Updated: Nov 21, 2022
One of the most often asked questions - “Is soy good or bad for my health?”
Whoever is claiming that soy products are absolutely bad or absolutely safe is underestimating huge individual variability in responsiveness to soy due to digestive/genetic/age/ethnicity and health status factors.
I’ve seen studies claiming both sides, and my conclusion- it depends!
Given the extent of soy and its derivatives hitting our food supply in many forms, it’s reasonable we ask questions, investigate more, investigate again and differently. However, I think it’s too early to put soy discussion to bed.
Soy is a complete protein that is cholesterol-free, lactose-free, high in fibre and rich in complex carbohydrates, antioxidants and unsaturated fats. But soy is also packed with phytoestrogens, which makes it a hormonally active food.
For many the consequences of this activity will be minimal, or even potentially beneficial, but for others the endocrine-disrupting properties of soy should not be discounted.
Phytoestrogens are naturally occurring plant compounds that are structurally and/or functionally similar to mammalian estrogens and their active metabolites.However, phytoestrogens are not the same thing as female estrogens. Soy foods do not contain estrogen.
The question of whether or not soy phytoestrogens are beneficial or harmful to human health is neither straightforward nor universally applicable to all groups!
Ultimately risk of harm comes down to two primary factors: dose and timing of exposure.
Few examples :
For women suffering from menopausal symptoms (hot flashes, breast pain, headaches etc), phytoestrogen diet might be potentially beneficial;
For infants and small children, consuming soy milk formula is of particular concern because their hormone-sensitive organs, including the brain and reproductive system, are still undergoing sexual differentiation and maturation. Societies including the American Academy of Pediatrics, the European Society for Pediatric Gastroenterology Hepatology and Nutrition Committee on Nutrition, Paediatric Specialist Group of British Dietetic Association have issued guidelines recommending against the exclusive use of soy formula except in the rare cases of true milk allergy or lactose intolerance;
If you have clinical or subclinical hypothyroid, compensatory iodine intake is advisable if soy is part of the regular diet. Both genistein and daidzein (bioactive elements in soy) potently block thyroxine synthesis by serving as alternate substrates and blocking thyroid peroxidase catalysed tyrosine iodination. Soy also decreases absorption of synthetic thyroid hormone potentially necessitating higher doses in hypothyroid patients. However, there's no evidence that people who have hypothyroidism should avoid soy completely.
In pregnancy it is even more complicated, because there is not enough research regarding the relationship between soy intake and thyroid levels during pregnancy. However, because thyroid hormone is essential for normal brain development, pregnant women regularly consuming soy should be particularly mindful of this endocrine-disrupting property of soy.
Consumed in high dose (45g/day) soy can also impact reproductive function in women. Few case studies reported uterine bleeding with endometrial pathology related to a high intake of soy products.
The beneficial effect of dietary soy food intake, especially for women diagnosed with breast cancer, is controversial. At this stage there is not enough data on the efficacy and safety of high-dose genistein and daidzein supplementation, in patients with breast cancer. Keep in mind that moderate amount of soy foods is probably a healthy option, while soy dietary supplements may not be.
There is a lot of soy protein consumption in the sport industry. And it’s true that soy protein is as effective as any animal source proteins if you want to gain muscle mass and strengths from resistance training. And there is no evidence that soy affects reproductive hormones in male - in normal dietary doses! But I guess the picture might be different for those having regular intake of phytoestrogens in high amounts from soy protein powders. And what if there is a cumulative effect of soy phytoestrogens with other endocrine disruptors (plastic, dioxins, BPA and other chemicals)?
It’s frequently argued that Asian populations have been consuming soy for centuries, with no obvious consequences.
True, but it’s probably also the reason why Asian population is most likely better adapted to soy products.
Imagine, if you grew up in a large urban city and suddenly started eating like Inuits or Masai tribes, chances that you’ll feel unwell are quite high.
We also have very different gut microbiome, and our genetic makeup shaped by our environment is most likely very distinct from Asian population.
Don’t forget that in a traditional Asian diet, soy consumption is moderate across the entire lifespan. In addition, in Asia people consume fermented types of soy products - it’s not difficult to imagine that fermented soybeans or natto (traditional Japanese food often served for breakfast) has a very different health impact compared to commercially available soy milk from your Tesco or Aldi shop.
Our diet and lifestyle differences may also be confounding when evaluating the potential health benefits and risks of soya. For example, Asian populations on traditional diets eat less processed foods, considerably higher levels of seafood and lower levels of animal fat than Western populations. So the interactions between various foods are also important.
Bottom line - if you are healthy, there is nothing to worry about occasional soy consumption. Enjoy edamame, miso, tofu, tempeh or even soy sauce in cooking, but remember, as with all plant foods, less processed is better. For more vulnerable categories of people (thyroid issues, lactation and pregnancy, estrogen disbalance) it’s probably safer to limit soy products such as soy milk or yogourt, but again, it’s all about the quantity and the quality. And finally if you are lactose intolerant, there are still other milk alternatives (almond, coconut, oat, rice).