Is Soy, a Poison?

These are the papers which people talk about mostly.

  1. Randomised Control Trial (RCT) on soya and patients with subclinical hypothyroidism

First, this is a relatively small study (48 participants completed the study) and the mean age of participants was 58 and about 85% of them were women. So this study was done on elderly female patients with subclinical hypothyroidism. Not the kind of audience that is concerned with this debate. Also, it’s important to note that participants in this study took manufactured soya supplements and not soya chunks or tofu. 

People keep mentioning that the participants developed hypothyroidism after taking just 30gm of soy protein. This is not right. The paper clearly mentions that all participants got 30 gm of soy protein, the difference was in how much phytoestrogens they got (2gm vs 16 gm). The study had nothing to do with soy protein.

Further, they carefully omits an important sentence from the abstract. I’ll add the whole paragraph for your reference:

“There is a 3-fold increased risk of developing overt hypothyroidism with dietary supplementation of 16 mg soy phytoestrogens with subclinical hypothyroidism. However, 16-mg soy phytoestrogen supplementation significantly reduces insulin resistance, hs-CRP, and blood pressure in these patients.

Read the paper here for yourself: https://academic.oup.com/jcem/article/96/5/1442/2833679

2. 1991 study of 30gm soya beans in 37 healthy participants

First, this study did not have a control group. A control group is particularly important in studies with just 37 participants across three arms. Second, they have given the participants around 30 gms of soya bean and not 30 gm of soy protein. For reference, 30 gm of soya bean has around 8-10 gm of soy protein. Seems too low to make a difference right?

Some lines from another research paper that critiques this 1991 study:

“Furthermore, these findings appear to be biologically implausible since it is difficult to comprehend how such small amounts of soy protein (approximately 8 g)  and isoflavones (approximately 30 mg) could result in marked antithyroid and goitrogenic effects in a population [Refering to Japanese population who have high soy diet and were the participants in the original 1991 study] that regularly consumes soy but does not have a high incidence of goitre.”

Link to the research paper which critiques the study detail: https://www.liebertpub.com/doi/abs/10.1089/thy.2006.16.249 

Link to original 1991 study: https://pubmed.ncbi.nlm.nih.gov/1868922/

(The original paper is in Japanese and only the abstract is available in English)

3. 2002 study on beef vs soya chunks in muscle cross-sectional area

Generally, people very cleverly cherry-pick and misrepresents data on a 50% increase in muscle cross-sectional area but omit to tell us that the difference was statistically non-significant. Statistically insignificant means that the difference was very likely a coincidence and not because of the interventions that are beef vs soya. They show you one data point on muscle cross-section, let me show you the results for all data points. Straight from this paper:

“Body composition, resting energy expenditure, and concentrations of muscle creatine, phosphocreatine, and total creatine did not differ significantly between groups [Beef vs Soya Chunks] or change over time.”

What was the conclusion of the researchers who wrote this paper? Read it for yourself:

“These results indicate that the predominant source of dietary protein (beef or soy), when consumed by older men in amounts that averaged 1.1 g protein · kg−1 · d−1, did not influence the extent of muscle hypertrophy during 12 wk of resistive training.”

Don’t take my word for it, read the paper yourself: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2556250/pdf/nihms58556.pdf

4. Soy vs milk beverage post-workout

In this 2007 study, there were three groups. One took 500 ml of skimmed milk post-workout, the other took 500 ml of soy protein drink and the last group was the control. As compared to the control, the soy and milk group had better muscle development. Milk however was better than soy in some measured parameters for muscle growth. 

Milk may be better than commercially available soy drinks. Who is surprised by these results and how is it relevant to the debate on hand? People list a couple of more papers that validate these results. He also adds that there is an equal number of studies that show that soy is equivalent to whey or milk-derived protein sources. Not sure how this supports their argument.

