Soy, soybeans, and soy protein: muscle-building alternatives to meat, healthy legumes, or hormone-altering cancer beans? Few foods are as controversial as soyfoods. As always is the case when it comes to controversial stuff, the truth is entirely black or white.
Soy means different things to different people. For the weight-lifting fitness aficionado, soy protein for muscle-building purposes is probably the first thing that comes to mind. For the vegan or vegetarian, soybeans are one of the few completely plant-based foods that offer a source of really high-quality protein.
Soyfoods have many proposed health benefits, in addition to their high-quality protein content. However, these are countered by a wide-spread concern of potential negative effects of soy consumption, because of certain natural compounds found in the soybean.
In this article, you will find everything you need to know about the soybean: the nutrients you get from it, any effects on your health, both positive and negative, not to mention how effective it is for building muscle.
What is Soy Protein?
Soy protein is a protein found in, not surprisingly, the soybean. It is the rare plant-based protein that gives you enough of the amino acids you need to build muscle. You get soy protein from the unprocessed soybean, from soyfoods based more or less on the natural legume, and in protein supplements separating the soy protein itself from the rest of the nutrients in the bean.
Soy protein supplements are the most popular alternative to protein powders based on animal proteins. Strength athletes use it for muscle-building purposes, and vegans and vegetarians looking to increase their protein intake with a high-quality plant-based protein source use it as a dietary supplement.
The Soybean: a Nutritious Little Fellow
The soybean is a species of legumes originally from East Asia, where they have been a staple food for thousands of years. Today, the soybean is a global crop, and a majority of the annual harvest comes from plantations outside of Asia.
Legumes in general are quite nutritious, but the soybean is unique. It gives you a lot more fat, quite a bit more protein, and high-quality protein at that, but at the same time not nearly as many carbohydrates as other beans, peas, or lentils.
Plant proteins are, with few exceptions, not considered complete or whole proteins.
A common misconception is that plant proteins lack one or more amino acids, and that’s why they are not considered complete. This is not the case, since all protein, regardless if you get it from animal or vegetable sources, provide you with all 20 amino acids. The difference is that most proteins from plants have too little of one or more of the amino acids. They don’t lack them completely but don’t provide you with enough of them. This means that your body can only use that protein to build and repair tissue until it runs out of the least abundant amino acid. The amino acids you don’t get enough of from plant proteins are called limiting amino acids.
Legumes contain a lot of protein, but the protein quality is limited by one or more amino acids, methionine in particular. However, soy protein is the exception. Soy protein gives you all the amino acids you need and enough of all of them. This qualifies soy protein as a complete protein.
Even when a protein gives you enough of all amino acids, it still might not be optimal for building muscle in response to strength training. Believe it or not, when you calculate protein quality, strength training is not a consideration. Instead, the focus is on things like life, health, and general growth.
Protein quality is often expressed in scores determined by the PDCAAS or DIAAS methods.
PDCAAS stands for Protein Digestibility Corrected Amino Acid Score. It is a method for measuring protein quality, considered the best such method by both the Food and Agriculture Organization of the United Nations and the World Health Organization since 1993. The method does not only evaluate human protein requirements but also how well we digest and use the amino acids from the protein we eat.
The PDCAAS of the soybean is somewhere between 0.95 and 1, 1 being the highest possible.1 Other legumes score between 0.6 and 0.7.
DIAAS is the acronym for Digestible Indispensable Amino Acid Score. Unlike PDCAAS, which looks at the entire digestive system, DIAAS measures the uptake of individual amino acids from the small intestine.
Today, DIAAS is considered the more reliable method. In January 2013, the Food and Agriculture Organization of the United Nations recommended switching over from PDCAAS to DIAAS when measuring protein quality.2
One of the main critiques of the PDCAAS-method is that it only measures protein quality up to the score of 1. All qualifying proteins get that score, but no higher, even if they differ in actual quality. The World Health Organization considers 1 “good enough” and every protein that actually would score higher, if possible, should be rounded down. This might indeed be good enough for life and health. However, if the topic of discussion is building muscle, every protein with a score of 1 might not be equal in practice. The limit makes it impossible to compare very high-quality proteins with each other.
That’s where DIAAS has the advantage. Using the DIAAS method, you can compare any protein with any other protein. To calculate the DIAAS score, you start with the number 100 and multiply it with the milligrams of a digestible essential amino acid per 1 gram of the test protein.
After that, you divide the result with the milligrams of the same total milligrams of the same amino acid in 1 gram of the protein. That way, you end up with the relative quality of the protein. If this all sounds confusing, don’t worry about it. We won’t quiz you about it at the end of the article.
However, the DIAAS method does not only have advantages. The downside is that we lack enough human data on how well we absorb and digest individual amino acids from the small intestine.
