Archive for the ‘Soy’ Category

Soy and Childlessness

Thursday, May 15th, 2014
Summary A cross-sectional study from AHS-2 has found an association between eating more soy and an increased rate of not becoming pregnant or having a child. The study was cross-sectional and had some limitations. It should not be considered proof that soy causes childlessness.

A report from Adventist Health Study-2 was recently released examining the association between soy intake (measured as isoflavones) and childlessness (1).

The authors write:

“The results suggest that when the isoflavone intake exceeds approximately 40 mg per day, the overall lifetime risk of never becoming pregnant increased by 13% and that of ever giving birth to a live child was reduced by approximately 3%. The associations were consistently found to be stronger in women who reported having had difficulties in becoming pregnant (1 straight year or more without success to become pregnant).”

40 mg of isoflavones per day is the equivalent of about 2 servings of soy per day.

There are some reasons not to conclude that soy causes childlessness:

• The author’s stated that, “One major limitation in our study is that we are not able to distinguish with certainty between involuntary and voluntary childlessness.”

• The finding for never becoming pregnant was 1.13 (1.02, 1.26) for women eating 40 mg of isoflavones per day compared to those eating only 10 mg. This is barely statistically significant.

• It was a cross-sectional study with relatively weak findings. Cross-sectional studies can indicate what should be studied prospectively and in clinical trials, but cannot prove causation.

• The researchers did a lot of testing with many different models, the results of which didn’t always agree. The authors stated in their discussion, “[G]iven the somewhat exploratory nature of our analyses and the many statistical tests conducted, we do not believe that [an inconsistency they found] deserves much emphasis at this point.” Indeed – that could apply to their main finding.

Despite the limitations, if someone is having a hard time becoming pregnant, cutting out the soy might be something to try.

The study is linked to in the reference below and is free to the public.

I have updated the VeganHealth.org article, Soy: What’s the Harm?

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References

1. Jacobsen BK, Jaceldo-Siegl K, Knutsen SF, Fan J, Oda K, Fraser GE. Soy isoflavone intake and the likelihood of ever becoming a mother: the Adventist Health Study-2. Int J Womens Health. 2014 Apr 5;6:377-84. doi: 10.2147/IJWH.S57137. eCollection 2014. | link

VeganHealth Update: Hexane in Soy

Friday, April 11th, 2014

 
Just added two links to both Soy What’s the Harm: Hexane and Response to Not Soy Fast:

Hexane in Soy Food – Berkeley Wellness | May 01, 2012
Do Veggie Burgers Contain Hexane? – Shereen Jegtvig, MS | Updated Feb 07, 2014

Thanks, Brenda!

Soy Protein, Weightlifting, and Testosterone

Monday, December 30th, 2013

A reader asked me to comment on a study comparing soy protein, whey protein, and a carbohydrate placebo and their effects on hormones (1).

The study was mainly conducted to see if soy increased estrogen levels. Unsurprisingly, it did not.

On all 3 supplements, testosterone levels went up during the workout. But compared to both whey and carbohydrate placebo, testosterone levels were not sustained as high post-workout. And cortisol, which can break down muscle tissue for energy, was higher on the soy and carbohydrate regimens than on whey.

The study was done on 10 young men who took each supplement for 2 weeks, with washout periods between regimens. The supplements were taken 30 minutes before a bout of exercise: 6 sets of squats, 10 repetitions each, 2 minutes rest between sets, weight was 80% of their maximum. The hormones were measured before, during, and at a number of times after the exercise up to one hour.

All 3 supplements were 80 calories worth of energy.

It’s possible that the lower testosterone levels are a result of either the much lower amounts of leucine in soy versus whey protein (about 39% lower) or due to the isoflavones. The authors of the study say, “The majority of evidence from previous research on isoflavones alone has shown no effect on testosterone,” making me think it’s the lower leucine leading to lower testosterone.

As for the cortisol, that soy didn’t perform as well as carbohydrate is odd and might be due to the fairly intense workout, soy and carbohydrate caused higher cortisol levels. When I lift weights, I try not to push my muscles to the point where I’m breaking down muscle for a significant portion of energy, and six intense sets of squats to 10 reps seems like a lot. The carbohydrate, or the extra leucine in the whey (which muscles can use for energy), could have been enough to prevent muscle from being degraded for energy.

It was not clear from the write-up whether the participants were blinded to which regimen they were on, though I assume they were. If not, that could have played a role.

