food variety, which we know is super important for gut health.
But I cannot remember the last time I went a full week without answering the following question,
“But doesn’t soy mess with your hormones?”
So considering that I am a dietitian recommending soy foods to pretty much any client that isn’t allergic, you can probably guess what conclusion I have come to on this. But it is a little bit more nuanced than that.
The Theory on Soy & Estrogen
Let’s take a step back and go over the theory. Where did the connection between soy and hormones, specifically, estrogen come from?
Soy & Phytoestrogens
Soy contains phytoestrogens, which is an isoflavone that is very similar to estrogen, the female sex hormone.
These phytoestrogens can actually act like estrogen in the body but with a much weaker effect. But this is what has sparked the most fear around soy products in general.
However, phytoestrogens don’t always mimic estrogen. In fact, they can have different effects on different tissues of the body and different effects on different people.
In pre-menopausal women, phytoestrogens can actually have an estrogen-blocking action. On the other hand, in post-menopausal women when estrogen levels have reduced, phytoestrogens can have the opposite effect.
In regards to different tissues in the body, phytoestrogens have been linked to a reduced risk of breast cancer. This may be due to the fact that soy has an estrogen-blocking effect on breast tissue.
A 2016 review on soy and health outcomes, warned against equating phytoestrogen to estrogen. The authors state that whilst the molecules are similar, they also have a vast array of differences, particularly in their role in the human body.
Soy & Feminizing Effects In Men
In 2009, Men’s Health Magazine published an article titled “Is this the most dangerous food for men?”.
This article spread like wildfire amongst the fitness community. Much of the article focuses on a case report published the year before of an older man who had developed feminizing effects, including gynecomastia (an increase in breast tissue), allegedly as a result of his soy intake.
Firstly, anecdotal evidence or one case report is hardly evidence to suggest that soy has feminizing effects in men. But it was definitely an attention-grabbing story.
Secondly, this man disclosed that he was drinking over 3L of soy milk per day. Even for me, who is on the higher end of soy consumption as a vegan powerlifter, that is far more than my normal intake.
So clearly, he was very much an outlier and not reflective of a moderate or usual soy intake.
The article also went on to cite a small pilot study that found soy intake was associated with lower sperm counts in men attending a fertility clinic. However, the same group of researchers would go on to refute the findings of this study with further research.
After going down the rabbit hole that is research on soy intake and isoflavones, there are cases where I would suggest that soy intake be moderated.
But it is absolutely clear with the current research that soy is not the monster it has been made out to be.
Soy Intake In Men
Soy, Estrogen Levels & Fertility
When it comes to soy intake and its impact on hormone levels, it seems that men are the least affected.
A 2010 review found that the data from nine different clinical studies showed that isoflavone exposure from eating soy did not affect circulating estrogen levels in men.
Specifically, in regards to fertility, a randomized control trial also from 2010 investigated the effects of soy isoflavones on semen quality in 32 adult men. Participants in the study consumed milk protein isolate, low-isoflavone soy protein isolate, and high-isoflavone soy protein isolate. After each intervention, urine and semen samples were collected. There was no meaningful difference was found between groups.
Current research also suggests that soy intake amongst men is not correlated with lower rates of fertility.
Soy & Prostate Cancer
Isoflavones, specifically genistein and daidzein which are found in soy have been associated with lower rates of prostate cancer. This was found in a meta-analysis of over 30 case-control and cohort studies from the US, Japan, Europe, and China.
This is likely due to the fact that soy isoflavones collect in prostate tissue and act as a weak estrogen and exert a protective effect against the development of prostate cancer.
So in general, it appears men can consume soy products without an effect on hormone levels or fertility. This means that an increase in breast tissue or other negative health effects shouldn’t be a concern for men wanting to include some more soy in their diet. In fact, including soy foods regularly may be protective against prostate cancer.
Soy Intake In Women
Soy & Breast Cancer
The largest and most detailed study to investigate soy intake and the risk of breast cancer is definitely The Shanghai Women’s Health Study.
This study followed over 70,000 Chinese women for over seven years.
A key finding was that the women who ate the most soy had a 59% lower risk of premenopausal breast cancer compared to those who ate the lowest amounts of soy.
Similar to prostate cancer, it is believed that the phytoestrogens in soy act on the breast tissue in a way that reduces the risk of cancer developing in the area.
It was once thought that even moderate soy intake would actually increase the risk of breast cancer. This is because high estrogen levels have been associated with an increased risk of developing this type of cancer.
However, the phytoestrogens in soy do not appear to have this effect.
Fertility & Pregnancy
All reproductive tissues in the body can theoretically be acted upon by phytoestrogens. This includes the ovaries and uterus.
Increased estrogen signaling in these tissues may disrupt the usual hormonal cascade and the menstrual cycle. Therefore, it is possible that phytoestrogens could impact fertility.