But back to soy protein! How to calculate and measure protein quality can be interesting, but it’s probably not the information you look for first when you decide which protein supplement to use. DIAAS as a method to measure protein quality is not yet common practice, but based on rodent studies, soy protein would score around 0.9.3
In practice, this means that if you choose soy protein, you’re giving your body and your muscles what they need, no matter how you look at it. The Food and Agriculture Organization of the United Nation considers a DIAAS score of 0.75 and above as high enough to cover all your protein quality needs.4 Soy protein easily exceeds this value, calculated using the recommended method of measuring protein quality.
Soybeans do not contain as many carbohydrates as other legumes. This makes them extra valuable for vegans and vegetarians looking to cover their protein needs without overloading on carbs. Most plant-based protein sources give you quite a lot of carbs along with the protein. While not a problem in and of itself, eating a high-protein diet using only plant-based sources without loading up on carbs and calories at the same time can be a challenge.
When you eat soybeans or soyfoods, a lot of the carbohydrate content comes from oligosaccharides, more specifically an oligosaccharide called stachyose.5 Oligosaccharides are a type of carbohydrate made up of 4 to 10 simple carbohydrate molecules, which act like fiber in your digestive system. Your intestinal enzymes have a hard time breaking them down, and they reach your colon relatively intact. There, they stimulate the growth of beneficial bacteria. Soyfoods, especially non-fermented soybean milk is associated with a healthy microbiota.6
The bacterial growth soybeans stimulate, while mostly beneficial, also has the unfortunate and sometimes painful side-effect of gas build-up, much like other beans and legumes. The processing of soy products like tofu and tempeh lowers the oligosaccharide content of the end product, reducing the risk of you ending up with flatulence and a gassy stomach after a soy-based meal.
The fat you get from soybeans is comprised of 10–15% saturated fatty acids, 19–41% monounsaturated fatty acids, and 46–62% polyunsaturated fatty acids.7 The fatty acid content varies to such an extent because there are many different types of soybean. Also, fat content and composition vary quite a lot from bean to bean even within a certain family of beans. In other words, you can’t be 100% certain that your particular bowl of tofu gives you a certain fatty acid composition.
The polyunsaturated fatty acids in the soybean come from linoleic acid and alpha-linolenic acid, which are omega-6 essential fatty acids and omega-3 essential fatty acids. Soybeans are one of few foods from which you get decently large amounts of both the essential fatty acids.
Vitamins and Minerals
On the whole, soybeans are comparable to other legumes when it comes to vitamins and minerals. At least there is nothing exceptional about soybeans in this regard.
A couple of micronutrients deserve a special mention, though, namely calcium and iron. Not because the soybean contains huge or tiny amounts of them, but because soy often replaces other good sources of these nutrients. Someone who switches to a vegan or vegetarian diet might replace meat and milk with soy for the protein, losing well-absorbed iron from the meat and a concentrated source of calcium in the form of milk.
While not a problem on paper, since plant-based diets usually provide at least as much of, for example, iron as a mixed died, the uptake might be an issue. In general, your body can’t absorb and use calcium and iron from vegetable sources as well as from animal-based sources.
However, this might not be the case with soy.
Calcium from soybeans and soyfoods are absorbed surprisingly well.8 Surprising, because soy contains both phytates and oxalates, so-called anti-nutrients that prevent your body from absorbing minerals. Research shows that you absorb and use calcium from tofu just as well as calcium from cow’s milk.9
The iron uptake from soybeans is also better compared to most other plant sources of iron, probably because the iron in soybeans comes from ferritin. Ferritin is a protein that gathers iron from the food you eat and releases it in a controlled manner, which makes it more readily absorbed.10 11 12
The number of micronutrients you get from soy varies between different types of beans. Various foods based on soy can differ even more in vitamin and mineral content. Also, the production of said foods affects the nutritional value of the end product. For example, the traditional Japanese food natto, made through fermentation, ends up with an extremely high vitamin K content.
Protein supplements from soy, like soy protein isolate, also provide you with micronutrients like manganese, copper, and iron. Thirty grams of soy protein isolate powder cover about 25% of your daily iron needs, for example.
When and How to Use Soy Protein?
Regardless of whether you get soy protein from soyfoods or soy protein supplements, you don’t have to treat it in any special fashion.
Just use soy products like tofu and tempeh, or natural soybeans for that matter, like you use any other source of protein in your diet and your cooking.
The same goes for soy protein isolate. A protein supplement isn’t necessarily a supplement per se, but rather food in the form of powder. The most common way to use it is probably after working out, but you don’t have to conform to that norm if you don’t want to.
You can increase the protein content of low-protein meals using soy protein isolate, or use is it as the main source of protein in a meal, maybe in oatmeal or your pancake batter. Use it any way you can imagine. It’s just bean protein, not something you need to treat in any particular way or with some special kind of respect.
How Much Soy Protein Should You Take?