More research is needed – we don’t even know if this would result in a noticeable difference in strength gains, particularly on a different weight-lifting program that isn’t quite so focused on so many reps for the same muscle groups.

But if you are a serious weightlifter and this worries you, adding some leucine to your soy protein powder, or a non-soy protein powder, might be the way to go. Or perhaps just carbohydrate instead, to prevent the cortisol response.

The Vegetarian Resource Group has information on where leucine is sourced in their 2011 article, Leucine, Isoleucine, and Valine May Be Derived from Duck Feathers or Human Hair.

Addendum 12-31-2013:

For testosterone, whey and carbohydrate achieved similar results. Soy was not as high from 5 to 30 minutes post exercise, but was about the same at 60 minutes. Although the differences were statistically significant, they were not all that great – the biggest difference was about 30% between soy and whey.

For cortisol, I had said that soy didn’t perform as well as carbohydrate, but the differences were so small that I shouldn’t have said this. Soy and carbohydrate resulted in statistically significant, higher levels than whey at 5, 15, and 30 minutes post exercise. The biggest difference between soy and whey was about 45%.

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References

1. Kraemer WJ, Solomon-Hill G, Volk BM, Kupchak BR, Looney DP, Dunn-Lewis C, Comstock BA, Szivak TK, Hooper DR, Flanagan SD, Maresh CM, Volek JS. The effects of soy and whey protein supplementation on acute hormonal reponses to resistance exercise in men. J Am Coll Nutr. 2013;32(1):66-74. | link

Meat Alternatives Associated with Lower Hip Fracture Risk in AHS-2

Thursday, October 10th, 2013

Today was a good day: As longtime readers of this blog will know, I love getting good news about my “bad” habits. :)

A study came out that fits in well with past findings, is well-written, and supports my proclivity to eat and recommend soy meats!

It was a report from Adventist Health Study-2 (AHS-2), conducted among caucasian Seventh-day Adventists living in the USA, comparing the intakes of many different foods and hip fracture risk after 5 years of follow-up (1).

First, there was a bit of bad news in that vegans had the highest rates of hip fracture at 3.0 per 1,000 person-years compared to 2.0 for non-vegan vegetarians (including semi-vegetarians) and 1.6 for non-vegetarians. The paper didn’t report whether this finding was statistically significant, but it was not a great trend, obviously. Read on for how to reduce your chances.

Here are findings from the fully adjusted model (2):

– Meat alternatives once a day or more (compared to less than once per week) were associated with a 66% reduced risk of hip fracture in the vegetarians (.34, .12-.95).

– Eating legumes once a day or more (compared to less than once per week) was associated with an 82% reduced risk in non-vegetarians (.18, .06-.54) and an 55% reduced risk in vegetarians (.45, .22-.94).

– Meat more than 3 times per week was associated with a 45% reduced risk in the non-vegetarians (.55, .36-.83), compared to less than once per week.

– Dairy, nuts, soy milk, and “tofu & soy cheese” were not associated with a lower risk. The average amount of tofu & soy cheese per day was only .1 among the vegetarians. They did drink close to a cup of soymilk per day (or “soya milk” as the authors, who apparently think they live in Europe, call it).

The authors emphasize the need for vegans to eat legumes in order to get enough of the essential amino acid lysine:

“[A]n individual who adheres to a vegan diet, which excludes meat and dairy products, will need at least two cups of cooked beans [per day] to meet the recommended lysine intake requirement…Lysine and hydroxylysine are the main amino acids in the cross-linking process of bone collagen…Lysine can also influence bone health through its end product carnitine. Carnitine supplements have been shown to improve bone density in some animal and human studies.”

Note that the requirement for 2 cups of cooked beans assumes no other lysine sources in the diet, which isn’t the case. You can can read more on lysine needs and how to meet them in Protein.

They go on to say:

“Among our participants, intake of meat analogues of at least one serving daily reduced the risk of hip fracture by up to 49%…The main protein ingredients in meat analogues are soya, wheat, gluten, eggs and milk. A typical serving of meat analogues (1 serving ~73 g in AHS-2) contains at least 10 g of protein, but can vary from 9 to 18 g.”

Or 30 g as Tofurky Italian sausage has!

They point out that their finding for meat being protective is backed up by other research but not all. After a brief analysis of the research they suggest that meat intake is associated with bone health when protein intakes are low. And, finally:

“Protein is recognized for its ability to improve [calcium] balance, suppress parathyroid hormone, increase lean body mass and increase production of the bone growth regulator insulin-like growth factor-1.”

