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Does Soy Protein Impact Thyroid Function?

The topic of soy protein tends to bring up lots of argument and debate and certainly there is some good reason for this.   Depending on a huge number of factors, soy protein can be good, bad or neutral.  In general, the extreme stances are just that and the reality lies in the middle.  But the topic of today is not soy protein in general (discussed in detail in The Women’s Book Vol 1).  Rather, I want to examine if soy protein impacts thyroid function.

To examine the topic, I want to look at the following recent review:

Messina M, Redmond G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid. (2006) 16:249-58.

A Primer on Thyroid Hormone Production

To give readers a brief background on the topic, the thyroid gland releases two primary hormones T4 and T3 (thyroxine and trio-iodothyronine respectively) in a ratio of roughly 80:20 in response to the signal sent by TSH (thyroid stimulating hormone).  That is to say, most of the thyroid released from the thyroid gland itself is the relatively inactive T4.  Most T3 is actually made in other tissues (especially the liver but also in many other cells) from the metabolism of T4 via an enzyme called 5′-deiodinase.

Now, I imagine most readers think of T3 in terms of its effects on body weight or body fat and it’s certainly true that T3, along with the catecholamines (adrenaline/noradrenaline or epinephrine/norepinephrine depending on which side of the pond you’re on) are two of the primary regulators of human metabolic rate.  Of course, thyroid controls about a billion other things in the body too and, as one example, low T3 status can cause depression.

I should mention that iodine intake plays a crucial role in thyroid metabolism with inadequate intake of iodine causing thyroid problems.  There are many other micronutrients that are involved in this conversion process as well; these include selenium and iron (iron deficiency can impair thyroid conversion, yet another reason to eat red meat while dieting).

Soy Protein and Phytoestrogens

Now soy proteins are known to contain hormonal mimics called phytoestrogens. This include genistien, daidzein and others.  A great deal of controversy exists over the impact of soy phytoestrogens in the human diet.  While phytoestrogens may have some beneficial effects (especially in post-menopausal women for whom low estrogen can predispose towards heart disease and bone loss) other research shows negative impacts.  A lot of whether positive or negative impacts are seen depends on what is being looked at, what population is being examined, and what dose is being given.

The effect of phytoestrogens in men is far less studied and understood.  While many are concerned that the phytoestrogens present in soy may negatively impact on testosterone levels the reality is that the studies done to date, using moderate doses of soy/phytoestrogens, have found little to no impact (higher doses are often seen to cause issues).

There is likely to be a sex and population specific response to these compounds and whether or not soy has an impact on anything at all depends heavily on the amount being consumed.  Small amounts of soy protein tend to have minimal or no effects on most things studied (such as testosterone levels) while large daily amounts are often seen to have an effect.

There’s an old saw in medicine that the dose makes the poison and this is certainly one of those situations.

Invariably backlash against soy protein occurs for the same reason it occurs for other nutrients.  Human nature dictates that whenever something is declared healthy that some people will interpret that as “consume as much as humanly possible.”  And soy is no different in this regard.

As soon as soy became a hot nutrient, consumption went up to enormous levels.  Now, as I’ll discuss below, the actual intake of soy foods among Asian cultures is actually not very high.  As well, when intakes of soy protein and phytoestrogens get above a certain point, they can cause problems.  And that’s what happened: people started overconsuming soy, got into problems and decided it was evil.

But the problem was never with the soy protein, it was with their mistaken overconsumption of it.

Effects of Soy Protein on Thyroid Status

So with that background out of the way, on to today’s research review, a review paper on the impact of soy protein on thyroid hormone status and metabolism.  As I noted above, there are two primary thyroid hormones, T4 and T3 and it appears that soy may have an impact on both.

Early work in animals had supported the idea that soy proteins could actually increase thyroid (mainly T4) output and this is likely where a lot of the pro-soy claims come from in terms of thyroid status (e.g. some will claim that soy will help fat loss by raising thyroid hormones).  But results in animals frequently don’t translate to humans and this is no exception.

However, as the paper points out,  studies suggests a very different effect in humans with soy protein having little to no direct impact on thyroid hormone output.  This is yet another place where extrapolating from animal research just doesn’t pan out.  Of course, there is also animal research suggesting a negative impact of soy protein, primarily the phytoestrogens, on animal thyroid status, something the pro-soy folks seem to ignore when they claim that soy will increase thyroid output.

Beyond that, for individuals with normal thyroid function, soy protein appears to have little to no impact on overall thyroid status.  The review examined 14 different studies (8 in women, 4 in men and 2 in both) and, with one exception, found little to no impact of soy intake on any measure of thyroid hormone status.  I’ll spare you all of the details, only the punchline is of any real importance.

In individuals with normal thyroid status, soy protein has no impact.

However, in individuals with pre-existing low-thyroid (hypothyroid) symptoms, soy proteins can cause problems. Research has shown that soy protein intake may increase the dose of thyroid medication needed (the soy appears to impair uptake of thyroid medication) and individuals who are on thyroid hormones may need to avoid soy protein immediately around the intake of their medication.