The most important number, by far, to keep in mind when planning your protein intake is the total amount you eat and drink over the day. Included in that number is all your dietary protein, both the protein you get from “real food” and the protein you get from supplements, like soy protein isolate.
To build muscle to the best of your capabilities, ideally, you want to aim for at least 1.6 grams of protein per kilogram of body weight and day, all sources included.13 If you end up eating more than that, no harm was done, but don’t expect any extra gains either.
Even though your total protein intake is the most important factor, the amount of protein you eat in any given meal is not irrelevant. You need a certain amount of protein each meal to stimulate muscle protein synthesis properly. At the same time, you don’t automatically build more muscle by adding even more protein to the plate, once you’re above a certain amount.
Compared to the most common protein supplement, whey protein, soy protein isolate stands out for a couple of reasons.
You need about 2.7 to 3 grams of the amino acid leucine from any given meal (or shake) if you want your muscles to add as much new protein as possible.14 Below that amount, you will still build muscle, but not as much as you could have. If you get more, nothing bad happens, but you don’t synthesize any more muscle protein either. Instead, the protein is used to build things like your gut, inner organs, and your skin, and any excess amino acids oxidize and you use them as energy.15
Let’s use whey protein as a comparative example. Whey is the most common type of protein supplement and a good point of reference for both the protein content of a meal and the amount of leucine in that meal.
You get 2.7 grams of leucine when you chug a whey shake providing 25 grams of protein. In other words, 25 grams of whey protein is enough to stimulate muscle protein synthesis as much as possible. If, however, you use soy protein isolate, 25 grams of protein won’t be enough. How come? Simply because the leucine content of soy protein is less than that of whey protein. Because of that, you need to increase the amount of protein per meal from 25 grams to 40 grams, if you use soy protein and want to stimulate muscle protein synthesis maximally.16
How much of your soy protein powder you need to use to get 40 grams of protein depends on how processed and “clean” your particular protein supplement is. If it contains a lot of fat and sugars, you will need more than if it is highly filtrated. Somewhere in the ballpark of 55 grams. The table of contents and a calculator should solve that dilemma.
Soy Protein and Your Muscles
How do your muscles use soy protein? What happens after you eat that tofu or drink a shake of soy protein isolate?
Muscle Protein Synthesis
Muscle protein synthesis is the process by which your body adds amino acids to your muscle fibers. When you eat or drink soy protein in some form, your digestive system breaks it down to free amino acids. They gradually appear in your blood, after which they are transported to where they are needed to build new tissue and repair old. This process starts whenever you eat some form of protein and is independent of the type of protein we’re talking about. If you want to stimulate this process as much as possible, you want the amino acids to flood your bloodstream in a fast and powerful manner.
Protein can be either fast or slow. This applies to how fast you break it down and how fast the amino acids appear in your blood.
You digest fast protein, as the name suggests, quickly. The amino acids appear in your blood shortly after eating the protein, stimulating your muscle protein synthesis in a powerful but short-lived manner. Whey protein is a prime example of a fast protein. It’s the only example of a truly fast protein. When you ingest whey protein, plenty of amino acids appear in your bloodstream after a very short while. They start signals telling your body to build new muscle protein. However, only an hour or two later, these signals have faded to nothing, along with the supply of amino acids, and your muscle protein synthesis returns to base levels.
Slow protein, on the other hand, is digested over a much longer time, which means that the supply of amino acids also lasts a lot longer. The level of amino acids in your blood never reaches the same heights and doesn’t stimulate muscle protein synthesis as powerful. On the other hand, it lasts a whole lot longer. Most protein sources in your diet provide you with slowly digestible protein. Soy protein is one of them.
Your body can absorb about 10 grams of whey protein per hour. The corresponding value for soy protein isolate is 4 grams.17 That’s a typical example of fast and slow proteins. This means that soy protein can’t match whey protein when it comes to building a lot of new muscle protein in a short time. However, it also means that a serving of soy protein keeps your muscle protein synthesis going for a lot longer. If the soy protein comes from an intact source, like the soybeans themselves rather than soy protein isolate, the difference is even more tangible. In that case, you also get fiber, fat, and other thingamabobs that slow down the uptake of the protein.
The Young and the Old
If we isolate the effects of a single protein serving, a fast protein is the best option for the elderly, while slow protein is better for young people.18
A fast protein, like whey, boosts muscle protein synthesis for a couple of hours. After that, it returns to fasted levels, where it stays until your next protein-rich feeding. Slow protein, like soy protein, keeps muscle protein synthesis going for a lot longer. The milk protein casein, for example, provides a steady supply of amino acids for your muscles for at least 5–7 hours. Casein is just a tad slower than soy protein, which means that you can expect soy protein to have similar effects on your muscles.