This study had something for everyone – low-fat proponents can relish in the findings for legumes and meat alternative eaters can smugly continue in their bad habits!

I have not yet updated the VeganHealth.org article, Calcium and Vitamin D, with this new study yet, but hope to do so in the next few days.

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References

1. . Lousuebsakul-Matthews V, Thorpe DL, Knutsen R, Beeson WL, Fraser GE, Knutsen SF. Legumes and meat analogues consumption are associated with hip fracture risk independently of meat intake among Caucasian men and women: the Adventist Health Study-2. Public Health Nutr. 2013 Oct 8:1-11. [Epub ahead of print] | link

2. *Adjusted for fruits and vegetables intake, age, height, weight, gender, energy intake, physical activity, smoking, health status and total calcium intake.

American Cancer Society on Soy & Breast Cancer

Tuesday, April 30th, 2013

If you’ve read my articles on soy (Response to Not Soy Fast, Soy: What’s the Harm), there’s nothing new here, but it’s good to hear it from an organization like the American Cancer Society (ACS):

The Bottom Line on Soy and Breast Cancer Risk
August 02, 2012
By Marji McCullough, ScD, RD

Excerpt:

“Bottom line: Even though animal studies have shown mixed effects on breast cancer with soy supplements, studies in humans have not shown harm from eating soy foods. Moderate consumption of soy foods appears safe for both breast cancer survivors and the general population, and may even lower breast cancer risk. Avoid soy supplements until more research is done. So, enjoy your occasional tofu stir-fry or tofu burger – they are unlikely to increase your risk of breast cancer and, on balance, are some of the healthier foods you can eat!”

Warning: There are the typical Weston Price Foundation-type comments after the article – the myths about Asians eating only fermented soy and fermented soy being significantly different than other soy. Too bad they allow comments which will possibly just serve to scare people. And I realize that I’m saying that as someone who allows comments on my own blog, but it’s one thing to be an RD blogger and another to be the strategic director of nutritional epidemiology for the ACS. Of course, that doesn’t mean Dr. McCullough is infallible, but if someone whose opinion is worthwhile has an objection to her article, they can contact the ACS behind the scenes and she can correct the article if they have a valid point. Just my two cents!

(Thanks, Matt.)

Dr. Oz on Soy

Thursday, March 21st, 2013

I have never watched the Dr. Oz show, but two days ago I got a phone call from my mother saying that Dr. Oz was telling people that processed soy can mess up your hormones.

Nerdles.com provides a run down on the show that includes:

“Now the #1 thing to avoid to keep your hormones in balance is processed soy. Processed soy is stripped of all the healthy omega-3s, fiber, and are ultimately left with carbs and soy protein isolate. Inside soy protein isolate, you’re finally left with estrogen and high-levels of this will interrupt your cycle….The gold standards of soy are miso, natto, and tempeh – get these into your diet instead for healthy hormone levels.”

But processed soy does not contain more isoflavones (the “soy estrogens”) per serving than unprocessed soy, so this doesn’t make much sense. Processing soy protein concentrates the protein, not the isoflavones.

And on Dr. Oz’s website, in the article, 4-Week Cyclical Hormonal Health Plan, they bring out the old Weston Price Foundation talking points, “Traditionally, Asian cultures consume no more than two teaspoons of fermented soy a day, which has been shown to be health-promoting, while more than that quantity becomes problematic.”

The idea that Asians limit their soy to two teaspoons a day, and that it is all fermented, is not backed up by the research as reviewed in my article, Soy: What’s the Harm? – Asian Intakes. By the way, two teaspoons of soy is only 1/12 of a typical 1/2 cup serving of tempeh – not even a serving per week.

Nor is there any evidence that more than two teaspoons a day is problematic. The entire article, Soy: What’s the Harm? is a discussion of how much soy is safe (or you can see the much abridged version, Response to Not Soy Fast).

Since I have not reviewed the large amount of research on soy and menstrual cycles in any rigorous way, I asked Dr. Mark Messina, one of the world’s leading soy experts, to comment on the statement that soy will interrupt your menstrual cycle and he said:

“Soyfoods and the phytoestrogens in soy have relatively minor effects on reproductive hormone levels in women. There is evidence indicating that soy may increase the length of the menstrual cycle by approximately one day. However, when only high-quality studies are considered, the effect of soy is no longer statistically significant. Whether a possible effect of soy on menstrual cycle length has clinical implications is unclear although longer cycles are associated with a lower risk of breast cancer.”

