For as long as I’ve eaten a plant-based diet, a nearly unquestioned assumption has underlain almost every conversation I’ve had about diet:
Soy is bad for you.
Of course, I’ve never really believed it in the strictest sense. Instead, as I often do when it comes to controversy around foods that people have eaten for thousands of years, I decided to believe a softer version:
Too much soy is bad for you.
And so my family has included soy (most often tofu, sometimes tempeh) in our diet over the past ten years — probably about once per week, and without fear that it’s harming our health.
This casual attitude of ours towards soy has worked well enough (no man boobs, yet!). But as I’ve come to think more carefully about diet — especially my children’s — I decided it was time to take a more scientific approach to soy.
Is soy actually bad for you?
Let’s start with the easy part: processed food is bad for you. So the soybean oil and textured vegetable protein that are in so many snacks and vegan meat substitutes aren’t good for you.
Not because they’re soy, but because they’re not whole foods.
But when it comes to less processed forms of soy: tofu, tempeh, or even whole soybeans, it gets more complicated.
The research falls on both sides, but once we look closely at how that research was done the, answer becomes more clear: yes, soy is good for you. It doesn’t cause breast cancer or prostate cancer (in fact, it may help prevent them), and it doesn’t give men man-boobs (no word yet on whether it helps prevent them!).
Let’s see why.
What is soy, anyway?
When soy products are so pervasive on ingredient lists of foods that look nothing like beans, it’s easy to forget that that’s what soy is: a bean.
When you order edamame as an appetizer before your vegan sushi, that is, of course, whole soybeans steamed in their pod.
Eastern cultures have eaten diets high in soy for thousands of years, and contrary to conventional wisdom that says soy is bad, soy is likely a contributing factor to the relatively good health of those cultures compared to that of the West.
This isn’t all that surprising. Soybeans are beans, and beans are incredibly rich sources of nutrients, including fiber, a known cancer-fighter.
But the controversy around soy isn’t without reason: it stems from the fact that soy contains isoflavones, which are a class of phytoestrogens. And estrogen sounds scary.
(It also stems from misinformation spread by the Weston A. Price foundation, according to one theory.)
But phytoestrogens aren’t the same as estrogen, and not all the effects of estrogen or estrogen-like compounds are bad.
According to Dr. Michael Greger:
Estrogen has positive effects in some tissues and potentially negative effects in others. For example, high levels of estrogen can be good for the bones but can increase the likelihood of developing breast cancer. Ideally, you’d like what’s called a ‘selective estrogen receptor modulato’ in your body that would have proestrogenic effects in some tissues and antiestrogenic effects in others. Well, that’s what soy phytoestrogens appear to be.
So the phytoestrogen in soy has much weaker effects than true estrogen, and because the isoflavones can bind to the body’s estrogen receptors, the result is either weak estrogenic or even anti-estrogenic activity.
The result is that soy appears to lower the risk of breast cancer, while also providing many other benefits to most people’s health, including the strengthening of bones, a reduction of hot flashes, a lowered risk of prostate cancer, and protection against cardiovascular disease.
This is very different from the messaging most of us have received from our friend the internet.
Why there’s so much conflicting information about soy
If soy appears to be so good for us, why do so many people say it’s not?
First, because much of the early research on soy was done on mice.
According to the American Institute for Cancer Research (AICR), the breast-cancer promoting effects of soy in early studies were due to the different way in which rodents metabolize isoflavones from the way humans do.
Since then, many studies on humans have shown either a protective effect of soy against breast cancer or no effect at all, so it’s still not entirely clear whether soy is good or simply neutral when it comes to breast cancer risk. (But again, because it’s a bean, soy in unprocessed forms provides some fiber, which is known to fight cancer of many kinds.)
Second, soy acts differently on different people, depending on ethnicity and hormone levels. According to the Harvard School of Public Health, soy may act more like estrogen in pre-menopausal women, and more like an anti-estrogen in post-menopausal women, so studies on different groups yield different results.
For example, it used to be thought that soy might be harmful following a breast-cancer or prostate-cancer diagnosis. But better, more recent studies have shown that it has no effect on the growth of these cancers.
Finally (again according to Harvard Health) the type of soy studied (whole, processed, fermented, unfermented, etc.) may yield different results. However, many people mistakenly assume that processed soy contains more isoflavones than unprocessed soy, when in fact the opposite is often true, as processing removes many of them. Again, it’s not that it’s bad because it’s processed soy; it’s just bad because it’s processed.
Is too much soy bad for you?
Sort of, but not in the way most people think.
It’s not bad because it’s soy… instead, it’s simply that too much of any one food is bad for you!
Eating too much of the same food prevents you from getting a variety of nutrients from other sources. Dr. Joel Fuhrman considers two to four servings of soy per week to be a healthful amount. The AICR defines moderate soy consumption as “1 to 2 standard servings daily of whole soy foods, such as tofu, soy milk, edamame and soy nuts,” and considers this amount healthy — and possibly even protective for breast cancer survivors a year after diagnosis.
Is soy bad for men? Does soy increase estrogen in males?
No, as long as it’s consumed in moderation. According to Dr. Fuhrman, the man-boobs hysteria (my phrase, not the good doctor’s) can be traced to a single case of a man who experienced breast swelling — after consuming three quarts of soy milk every day for six months straight! (And the issue, by the way, resolved after the man stopped drinking so much soy milk.)
Soy protein powder and IGF-1
Soy protein is known to be the most complete of plant proteins, meaning it provides all 9 essential amino acids in similar ratios to animal proteins. But for a while, there was some concern that the “completeness” of soy protein (and the concentration of this protein in soy protein isolate) led to increased levels in the bloodstream of a hormone called insulin-like growth factor 1 (IGF-1) in the blood just like animal protein, which has been linked to growth of cancerous tumors.
However, it has since been established that soy and other plant proteins do not significantly raise levels of free IGF-1 in the bloodstream, because they tend also to increase IGF-binding proteins along with IGF-1.
Still, even though it doesn’t raise serum IGF-1, remember that soy protein powder is not a health food in the way that unprocessed soy is. For certain types of people, supplementation with protein powder might be the best option, but it’s never as healthy as getting your protein from whole foods.
Which brings us, finally, to the question nearly every vegan wonders about at some time or another…
Is tofu healthy?
Yes! Dr. Michael Greger considers tofu and tempeh, which undergo mild processing, to be about half as nutritious as whole beans… but because whole beans are so healthy, he recommends tofu and tempeh as health foods.
And since, as we’ve established above, the isoflavones in soy are nothing to fear, this means we can eat our beloved tofu several times a week (maybe even more than that) without worrying about it.
But no, this still doesn’t make our even-more-beloved takeout Thai coconut curry with deep-fried tofu a good idea. Once in a while, but not every day.
And so, we see that that old strategy of moderating — not avoiding — controversial foods has worked again.
And in fact, in the case of soy, I probably didn’t need to feel guilty about those busy weeks here and there when, as a matter of convenience, we ate tofu stir-fry three or four times for dinner and had it leftover a few times for lunch.
Soy shouldn’t be the center of your diet. No single food should be. But as part of a varied, whole-food-centric plant-based diet, we have our answer: soy isn’t bad for you, it’s good.