If you are older, say 60 years or older, your muscles don’t respond quite as well to slow proteins. Sometime around that age, so-called amino acid resistance starts to creep in. That’s a natural, age-related condition where you don’t get the same anabolic response from a protein-rich meal as when you were young.19 Because you digest soy protein slowly, you probably don’t synthesize as much muscle protein from eating it, as you did in your youth.
You can counteract this phenomenon in several ways.
You can simply use a fast protein instead. That way, you still get a robust muscle-building effect when the amino acids promptly appear in your blood. That’s the reason fast protein gives you a better so-called muscle protein balance, the difference between muscle protein synthesis and muscle protein breakdown, if you are elderly, even if the time of reference is more than an hour or two. The problem with this method is that you can’t apply it to soy protein, which is slow by nature.
Another way is to eat more of the slow protein. By doing that, you also get more of the leucine that triggers muscle protein synthesis. In other words, eat more soy protein in a particular meal, and your muscles benefit from it even if you are older.
Seeing as a young person requires about 40 grams of soy protein for max muscle protein synthesis, the amount you would need as a senior citizen could potentially get out of hand. An elderly individual needs almost twice as much protein as a young for the same anabolic effect. We’re talking about close to 100 grams of protein powder or quite the heaping plate of tofu here.
Of course, this could be both a good or a bad thing, depending on your appetite, hunger, and when you eat or drink your protein. In most cases, though, appetite decreases with age, possibly making soy protein problematic for the elderly man or woman who wants to build as much muscle as possible. For the average person, though, it’s likely a non-issue.
Soy Protein and Strength Training
Several studies suggest that soy protein helps your muscle recover and grow from your strength sessions, both long-term and after a particular workout. Several studies compare soy protein to other popular protein supplements, like whey and casein protein.
A 2018 meta-analysis summarizes these findings.20 Five studies compare soy protein to whey protein, and nine studies compare soy protein to animal protein in general, meaning beef protein, milk protein, and whey protein in this case. Of course, whey protein is a milk protein, but this meta-analysis reviews whey and milk protein as in 80% casein + 20% whey separately.
Studies that only look at muscle protein synthesis throughout a few hours suggest, almost without exception, that whey protein is a better choice than soy protein. Those results led to early speculations that soy protein also is the inferior choice when it comes to build muscle and gain strength over time. However, more recent research shows that a temporary increase in muscle protein synthesis, like after drinking a protein shake, is not associated with long-term muscle growth.21
The 2018 meta-analysis supports this conclusion. It does not find any differences between soy protein and whey protein in regards to gaining muscle and strength. Both your muscle mass and your strength in exercises like the squat and the bench press seem to benefit from strength training regardless of your protein of choice, be it soy protein, whey protein, or some other kind of protein.
Individual studies are more ambivalent. Some show greater muscle growth with whey protein supplementation compared to soy protein, while others fail to see any difference. Looking at the complete picture in the form of reviews and meta-analyses, you can likely expect similar results from your efforts in the gym no matter which protein supplement you use.
The 2018 meta-analysis has several shortcomings, which the authors acknowledge. The included studies are far from homogenous regarding study protocol, methodology, and the training status, age, and sex of the subjects. Protein supplementation research with untrained subjects is notoriously useless. Not because the protein supplement itself lacks effect, but because untrained individuals get such great muscle growth when they take up strength training, from the training itself, that a protein supplement does not add anything extra. You can only gain a certain amount of muscle in a certain amount of time, and strength training itself causes those kinds of gains in beginners.
Other issues include a low subject count and short study duration. All these issues combine to give the conclusions of the meta-analysis low statistical power. Differences in training results might have shown up depending on the choice of protein if only the studies had been longer and utilized more subjects in the trials. Differences that currently appear to be non-existent might materialize if that had been the case.
The meta-analysis itself can’t be blamed for these issues. The available research is what it is. In any case, there is no doubt that you can use soy protein as a protein source for gaining muscle and becoming stronger.
Isoflavones belong to a group of so-called phytochemicals, natural chemical compounds found in plants but aren’t considered nutrients. You find isoflavones in legumes, soybeans in particular. The soybean contains a uniquely high amount of isoflavones compared to all other beans and foods based on beans.
In Asia, where soybeans are much more of a dietary staple than in the west, the average isoflavone intake is also a lot higher. The average adult Japanese man or woman consumes between 30 to 50 milligrams of isoflavones per day. The corresponding amount for adults in the US and Europe is barely 3 milligrams per day.22 23
One gram of protein from soybeans gives you about 3.5 grams of isoflavones.24 When you eat 100 grams of tofu, bean curd made from coagulating soy milk, you get about 25 milligrams of isoflavones as a bonus. Heavily refined soy products, like soy protein isolate, come loaded with a much lower isoflavone content, due to the manufacturing processes.25
The chemical structure of soy isoflavones is similar to the female sex hormone estrogen. They bind to estrogen receptors called ER Alpha and ER Beta.26 27 This lets them exert estrogen-like effects in animal and test-tube studies, mechanisms classifying them as phytoestrogens. Even though these effects are not nearly as powerful as those of real estrogen, phytoestrogens are highly controversial. Speculation regarding their health effects, both positive and negative, abound.