In addition to the information on soy, that entire article, 4-Week Cyclical Hormonal Health Plan, sounds pretty far-fetched.

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Soy vs. Cow’s Milk and Bone Mineral Density Take Two

Wednesday, March 6th, 2013

Yesterday, I wrote about a clinical trial comparing soy vs. cow’s milk’s impact on bone mineral density (BMD) in Chinese women (link). To sum it up, the cow’s milk fared better than soy, which fared better than nothing.

Today I found a prospective study from the Adventist Health Study-2 (2011) comparing soy vs. cow’s milk (1). They followed 337 post-menopausal, Caucasian women, who had not been previously diagnosed with osteoporosis, for two years. 61% were considered vegetarians because they ate meat less than once a month.

The authors sum up the results:

“Those drinking soy milk at least once a day or more (highest category) were 57% less likely to have osteoporosis than those who never used soy milk [.43 (.21–.89)]. Likewise, those eating dairy products at least once a day or more were 62% less likely to have osteoporosis than those consuming dairy less than twice a week [.38 (.17–.86)].”

There was not a separate soymilk group and cow’s milk group, both findings were comparing the entire group of women to each other, though the results for each milk were adjusted for intake of the other milk.

The researchers did not measure total calcium intake, but they seemed to assume that most of this soymilk was calcium-fortified. They also found no correlation between calcium or vitamin D supplement usage (amounts not measured) and better bone health.

Interestingly, cheese was the only dairy product that, when separated out, was correlated with better bone health (the other categories were “milk,” and “yogurt/ice-cream”). The authors speculated that it was due to the higher calcium and protein content of cheese.

In conclusion, it appears to be about equally beneficial for bone health for Caucasian, postmenopausal women to be drinking either soymilk or cow’s milk.

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Reference

1. Matthews VL, Knutsen SF, Beeson WL, Fraser GE. Soy milk and dairy consumption is independently associated with ultrasound attenuation of the heel bone among postmenopausal women: the Adventist Health Study-2. Nutr Res. 2011 Oct;31(10):766-75. | link

Soy vs. Cow’s Milk and Bone Mineral Density

Tuesday, March 5th, 2013

I’m going through my backlog of research on bones in order to write a reader-friendly version of the tome that is Bones, Vitamin D, and Calcium on VeganHealth.org.

In January 2012, a report from China was released of a randomized clinical trial comparing the effects of cow’s milk and soymilk on bone mineral density (BMD) in postmenopausal women (1).

The women were divided into three groups: soy, cow’s, and control. The women knew which treatment they were receiving, while the control group got nothing. The milk and soymilk had 250 mg of calcium and they drank one serving per day.

BMD was measured in the spine, hip, and femur at 6, 12, and 18 months.

In the cow’s milk group, BMD increased in the hip and femur. In the soymilk group, BMD did not change significantly, but showed a downward trend in all three locations. In the control group, there was a downward trend in all three locations that only reached significance in the femur at 18 months.

It should be strongly noted that the participants were asked to avoid calcium from other milks and supplements during the trial which means their total calcium intake was probably quite low and I’m somewhat amazed that their BMD didn’t decrease more or in all three groups. Total calcium intake during the study period was not reported.

Compliance was 93.1% in the cow’s milk group and 82.5% in the soymilk group. This might explain a small amount of the differences between those groups. Other possibilities are that calcium in fortified soymilk is known to quickly settle to the bottom of the milk where it might not be ingested but left in the container, and that milk contains phosphorus and growth factors that might increase BMD.

This is just one small study and more work needs to be done before any conclusions should be drawn.

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Reference

1. Gui JC, Brašić JR, Liu XD, Gong GY, Zhang GM, Liu CJ, Gao GQ. Bone mineral density in postmenopausal Chinese women treated with calcium fortification in soymilk and cow’s milk. Osteoporos Int. 2012 May;23(5):1563-70. Epub 2012 Jan 27. | link

Soy Allergies

Tuesday, December 18th, 2012

This is an article written for the Food Empowerment Project’s Food Chain newsletter Issue #11 (not yet published).

Soybeans and their products are often common ingredients in plant-based diets. Soyfoods include edamame, tofu, tempeh, soymilk, soy meats, soy ice cream, soy-based mayonnaise, miso, soy sauce, and many others. Soy is also one of the richest sources of protein in the plant kingdom. Because soy is so common, people with soy allergies sometimes wonder how they can possibly be vegan.

A true soy allergy can result in hives, itching, swelling, wheezing, and digestive upset. In very rare cases, a soy allergy can be life threatening and require immediate medical attention.