Another review (Doerge DR. Goitrogenic and estrogenic activity of soy isoflavones. Environ Health Perspect. (2002) 110 Suppl 3:349-53.2002) has shown, using mainly animal work, that the phytoestrogens in soy can impair the enzyme (thyroid peroxidase) responsible for proper thyroid hormone production. That same review found that while soy protein itself could not induce a hypothyroid state, a high phytoestrogen intake coupled with a low iodine intake could.

Again let me note that animal research often does not carry over to humans.  In my experience, it usually doesn’t.

Soy Protein Intake and Iodine Status

I bring up this last point because one of the main providers of iodine in the modern diet is iodized salt and even there, diet surveys have shown a downward trend in overall iodine intake (due to a reliance on processed food and less iodinization of salt).

It’s worth noting that seaweed (another staple in Asian culture) is another good source of iodine. Even if Asian cuisine did contain a tremendous amount of soy, it would seem that the intake of seaweed, by providing iodine, would help to prevent problems from occurring.

In contrast, many “health conscious” people who consume a lot of soy are often found to be minimizing their sodium intake.   A consequence being that they put themselves at potential risk for consuming insufficient iodine.  That combination of an increased soy protein intake and decreased sodium/iodine intake could very well cause problems with thyroid status and metabolism.

But, and this goes to my comments earlier in the article, this is only an issue with people insistent on taking aspects of their diet to extremes.  People who are really consuming a massive amount of soy protein on a daily basis (that intake level not being seen in the Asian cultures in the first place) who also are trying to minimize sodium intake could be putting themselves at potential risk. And this is moreso the case if there is a pre-existing problem.

Does Soy Protein Impact on Thyroid Function?

So we have several different issues at stake here in terms of how soy protein might impact on thyroid hormone status.

Clearly individuals with no pre-existing thyroid problems don’t need to worry much about soy.  And, no, I’m not saying that folks should therefore eat as much of it as possible.  Just that they needn’t go out of their way to avoid any and all source of soy in their diet.

But what about people who do have a pre-existing thyroid problem?

First and foremost, anybody who is on thyroid medication should avoid consuming soy immediately before or after taking their medication as soy protein appears to impair absorption of thyroid medication.  Quite in fact most things seem to impair thyroid medication uptake and it’s probably best to take it on an empty stomach to begin with.    Individuals insistent (for whatever reason) on consuming soy near to the intake of their thyroid medication will need to increase their thyroid dose to compensate.  This, of course, should be dealt with through your medical provider/health professional.

As well, individuals with pre-existing thyroid problems may need to limit their soy intake on a day to day basis.  This is especially the case for individuals intent on reducing their sodium intake.  I’d mention that women are far more likely to have thyroid problems than men which makes them at a higher risk for the particularly bad combination of too much soy and too little sodium/iodine.

My Recommendations for Soy Protein and Phytoestrogen Intake

So what do I recommend so far as soy intake?  Well, mainly avoid the extremes.  Trying to avoid every last bit of soy intake (for example, a typical soy protein fortified cereal may contain a few grams at most of soy) seems misguided to me.

Most studies examining a variety of endpoints find that it’s only when soy protein intake is excessive that any sorts of problems start.  Even in the case of hypothyroid individuals, soy only appears to be a problem when iodine intake is insufficient in the first place.   Either eat some seaweed or make sure you get enough iodized salt.

Clearly, living on nothing but soy foods and soy fortified products is misguided as well.  As I mentioned above, the soy intake among most Asian cultures isn’t actually that massive in the first place and I suspect that much of the backlash against soy is primarily to do with people taking a little of a good thing, assuming a lot was better, and causing themselves problems because of it.

To me a happy medium seems the best; assuming no pre-existing thyroid problems, soy products are probably safe in moderation.  What’s moderation?  In The Protein Book, I suggested a daily maximum of perhaps 20-25 grams of soy protein on a daily basis.   I echoed this suggestion in The Women’s Book Vol 1.

Based on the average phytoestrogen content of most soy proteins (generally 2-3 mg phytoestrogen per  gram of soy protein), that will provide 40-75 mg of phytoestrogens which is right in the sweet spot of what’s been found to have benefits without negatives.

I should mention that many foods are currently being fortified with soy protein (check the labels) and people may already be consuming soy protein in some amounts without knowing it.  Adding more (e.g. through a soy protein powder) may very well take people above the level I suggested above.

Frankly, unless someone is a fairly strict vegetarian or vegan, there are enough other high quality protein sources (such as meat, dairy products, whey, etc.) that I don’t see the need to consume massive amounts of soy in the first place.  But neither do I think it’s a horrible protein that no-one should ever eat because it will make their testosterone drop and give men boobs.

Soy, like all proteins has a variety of pros and cons and, in moderation, can make up part of a healthy or sports oriented diet.  Thinking that it is evil and must be eliminated is as silly as thinking it’s the best protein ever and people should consume tons of it.

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