Some theories and speculations about isoflavones and phytoestrogens suggest positive health effects, from protection from heart disease and certain types of cancer to counteracting aging and improving memory. However, not many human trials lend convincing support to such claims. While these theories are likely not directly applicable in practice, it has not stopped the marketing of isoflavones in the form of dietary supplements claiming various health effects.
At the same time, there is some concern about negative effects on human health from a high intake of isoflavones. One of these concerns is an increased risk of cancer, especially in post-menopausal women. However, a comprehensive review published by the European Food Safety Authority (EFSA) recently concluded that isoflavones do not increase the risk of any of the three suspected forms of cancer: breast cancer, thyroid cancer, or endometrial cancer.28 The review by EFSA included studies over long periods and isoflavone intakes of up to 150 milligrams per dag, amounts you’d need supplements to realistically reach.
In summary, little evidence supports claims that you need to worry about health risks from soy isoflavones. On the other hand, there is equally scarce evidence that you will gain any dramatic health benefits from spending money on isoflavone supplements either.
Soy and Your Hormones
The fear of untoward hormonal disturbances from soy is a common reason why many, mostly men, avoid soy protein and soyfoods in general. These fears are not entirely plucked from thin air. The mechanisms are there, at least on paper and from animal observations. Give mice large amounts of soy isoflavones, and their testosterone levels drop.29 Also, as we said earlier, soy protein supplements might boost estrogenic signaling due to the phytoestrogenic isoflavone content.
Rodents and test tubes are all very well, but the important thing here is how your body reacts in real life.
A common notion proclaims that men run the risk of lower testosterone levels if they use soy protein. Is there any truth behind this notion?
Research Supporting Lower Testosterone Levels From Soy Protein
One study often presented as proof of the testosterone-lowering effects of soy protein is from 2007.30 Twelve healthy men between 25 and 47 years of age took 2 scoops (56 grams) of soy protein powder a day for 4 weeks. Their testosterone levels dropped by 19% during this time, only to recover in a couple of weeks once they stopped using the soy protein supplement.
This might sound terrifying, but probably only if you only read the abstract, not the entire study. If you do read the whole thing, you’ll find that the cause of this effect was a single person dragging the entire group down with him. His testosterone levels were twice those of the other subjects and 50% above normal reference levels. The results of the study probably indicate that, yes, you might lower your testosterone levels if you use a soy protein supplement, but only if your levels are abnormally high to begin with. Of course, this is not something to scoff at. If your natural hormonal balance allows you to build muscle at a faster rate than your peers, you probably want to keep it that way. However, this is not something replicated in later studies. Who knows, maybe this man was exceptional or had some rare condition?
Another, 20-year-old study, shows an association between soyfoods in general and lower testosterone levels in Japanese men. 31 Sixtynine men self-reported their food intake using food frequency questionnaires, and after evaluating the results, the researchers found that a high intake of soyfood was associated with lower testosterone levels. After adjusting for age, BMI, alcohol intake, and smoking, the association remained, but close to non-significance. Also, being a study of associations based on self-reporting, it can’t determine cause and effect.
Many studies show that you can use both soy protein supplements and soy foods in general without worrying about your testosterone.
Research Related to Strength Training
Studies looking at the effects of soy protein on the testosterone levels of strength-training subjects are far and few between. We only have a couple at our disposal.
One of the most well-known of these is from 2007.32 Twenty resistance-trained men were split into 4 groups, all of which received 50 grams of protein in supplement form for 12 weeks. During these weeks, they all engaged in strength training. One group received soy protein concentrate, one received soy protein isolate, one received soy protein isolate + whey protein, and the fourth group received whey protein only. All four groups added significant amounts of fat-free mass with the combination of strength training and protein supplementation, but the type of protein made no difference. Also, the researchers could not find any evidence of a testosterone-lowering effect in the men who received the soy protein. This study was funded by the soy industry, but it was blinded, randomized, and appears to be methodologically sound.
The latest published study featuring weight-training subjects was published in 2018.33 The participants spent 3 days a week for 12 weeks in the gym lifting stuff and boosted their daily protein intake with either soy protein, whey protein, or a placebo during this time. The testosterone levels in the soy protein group showed no signs of declining. On the other hand, they increased in the whey protein group. This could indicate some benefits of using whey protein, but not any negative effects of soy protein.
Meta-Analyses and Reviews
The evidence from meta-analyses, where you add the results from several studies together and look at the complete scientific picture, is worth more than the results of individual studies.