Soy allergies occur in 1 to 2 out of every 1,000 people. Soy allergies are often developed in infancy and many children grow out of the allergy as they age. In comparison to other typical food allergies, soy allergies tend to be slightly less prevalent and also less intense (on average).

If you think you might have a non-severe soy allergy, one way to test it is to stop eating soy for 3-4 weeks, and then try a small amount of soy and see what happens in the next 24 hours. If you think you have a severe soy allergy, you should not try soy without supervision by a medical doctor.

The good news is that although soy is very common in plant-based diets, it is not necessary. Many vegans have existed without soy. Other than soy, the best sources of protein are other legumes. Legumes include garbanzo beans (chick peas) which are used to make hummus and falafel, pinto beans used in most refried beans and burritos, black beans, lentils, split peas, green peas, black eyed-peas, and peanuts. Quinoa, seitan, and pistachios are also high sources of protein for vegans. If you eat a few servings of these foods each day, you should be getting plenty of protein.

And remember that in addition to soymilk, there is rice and almond milk, and in addition to soy ice cream there is rice, almond, and coconut ice-cream. Check labels as some might contain some soy. Daiya cheese, one of the most popular non-dairy cheeses that melts when heated, contains no soy. Field Roast meats also contain no soy.

References

Cordle CT. Soy protein allergy: incidence and relative severity. J Nutr. 2004 May;134(5):1213S-1219S. | link

Soy Allergy. MayoClinic.com. Accessed 12/12/2012. | link

Tyler, Steve. Estimating Prevalence Of Soy Protein Allergy. SoyConnection.com Accessed 12/11/2012. | link

Response to Not Soy Fast

Tuesday, April 5th, 2011

This article is a response to Not Soy Fast, written by Kristin Wartman (December 9th, 2010). It also appears on CivilEats as Another Look at the Evidence on Soy.

More details on every topic below, as well as many others, can be found in the companion article to this piece, Soy: What’s the Harm?, just published on VeganHealth.org. The reference numbers in the article below link to the references of Soy: What’s the Harm?

I approached each subject regarding soy without a foregone conclusion, trying to get to the truth rather than putting a positive spin on the results. While I think there is a limit on how much soy is safe to eat each day, that limit has not yet been determined. There is a great deal of evidence, however, that unless you have thyroid issues or are allergic to soy, two servings per day is perfectly safe.

I want to give special thanks to Jean Bettanny for her careful editing of Soy: What’s the Harm?


In her article, Not Soy Fast, Kristin Wartman argues that “…the research is mounting that soy foods are not only questionable in terms of their benefits, but in fact, may be hazardous to your health.”

Wartman describes the Cornucopia Institute’s recent report in which they describe finding residues of hexane in some soy food ingredients. The Cornucopia Institute gave few details about how much hexane they actually found and there is no evidence that the amounts typically found in soy foods are harmful to consumers. However, in the interest of worker and environmental safety, as well as trying to limit any potential harm from hexane residues, I cannot fault anyone for avoiding soy products produced with hexane. As Wartman points out, there are companies that make alternative meats without using hexane, such as Tofurky and Field Roast. (Also see Hexane in Soy Food and Do Veggie Burgers Contain Hexane?)

But this is where Wartman and I part ways. While there are legitimate concerns regarding soy, Wartman cherry-picked the studies and ignored the vast majority of research. For the topics in this article, I will cover the full range of research findings, both pro and con.

A little background: Soy contains isoflavones which have the ability to bind to estrogen receptors and can affect thyroid hormone (especially if someone has iodine deficiency). There are about 25 mg of isoflavones in one serving of soy.

Breast Cancer

Wartman implicates soy as a cause for breast cancer, mentioning only one study. Unfortunately, she didn’t cite the study correctly, so it is not clear to which she was actually referring. In any case, here is a run down of the research.

Case-control studies on soy and breast cancer have been generally encouraging to those with soy in their diets, with about half associating soy with a lower risk for breast cancer and the other half showing no effects.

Prospective studies, which are generally a higher level of evidence than case-control, have also been very positive. Of the six studies done on populations with higher soy intakes (about one to two servings per day is the typical upper intake amount), the Singapore Chinese Health Study (21), the Shanghai Women’s Study (22, 23), and the Japan Public Health Center study (26) all found that higher intakes of soy were associated with a reduced risk. The Japan Collaborative Cohort Study (17) and the Japan Life Span Study (30) found no association. The European Prospective Investigation into Cancer-Oxford (24), which contained a large number of vegetarians, also found no association. Regarding the lack of association in the European study, some have speculated that in order to receive benefits from soy, exposure must occur during adolescence when breasts are developing, while Western vegetarians often adopt the diet as adults.

