Friday, July 25, 2008

The Soy and Male Fertility Study

(Photo compliments of Image*After, and I'm pretty sure these are tadpoles, but you get the idea!)

If you're a veg*n, chances are that someone forwarded you one of the rather warped interpretations of a recent study suggesting that soy products have a negative impact of male fertility. If you're not a veg*n, you probably heard about it anyway, because people love to spread bad news about diets they know they should be following for optimal health.

Published in the journal Human Reproduction, this study was carried out by Jorge Chavarro and colleagues from the Harvard School of Public Health, Brigham and Women's Hospital, Massachusetts General Hospital, and Harvard Medical School. The study was funded by the National Institute of Environmental Health Sciences, the National Institute of Diabetes and Digestive Kidney Diseases, and the Yerby Postdoctoral Fellowship Program.

This cross-sectional study investigated a connection between sperm count and intake of isoflavones (phytoestrogens found in some plant materials including soy) and soy foods. 598 men who were being evaluated for infertility were recruited from the Mass General fertility clinic between 2000 and 2006. Those who were actually eligible for the study totaled 99.

The 99 men were weighed and measured, and asked to provide a sperm sample, a medical/lifestyle history, and diet records (a food frequency questionnaire) that indicated how often they consumed 15 soy-based foods within the last 3 months.

The researchers analyzed the association between soy intake and ejaculate volume, total sperm count, sperm concentration, sperm motility, and sperm structure. They controlled for other factors such as body mass index (weight for height), age, abstinence time, and intakes of caffeine and alcohol.

Of the 99 men who took part, the majority (72%) were overweight or obese. In terms of sperm count, 42% had normal levels while 10% had very low sperm counts (defined as being below 20 million/ml). Just over half the men (55%) had sperm with low motility (poor movement).

There was a significant relationship between soy intake on sperm count, with men in the highest intake category having an average of 41 million sperm/ml less than those who did not eat soy. The researchers also found that men with higher sperm counts had a stronger relationship between sperm quality and count to different levels of soy intake. (Meaning, men with high sperm counts were affected by soy more than men with low sperm counts.)

So what does this all mean? Is the relationship really there? We have to be careful in our interpretation of this study.

First of all, it says nothing about vegetarianism. Some vegetarians eat no soy at all, and some meat eaters eat a lot of soy. The articles that hint about some sort of danger of vegetarianism are all about hype and selling ads.

Second of all, if soy products really rendered men infertile, how could one explain the population issues in Asia, where soy is consumed several times a day?

Third, one must look at the study design: cross-sectional means that it is impossible to establish a causal link between sperm quality and diet. In other words, we have no idea whether the soy intake necessarily preceded problems with sperm count and quality, or whether it is the only or major factor responsible. In addition, a study with a sample size of 99 cannot possibly be used to develop healthy eating guidelines. There are too many other factors that may be affecting the findings.

Fourth, the majority of the participants were overweight or obese, and the researchers acknowledge that it is possible that excess body weight affects the relationship between soy intake and semen quality. For this reason, it is not possible to apply these results to average-weight men. That, and the fact that they were recruited from a fertility clinic, suggests a possible impact of soy on infertile, fat men, rather than an impact on the overall general male population.

Fifth, retrospective (recalling the past) data were used to determine food intake (i.e. the participant's responses to a questionnaire about how much soy they had previously eaten) using an unvalidated questionnaire (one that was never before tested). Thus, we can't say for certain how accurate this tool is in assessing soy intake.

Sixth, while isoflavones are found in non-soy-based foods as well, the study did not examine isoflavone intake from non-soy sources. Chances are that if this were recorded, the association between isoflavone intake and sprem count would be weaker.

This study, while legitimate and important, was taken out of context by the mass media and used as yet another excuse for the exclusion of health-supporting foods in one's diet.

Here is my take: First, obviously, men who eat soy father children. There are billions of examples in Asia, and millions more in the US (meet my vegan husband and child who was a bit of a surprise). Second, consuming soy foods has proven benefits, one of which is reducing risk of heart disease. The number one cause of death in the US is heart disease. So let's look at the big picture: Skip the soy because of one tiny study that says the sperm in soy eaters is a bit lower, or eat it and reduce the risk of a deadly disease that affects about 81 MILLION people a year in the US?

5 comments:

Alanna said...

Thank you for this post. What a great review of this highly-publicized story. Our vegan family is yet another testament to the fact that soy consumption does not lead to infertility!

Luke said...

Great article! Well written and clear. Thanks!

grey said...

Great post. Chavarro led a longitudinal study which found that women on a diet including meat were more likely than vegos to experience infertility (Protein intake and ovulatory infertility, Chavarro JE, et al, Am J Obstet Gynecol, 2008; 198(2): 210.e1-7). This shows at least that he isn't biased towards vegos, but I wonder if his sampling methods were any better here?

Mike Hussey said...

Doctors don't know what causes infertility in about half the cases in men, but think many men's infertility problems may be rooted in their genes.

erio said...

and the 7th point should be that considering most of the soy that's been consumed around the world for years has been GMO, some consideration should be made that genetic modification of the soy is the real culprit. And I would imagine meat induced infertility could be linked as well--GM corn & soy for feed anyone?