One such meta-analysis from 2010 compiled the results of 15 placebo-controlled studies and 32 less carefully controlled studies and examined the effects of soy protein or soy isoflavones on the reproductive hormones testosterone and sex hormone-binding globulin.34
The analysis could not find any negative consequences regarding testosterone from soy protein use. Sure, a study here and there suggested such effects, but rarely does every study included in a meta-analysis support the conclusion. Taken as a whole, the evidence pointed to soy protein and isoflavones not affecting male sex hormones, including testosterone. Also, the soyfood and isoflavone intake in many of the included studies was a lot higher than you’d get from the average Japanese diet. And keep in mind that a Japanese diet, in turn, is soy-based to a much higher degree than an average Western diet. The chances that a moderate intake of soy protein will lower your testosterone levels are likely quite slim.
Following that meta-analysis, it took quite a while until the next one.
At the time of writing this article, a brand new meta-analysis saw the light of day. During the last days of 2020, US and UK scientists provided an updated scientific landscape with an expanded meta-analysis, in which they re-examine the studies of the 2010 meta-analysis, in addition to 10 new studies published since then.35
Ten years of additional research did not reveal anything sensational. The previous conclusion remains: soy, soy protein, and soy isoflavones do not affect total or free testosterone in men, regardless of dose and regardless of the length of the studies. Four of the included studies lasted at least a year without any signs of a testosterone-lowering effect. Usually, these kinds of effects appear pretty fast. For example, if you drink alcohol 3 days straight, your testosterone levels start to decline. If you keep that practice up, you’ll probably reach the levels of a chronic alcoholic a month later. It seems unlikely that testosterone levels would remain stable for an entire year of high soy consumption, and only start to drop after that for some reason.
In conclusion: some research supports a testosterone-lowering effect from soy protein consumption. However, most studies that come to such conclusions are of questionable quality, with limitations and problematic methodology. The meta-analyses reviewing the available research do not support claims that your testosterone levels are at risk if you use soy protein to boost your protein intake.
Soy, Estrogen and Feminizing Effects in Men
This might very well be the most widespread fear that makes men avoid soy. Who hasn’t heard or read the expression “soy boy” as a negative term to describe men lacking masculine features?
Two case studies are often presented as evidence of the feminizing effects of soy.
A 60-year old man drank close to 3 liters of soy milk every day, after which he developed female breast tissue and suffered erection problems.36 Once he gave up guzzling soy milk, the issues went away.
A 19-year old man switched to a vegan diet and started eating large amounts of soyfoods. Soon after, his testosterone levels crashed, his sex drive declined, and he developed erection problems.37 He quit the vegan diet, stopped eating soy products, and a year later all his hormonal problems resolved themselves.
These two studies show associations between a high soyfood intake and feminizing effects. At least in these two particular men.
We could be witnessing cause and effect here, that the high soy intake caused the issues the men experienced. However, we need to take a couple of things into consideration.
First, the number of isoflavones the men consumed through their soy intake was 9 times larger than what an average Japanese man eats daily. And remember that his daily isoflavone intake is a lot higher than the average man in the Western hemisphere. We’re not talking about your average soy intake here.
Also, soyfoods provided the majority of their caloric intake. That’s not normal, and likely indicates a pretty unbalanced diet.
In other words, it’s quite possible, even likely, that these two isolated case studies show that an extreme, nutrient-deficient, and unbalanced dietary intake of something, soy, in this case, can cause health issues. We can’t say, based on those two results, that a moderate or even high soyfood intake has any feminizing effects.
As we saw earlier, there is no scientific evidence supporting a testosterone-lowering effect of soy. Another review, an industry-funded one, concludes that soy isoflavones have no effects on male estrogen levels, not even a very high daily intake over a long time. Also, studies find no negative effect on sperm quality.38
That review is a bit problematic, since it is funded by the soy industry and written by the Executive Director of the Soy Nutrition Institute. Up until the writing of this article, there have been no other reviews or meta-analyses available. The fact that a scientific article is funded by someone with an economic interest in a thing does not have to mean unreliable conclusions. Usually, someone has to fund such articles, or they won’t get written in the first place. However and unfortunately, it can lead to cherry-picking and negative effects being dismissed or not mentioned at all.
When 2020 turned into 2021, we suddenly got that new review, in the form of the meta-analysis we talked about earlier in the testosterone chapter.39 It also looked at other hormonal effects of soy and isoflavones, and unlike the 2010 analysis, it included estrogen in the analysis.
The new meta-analysis concludes, based on research dated up until April 2020, that neither soy protein nor soy isoflavones affect male estrogen levels.
Soy and Your Health: Positive Effects
Not only do you get high-quality vegetable protein from soybeans, but some scientific evidence suggests that it provides you with certain beneficial health effects as well.