However, we also know that whilst phytoestrogens and estrogen are similar, they are not the same. Phytoestrogens have a much weaker effect on the body.
Soy foods appear to increase the length of the menstrual cycle although ovulation is not prevented, it is simply delayed by one day. Interestingly, this minor effect on menstrual cycle length could be a factor in the reduction of breast cancer risk.
In regard to fertility research, it seems that moderate intakes of soy may be beneficial to fertility, whilst a very high intake may reduce fertility in some people.
One of the most significant longitudinal studies referenced throughout fertility research is the Nurses’ Health Study II (NHS II).
Researchers in this study collected data on over 100,000 female nurses between the ages of 25 and 42. When analyzing contraception rates, it was found that a higher intake of animal proteins increased the likelihood of infertility. As such, it was suggested that women who are conscious of their reproductive health include some plant-based proteins in their diet, including soy.
Furthermore, another study found that fertilization rates, clinical pregnancies, and live births were all higher among women who consumed more soyfoods. This study was specifically based on women undergoing assisted reproductive therapy.
Overall, the data from studies investigating soy isoflavones and fertility rates, suggests that moderate soy intakes similar to a traditional Asian diet, would likely have very little impact on reproductive health and it may even have a small positive impact.
On the other hand, physicians at SUNY Downstate Medical Center submitted a 2008 clinical case report when 3 women (aged 35–56 y) were treated for a similar suite of symptoms, including abnormal uterine bleeding, endometrial pathology, and dysmenorrhea.
In all 3 cases, symptoms improved after soy was withdrawn from their diet, suggesting that a high intake of soy isoflavones can compromise female reproductive health.
Isoflavone intake was not quantified but was estimated to exceed >100 mg isoflavones. In food terms, that is around 4-5 servings of soy foods per day depending on the types of soy foods consumed.
Each gram of soy protein in soybeans and traditional soy foods is associated with approximately 3.5 mg of isoflavones. In more refined products such as processed mock meat, as much as 80-90% of the isoflavone content can be lost in processing.
Therefore, if someone is looking to fall pregnant, they likely should avoid very high intakes of soy products and limit it to 2-4 servings per day.
Effects Of Soy In Menopausal & Post-menopausal Women
The North American Menopause Society concluded that soy isoflavones do not increase the risk of breast or endometrial cancer.
Actually, soy intake may be particularly beneficial for those who are going through menopause and those who are post-menopausal.
The evidence surrounding soy intake and its impact on hot flushes in menopause isn’t conclusive. However, the research we do currently have seems rather positive.
A review of 43 randomized controlled trials examined the effects of soy phytoestrogens on hot flushes and night sweats in perimenopausal and postmenopausal women.
Four trials found that 30 mg or greater of isoflavones in supplement form consistently reduced the frequency of hot flashes.
Other trials using dietary soy or soy extracts suggested a reduced frequency and severity of hot flashes and night sweats when compared with placebo, but these trials have been criticized for a potential placebo effect.
Due to declining estrogen levels, post-menopausal women have significantly higher calcium requirements. This is to try to maintain good bone health as a decline in estrogen can lead to significant bone loss in the years following menopause.
Since estrogen therapy reduces postmenopausal bone loss, it has been speculated that soyfoods may be beneficial for bone health due to containing phytoestrogens.
In one cross-over study, over a 50-day period, isoflavones (105 mg/day) increased bone calcium content by 7.6%, approximately half the increase noted in response to risedronate, a treatment commonly used in cases of osteoporosis.
Since postmenopausal women do not have to worry about the potential infertility issues that may arise with an intake of over 100mg of soy isoflavone per day, it could be a great way to support bone health.
Although getting adequate calcium and vitamin D should still be the first priority.
Soy & Thyroid Function
For those with a happy, healthy functioning thyroid, there is no cause for concern in regards to soy consumption.
However, people with hypothyroidism and who are taking thyroid medication may need to be mindful of their soy consumption as it may interfere with the thyroid medication.
Generally, it is recommended to avoid soy foods 3-4 hours around the time of your medication and to not abruptly increase overall soy intake.
This is very much on a case-by-case basis though. If you are on thyroid medication, you should consult with your doctor for a more personalized approach.
The Bottom Line
Soy has definitely developed a pretty negative reputation over the years. But research on soy and health outcomes tells a pretty clear picture.
Moderate soy intakes, given that you are not allergic, can provide positive health benefits to men and women of all ages.
However, those looking to become pregnant should likely be mindful of not having too much soy on a daily basis.
- >100mg of isoflavones from soy may interfere with female fertility
- Feminising effects in men have not been found in the research
- Phytoestrogens in soy may be protective against breast and prostate cancer
- In post mensopausal women soy intake may reduce hot flashes and assist with maintaining bone health
- For those taking thyroid medication and that wish to consume soy products should consult with their doctor first for individualised advice