As for women with breast cancer, including those with tumors that grow in response to contact with estrogen (known as estrogen receptor positive), the authors of the recently published Women’s Healthy Eating and Living Study (121) write:

Our study is the third epidemiological study to report no adverse effects of soy foods on breast cancer prognosis. These studies, taken together, which vary in ethnic composition (two from the US and one from China) and by level and type of soy consumption, provide the necessary epidemiological evidence that clinicians no longer need to advise against soy consumption for women diagnosed with breast cancer.

Also see The Bottom Line on Soy and Breast Cancer Risk by Marji McCullough, ScD, RD of the American Cancer Society.

Dementia

Wartman cites the 2000 Honolulu-Asia Aging Study, saying that it linked soy with an increased risk of Alzheimer’s disease and brain shrinkage. Actually, this study does not mention Alzheimer’s Disease, although it did measure cognitive function. Let me sum up the evidence on soy and mental cognition.

There have been twelve short-term (lasting one week to a year) clinical trials looking at the impact of soy on cognition, and all have shown soy to be helpful (44, 45, 48, 49, 47 50, 54, 55, 56) or neutral (51, 52, 57).

Epidemiological studies (unlike clinical trials), examine patterns of soy consumption and cognition in specific populations. One such study found tempeh (a fermented soy food) to be associated with improved cognition (9). Three reports from epidemiological studies have associated tofu with reduced cognition in some groups (2, 9, 53), but increased cognition in another group (42), and neutral in others (42, 53). The harmful findings for tofu in the epidemiological studies are likely due to confounding caused by the fact that people of lower economic status have traditionally eaten more tofu in Asian cultures as well as the fact that some tofu has been prepared using formaldehyde (at least in Indonesia from where some of these reports have come). The research as a whole provides little cause for concern.

Infant Formulas

Wartman suggests that soy-based infant formulas are “Perhaps the most alarming…” While I can understand the concern given that some infants are eating nothing but soy, the most important study to date, tracking adults who were fed soy formula as infants, provides assurance that there is no reason to be concerned about thyroid or reproductive function (95). Furthermore, the American Academy of Pediatrics and the National Toxicology Program considers soy formula safe.

The Beginnings Study is an ongoing study examining the effects of formula on child development (86). It is in its early stages with findings from children only a year old, but to date no negative effects of soy have been found on growth, sex organs, or neurological development compared to children on cow’s milk formula.

Some research shows that is best to choose a soy formula with DHA, and it is important to note that soy-formula is not intended for pre-term infants.

Feminizing Characteristics

Regarding the concern that soy could cause feminizing characteristics in men, there have been two case studies. In one, a man eating twelve servings per day of soymilk developed enlarged, sensitive breast tissue (123). In another, a man with type 1 diabetes was eating 14 servings per day of mostly processed soy foods for one year and developed erectile dysfunction (10), which normalized after ceasing the soy. While I would not recommend eating this much soy, one study used even much higher amounts of isoflavones and found no problems for most men (124).

As for sperm quantity and quality, while one epidemiological study raised concerns, albeit minor, about soy and sperm quantity (14), two clinical studies have shown no effects of soy (15, 126).

Thyroid

I do want to address one more issue that has recently arisen with the publishing of a clinical trial this year in which 16 mg/day of isoflavones in people with mild hypothyroidism appeared to cause an increased rate of advancing to overt hypothyroidism (78). Nine other clinical trials showed no effect of soy on the thyroid compared to placebo in people with presumably healthy thyroids (13, 60, 63, 64, 66, 68, 69, 71, 75) while the remaining five studies found small changes, all without physiological significance (61, 65, 70, 73, 74). People without hypothyroidism should have no problems with soy as long as they get enough iodine, but until we know more, people with mild hypothyroidism might want to avoid soy just to be safe.

Miscellaneous

In Soy: What’s the Harm?, I address other issues Wartman raised, such as mineral absorption (a non-issue) and traditional Asian intakes.

Conclusion

In addition to reducing the risk for breast cancer as mentioned above, soy also provides benefits for preventing prostate cancer, lowering LDL cholesterol, and improving menopausal symptoms. When you add up all the research on soy, there is no reason to think that two servings per day are harmful to most people, and good reason to think soy will provide some health benefits.