Several meta-analyses show that a soy-rich diet can lower your blood pressure. This goes for both soy protein and soy isoflavones.40 41 42 One study showed a very large effect by replacing one liter of cow’s milk with the same amount of soy milk daily.43
The beneficial, blood-pressure-lowering effects are usually apparent only in people who already suffer from hypertension (high blood pressure). If your blood pressure is normal, don’t expect anything special.
Elevated levels of LDL-cholesterol are one of the major risk factors for heart disease. Soy protein, soyfoods, and soy isoflavones purportedly improve blood triglycerides.
A scientific review of 35 studies lasting between 4 and 52 weeks saw lower levels of LDL-cholesterol, the “bad” cholesterol, and an increase in HDL-cholesterol, the “good”, with a regular intake of soy. This positive effect was most apparent in people with unhealthy cholesterol levels. Healthy individuals did not benefit particularly in this regard. Fairly unrefined soy products, like soy milk and soybeans, improved triglyceride levels more than refined soy products and soy protein supplements.44
Several other meta-analyses lend support to these findings and also conclude that soy protein lowers LDL-cholesterol.45 46 47 Actually, there are even more than those, but they are some of the major ones.
The United States Food and Drug Administration (FDA) also concluded, at the end tail of the last century, that soy protein, as apart of a healthy diet, may improve triglyceride levels and even reduce the risk of heart disease.48 Canada followed suit 20 years later.49 However, the FDA is currently in the process of reassessing the 1999 statement and the role of soy protein in the management of blood lipids, because of inconsistencies in the available studies.50
In summary, there is scientific evidence that soy protein exerts beneficial effects on your blood lipids, especially if you are already having issues with them. Currently, there is an ongoing debate on how solid that scientific support is, though.
Eating a high-protein diet can aid you during weight loss, for several reasons. It helps maintain your muscle mass, and you feel fuller with plenty of protein in your diet. Some studies suggest weight loss during a high-protein diet without the need to count calories.
A couple of studies find that a mixed diet high in soy protein is just as effective as a high-protein animal-based diet.51 52
However, a recent meta-analysis compiling the results of all available studies is not quite as enthusiastic.53 The authors did not find any positive effects on body weight, body fat, or waist circumference that could be traced to dietary soy and soy isoflavones. Meta-analyses trump individual studies, and also, in comparison, subjects using whey protein can drop 2 kilograms more body weight and body fat in 20 weeks, compared to those using soy protein.54
In summary, this means that you should feel free to use soy protein during a weight loss diet. It won’t slow down your weight loss. On the other hand, don’t expect it to help you along either. Whey protein is more closely associated with improved weight loss than soy protein. Some research support soy protein, but the evidence as a whole suggests that soy protein does not do anything special in this regard.
The female population in countries where people traditionally eat a lot of soyfoods has a significantly lower rate of breast cancer than women in the West. You might think this means that you too can lower your risk of breast cancer by increasing the amount of soy in your diet. However, observations like this do not always translate into cause and effect.
Epidemiological analyses indicate an association between a high intake of soy and a substantial reduction in the risk of breast cancer. Before jumping to conclusions here, we need to keep one important thing in mind. The majority of the research look at women in Asian countries. They have been eating large amounts of soyfoods since childhood. Even if the association between soy and breast cancer is cause and effect, and a high intake does reduce the risk, it’s probably only relevant when said intake is life-long.
Can you increase your soyfood intake dramatically in middle age and catch up to someone who has eating soy their entire life? Quite unlikely. Accordingly, clinical studies find no reduction in breast cancer risk by increasing soy isoflavone intake in adulthood.55
The fact that soy isoflavones exert some estrogen-like effects has led to speculations that soy might be harmful to women being treated for breast cancer. Rat studies lend support to this theory, and tumor growth is apparent in rodents treated with massive amounts of isoflavones. However, observational studies find no such effects in humans. If anything, soy isoflavones and a high intake of soyfood are associated with a lower risk of recurrence in women with breast cancer, when cancer comes back after treatment.56 57 Both the American Cancer Society and the American Institute for Cancer Research consider soyfoods safe for women who have or have had breast cancer.58 59 Also, the World Health Organization recognizes a possible association between a high soy intake and an improved breast cancer prognosis.60
Just like with breast cancer, prostate cancer frequency is much lower in countries where the population traditionally bases its diet on soyfoods to a higher degree. Meta-analyses show a clear association between the total amount of dietary soy-based food and a lower risk of prostate cancer.61
This association is apparent in both in vitro studies and in vivo studies, meaning both in test tubes and in real-life scenarios involving the male population of different countries.
Most of the available research examines the total intake of soyfoods, not any particular food or product based on soybeans. A couple looks at tofu, and the results suggest a protective effect substantial enough that it can’t be coincidental.
The cause of the observed risk reduction is currently not known. Early speculations that soy lowers testosterone levels, and by proxy, the risk of prostate cancer, is not relevant, since research has found no such hormonal effects from eating soyfood.
