does soy cause man boobs? the scientific term is gynecomastia and it can be caused by exposure to
chemicals or hormones. but can eating soy also cause gynecomastia? breasts on men? soy contains
molecules called phytoestrogens. 'phyto' means from plants and estrogen of course after the female
hormone. phytoestrogens get their name because their chemical structure is similar to estrogen
but the man boob story really took off in 2008 after a couple doctors in Texas described a
case of gynecomastia in a man drinking a lot of soy milk. three quarts a day which
is almost three liters. scientists have disputed the cause of gynecomastia in this isolated case
which is not necessarily due to the phytoestrogens in soy. i suspect in that
individual it probably was related to the soy drink in some way because when he stopped it
his gynecomastia resolved, so let's say it was the phytoestrogens for the sake of the argument
how common is this? gynecomastia tied to soy consumption? it's so unusual they named the report
"an unusual case" and just in case the title wasn't clear enough their conclusion states it again: "this
is a very unusual case". i searched the medical literature and i could only find a couple cases
ever described tying gynecomastia to soy. there is a 2020 report of an eight-year-old boy with
gynecomastia who was eating a lot of processed soy foods: 10 to 12 soy nuggets a day 4 days a week,
soy crumbles 2 days a week and one to two cans of soy chili 2 days a week. and just
like the Texan man, it completely reversed when he changed his diet. that's probably why this is so
unusual, you need the combination of a particularly sensitive individual together with an unusually
huge level of consumption and the two together, perfect storm. see, the problem is most reputable
sources will just tell you, yeah soy's a healthy food, it's a good idea to include some in your diet.
and that's the gist of it, that's good information, the problem is it doesn't cover the oddities so
it leaves the door open for confusion when people hear these stories online and go, oh,
it gave this dude in Texas man boobs once, my doctor didn't tell me that! and confusion sets in
when we put all of the information in context, number one, it's incredibly rare. billions of
people throughout the world eating soy regularly and we literally have a couple cases described,
ever. number two, in both those cases it's tied to processed soy foods, not soy itself. number three, it
seems to require a huge amount, unusual levels of consumption. and number four, even in those
rare cases where we do see it, it's completely reversible. now, one argument you'll hear sometimes
is, if soy caused gynecomastia we'd have millions of japanese men walking around with double Ds
but of course japanese men don't have breasts. well, most don't. okay, but maybe there are genetic
differences. maybe Western man can't handle the phytoestrogens as well as Asian men. possible. this
is why trying to answer these nutrition questions by comparing entire countries is really tricky
because a thousand things change. we need something more firm. yeah bad choice of words. we need better
support. nope, still weird. we need more data. there we go. careful studies with actual controls. lucky
for us, those have been done. this randomized trial took 200 western men and fed them soy
with or without phytoestrogens and there were no changes in volume of breast tissue in either group
okay, maybe adults can handle the phytoestrogens but kids are more sensitive because they're
still developing? this study looked at children drinking soy formula compared to a control group
that was not fed soy. none of the boys developed gynecomastia and none of the girls showed signs of
early puberty. they conclude that long-term feeding with soy protein formula in early life does not
seem to produce estrogen-like hormonal effects. okay, maybe eating soy doesn't give me man boobs
but that's a pretty high bar. how about more subtle changes? maybe it just reduces my testosterone? in
fact some animal studies suggest that. rats fed phytoestrogens have lower testosterone
animal studies raise interesting questions but we always need to look at human data. it's common for
something to be beneficial in animals but harmful in humans or vice versa. plus lab experiments
usually use the purified molecule which doesn't really reflect the foods we eat, and they usually
expose the animals to huge doses. in the rat study we just looked at the amount of phytoestrogens
that lowered testosterone was extremely high compared to normal human consumption. okay, so what
happens in human trials? does soy and phytoestrogen lower testosterone? that's the question. this large
analysis of 32 human trials found no significant effect of soy protein or isoflavone (that's the
phytoestrogens) on testosterone level. neither soy protein nor isoflavones affect reproductive
hormone concentrations in men even when intake greatly exceeded typical dietary japanese intake
and they included trials looking at soy foods like tofu and soy milk, soy grits, soy flour as well as
phytoestrogen supplements, so it was a pretty good coverage. this overview of the literature
reached a similar conclusion: in contrast to some rodent studies (we looked at those), neither
isoflavone supplements nor soy itself affect testosterone levels. similarly, there is essentially
no evidence that isoflavone affects estrogen levels in men. they conclude that isoflavones do
not exert feminizing effects on men at intake levels equal to and even considerably higher
than typical for asian males. and they really drive the point home: men can feel confident that making
soy a part of their diet will not compromise their virility or reproductive health. so while it is
true that soy contains phytoestrogens that kind of look like estrogen, the actual physiological roles
are very different. in fact, in clinical trials phytoestrogens have no effect on many biological
processes known to be affected by estrogen. in some cases, phytoestrogens can even have the opposite
effect of estrogen. so we always need to look at the actual health effect of a food in living
breathing humans, not assume from mouse data, petri dish experiments or chemical structures. you know,
it's really funny that people associate soy with gynecomastia because there's other foods linked
to a lot more cases. for example, gynecomastia was reported in young boys in Puerto Rico and they
suspected meat samples with higher estrogen levels. and it wasn't one isolated individual,
the report wasn't called "an unusual case", it was called "an epidemic". another outbreak in Italy with
breast enlargement in very young girls and boys with poultry and beef suspected to be the cause
and again it wasn't one case, once again they called it "an epidemic". of course in the grand
scheme that's still super unusual, billions of people eating chicken and beef regularly so those
cases are a tiny fraction, but it's just funny how these myths arise with some foods, in some
cases, but not others, no rhyme or reason. sometimes it's cultural or preconceived notions more than
evidence per se that propagate these ideas. man boobs are an extremely common issue in our society
but they're not coming from soy. as this article put it, men who want to avoid fat tissue on their
chest might want to avoid fast food in general rather than pinning their anxieties on a vegetable
so for most of us soy is health promoting, we'll talk about exceptions like allergies in a second
but for majority of us, men or women, it's fine to include it in your menu, including daily if you
want. ideally favor the minimally processed forms like edamame, tempeh, tofu. ultra processed forms
like imitation meats are okay here and there but avoid making it your number one go-to for protein
some soy milk is okay too, most people are not gonna have three liters every day so i don't think
it's a big problem. now, if you're allergic or you can't have soy for some reason that's okay, soy is
not essential, just like every other food, you can have a very healthy dietary pattern without soy
you're fine, just have the other 20 types of legumes. all right guys, we focused on gynecomastia
today but we have a lot more soy-related content in the pipeline so let me know your questions
below, we're going to try to touch on all the major issues. here's some more detail on soy-based
imitation meats and for other ideas here's my top 7 sources of plant protein. see you next time