Read the paper here: https://academic.oup.com/ajcn/article/86/2/373/4633050?login=true#111196238

5. Isolated soy protein-based formulas for infants

People use this generally: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3443604/

This is the 2008 recommendation by the American Academy of Paediatrics:

https://publications.aap.org/pediatrics/article/121/5/1062/73488/Use-of-Soy-Protein-Based-Formulas-in-Infant

“In term infants, although isolated soy protein-based formulas may be used to provide nutrition for normal growth and development, there are few indications for their use in place of cow milk-based formula. These indications include (a) for infants with galactosemia and hereditary lactase deficiency (rare) and (b) in situations in which a vegetarian diet is preferred.”

We don’t disagree with this. We never recommended soy-based infant feeding formula to our 1-year-old clients.

6. Meat replacement with soy on male hormone 

“Replacement of meat protein with soybean protein, like tofu, may have a minor effect on biologically-active sex hormones, which could influence prostate cancer risk”

I could go on about how the sample size was just 45, the difference was marginal, no significant difference was seen in many other sex hormones and so on. But I’d do something better. They cite one RCT, I’ll show you the latest 2020 summary of 38 RCTs on soy and testosterone and estrogen level in men. The researchers who did this meta-analysis of 38 clinical trials say:

“The results of this meta-analysis confirm the findings of a meta-analysis published in 2010 that found neither soy nor isoflavone intake affects total or bioavailable circulating testosterone concentrations in men.”

One single RCT: https://www.cambridge.org/core/journals/british-journal-of-nutrition/article/effects-of-replacing-meat-with-soyabean-in-the-diet-on-sex-hormone-concentrations-in-healthy-adult-males/50C464775151BA6ECBF767221F763AFF

Our citation of a meta-analysis of 38 RCT: https://www.sciencedirect.com/science/article/pii/S0890623820302926 

This is a good video essay on the history and the origins of the myth about soy and testosterone: https://youtu.be/C8dfiDeJeDU 

7. Case studies

Case studies are as good as anecdotal evidence. My neighbour eats 10 vada pavs every day and deadlifts 200kg. Would this convince you to start eating vada pavs for better gym performance?

In the scientific community, case studies only have value for rare diseases and unknown disorders where proper RCTs are not done or are not possible. When evidence from 38 trials with thousands of participants on the impact of soy is available, it’s foolish to cite or consider case studies. 


People mention that soy has a terrible track record when it comes to scientific research. This is not true and more importantly, this is unscientific. There are hundreds of soy related research papers. If you cite 3 against it, I can cite 30 for it. It’s a bottomless pit. 

This is why, in scientific literature, there are systematic reviews and meta-analyses that aggregate all available research studies on soy and come to a conclusion. Think of it as an accounting ledger, you try to add up all studies and see if the existing evidence points towards benefit or harm. 

So what do these systematic reviews and meta-analyses say? Let’s see. 

A Meta-analysis of 18 studies: 

“Soy supplementation has no effect on the thyroid hormones and only very modestly raises TSH levels, the clinical significance, if any, of the rise in TSH is unclear.”

Link: https://www.nature.com/articles/s41598-019-40647-x

A review of 14 trials:

“The preponderance of evidence from clinical trials involving healthy adult men and women indicates that neither soy protein nor isoflavones adversely affect thyroid function”.

Link: https://www.liebertpub.com/doi/abs/10.1089/thy.2006.16.249

A meta-analysis of 15 placebo-controlled treatment groups

“The results of this meta-analysis suggest that neither soy foods nor isoflavone supplements alter measures of bioavailable testosterone concentrations in men”.

Link: https://www.sciencedirect.com/science/article/abs/pii/S0015028209009662 

At PFC, we recommend using soy products like soya chunks as one of the many protein sources that you should have in your diet. This is advice is specifically relevant for folks who are vegetarian or vegan and don’t have access to animal protein sources like eggs and chicken. 

If a bowl of soya chunks with Maggi masala is the difference between you meeting your daily protein goals or not, then say yes to soya chunks! They are inexpensive, safe and healthy! Go through the papers we have cited and ask yourself:   

Who would you believe? Your call.

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