Correlation or causation? Associations or cause and effect? Currently, there is no way to tell. We need more research to potentially establish the latter. The association between soy and a lower risk of prostate cancer is real, though.
The supplement industry frequently markets soy isoflavones to menopausal women. The purported mechanisms behind this are as follows:
- Hot flashes are the most common problematic symptom of the menopausal transition. Usually, they pass over the course of 6 months to 2 years, but they can go on for a lot longer, sometimes decades.
- Japanese women suffer hot flashes to a much lesser degree than women in Western countries.
- Japanese women eat more soyfoods than Western women.
- Estrogen therapy offers relief from bothersome menopausal hot flashes.
- Soy contains a lot of isoflavones that interact with estrogen receptors in the body.
- Thus, soy isoflavones should prevent or relieve hot flashes just like estrogen therapy.
In the mid-90s, research on this topic took off, and since then, more than 25 studies have reviewed the above hypothesis.
The results are ambiguous. Some studies find that isoflavones reduce menopausal problems, others do not. A 2010 meta-analysis of the then-available evidence supports the theory. It gathers almost 20 randomized double-blind and controlled studies and concludes that soy isoflavone supplementation reduces hot flashes effectively. Both women in the middle of menopause and postmenopausal women experienced a reduction of hot flashes frequency and intensity of 21% and 26%, respectively.62 In particular, supplements containing large amounts of the phytoestrogen genistein proved the most effective.
Soy and Your Health: Negative Effects
In addition to the purported negative effects mentioned earlier in the article, from the feminizing effects in men to the increased risk of breast cancer, both of which are doubtful at best, there are speculations that soyfoods have other untoward health effects.
Animal studies and test tube research show that soy reduces the functionality of the thyroid gland and worsens hypothyroidism. Human research does not support these findings, especially if your thyroid is working as intended.63
Infant Soy Formula Effects
Soy formula based on soy and soy milk has been readily available for at least 100 years. During these decades, it has been a target of debate and scrutiny. Results from animal trials have led to suspicions that soy-based infant formula impairs the future reproductive capabilities, neurological development, immune system, and thyroid function of the child. No studies support such claims, and the available research does not find harmful effects in children given soy at an early age.64 65
While doing research and gathering material for this article, I noticed something. A substantial number of the reviews highlighting positive aspects of soy and soy protein are funded by the soy industry. In particular, the name Mark Messina shows up again and again, as the author of one article after another. He is the Executive Director of the Soy Nutrition Institute.66
What does this say about the quality and reliability of the available research? I don’t know. Maybe nothing, maybe a lot. I am not qualified to analyze that aspect. However, I think it’s something worth mentioning.
Mr. Messina is rarely involved in the studies themselves, but his contributions to reviews and articles summarizing those studies are all the more substantial.
This phenomenon is not isolated to soy research. On the contrary, it’s common practice in food and protein science. For example, the dairy industry often funds studies investigating the effects of whey or casein protein in athletes. It seems to happen to a surprisingly larger extent when it comes to soy, though, as far as I can tell.
- The nutrient composition of the soybean is not like that of other legumes. It provides you with more protein, fewer carbohydrates, and more fat, including fat from both omega-3 and omega-6.
- Soy protein supports life and health just as well as animal protein and is an excellent alternative if you chose not to eat protein from the animal kingdom.
- The quality and ability of soy protein to support muscle protein synthesis are somewhat lower than that of, for example, milk proteins. This difference does not translate into inferior muscle growth in the long run. To be on the safe side, you can counteract it simply by eating larger servings of soyfood and soy protein.
- Soyfood contains isoflavones in abundance. Isoflavones have several purported positive health effects. These have some scientific support, but the quality of the evidence varies.
- There is no evidence that soyfood and soy protein exert a feminizing effect in men. Also, research lends little to no support to claims that soy consumption leads to lower testosterone levels.
Soybeans are great sources of nutrients and natural chemicals with potentially beneficial health effects. They provide plenty of high-quality protein, suitable for strength-training individuals looking to gain muscle mass and strength. Soyfood is associated with health benefits like a lower risk of several types of cancer, an improved lipid profile, protection from heart disease, and reduced frequency and intensity of menopausal symptoms.
Controversial health issues associated with soy, like an increased risk of breast cancer in women and male feminization, lack convincing evidence. In some cases, the opposite seems to be the case.
Soy is a controversial topic. If you read or hear that soy or soy protein gives you cancer or prevents cancer (both camps have their proponents), be wary of taking everything you see as facts at first glance. Remember that any studies supporting dramatic claims like that are usually correlational studies that can’t prove any kind of cause and effect.
If you’re looking for a plant-based source of quality protein, you’ll find it in the soybean and products based on the soybean. Soy protein is one of the very few choices you have if you want a vegetable protein that is complete on its own. Soybeans not only provide you with a nutritious protein source, but you might even gain some health benefits as well.
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