Soya-based food and male fertility

The Daily Mail today reports on, “Why a vegetarian diet may leave a man less fertile.” It said research has found that eating tofu can significantly lower your sperm count. The Guardian also covered the study and reported that men who eat more than two portions a day have, on average, 41 million less sperm per millilitre of semen than men who've never eaten tofu. It said that although soya (tofu is made from soya beans) is unlikely to make healthy men infertile, it could have a significant effect on men who already have lower-than-average sperm counts.

The study behind this news had some limitations: it was small, and mainly looked at overweight or obese men who had presented to a fertility clinic. It focused only on soy (soya) intake, and the Daily Mail’ sclaim that there is a causal link between eating a ‘vegetarian diet’ and reduced fertility is misleading.

The idea that soy affects male fertility is not new, and there is a growing body of research on it. However, to date there is little consensus from the research and any relationship is unclear. More human studies are needed to determine if there really is a link.

Where did the story come from?

Dr Jorge Chavarro and colleagues from the Harvard School of Public Health, Brigham and Women’s Hospital, Massachusetts General Hospital, and Harvard Medical School carried out the research. 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. The study was published in the peer-reviewed medical journal Human Reproduction .

What kind of scientific study was this?

This cross-sectional study explored the association between low sperm count and intake of isoflavones (phytoestrogens found in some plant materials including soya) and soya products.

All men who presented for evaluation between 2000-2006 at an infertility clinic in the Massachusetts General Hospital Fertility Centre were invited to participate. Men who agreed were asked to provide a semen sample at the clinic which was then analysed for sperm count and movement, using computer-assisted sperm analysis [CASA]. The men were also asked about their medical history, lifestyle factors and length of abstinence from sex prior to providing the sperm sample. Their height and weight was measured.

A shortened food frequency questionnaire determined how often, on average, the men consumed each of 15 soy-based foodstuffs in the past three months (never, less than twice a month, twice a month to twice a week and more than twice a week). The soy foodstuffs included tofu, tempeh, veggie or tofu burgers, miso soup, soy milk, soy ice cream, soy nuts, etc. The men were also asked to describe their serving sizes in relation to an average size (i.e. whether they ate more or less than this in each serving). A database of foodstuffs was used to determine the content of isoflavones in the foods.

Of the 598 men who were approached to participate, 99 men were available for analysis (because they had provided a sperm sample and had responded to the food frequency questionnaires). They were mainly Caucasian and averaged 36.4 years old.

The researchers examined the association between soy foods overall (in terms of the four frequency categories) and ejaculate volume, total sperm count, sperm concentration, sperm motility, and sperm structure. They also explored any links between these factors and the intake of four particular isoflavones. Other factors that may have affected the link were taken into account. These included smoking, BMI, age, abstinence time, and intakes of caffeine and alcohol.

What were the results of the study?

Of the 99 men who took part, the majority (72%) were overweight or obese and 74% had never smoked. 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 effect of soy intake on sperm count, with men in the highest intake category having an average of 42 million sperm/ml less than those who did not eat soy. There was a similar trend observed with the individual isoflavones, but these links were not statistically significant. The researchers note that there was a ‘suggestion’ of an effect of weight (i.e. that more overweight or obese men were more likely to have a link between soy intake and sperm problems), but this was not statistically significant. They also found that men with higher sperm counts had a stronger relationship between sperm quality and count to different levels of soy intake.

What interpretations did the researchers draw from these results?

The researchers concluded that their study demonstrated that dietary intake of soy and isoflavones is ‘inversely related to sperm concentration’, even after adjusting for multiple factors that may affect this link (i.e. the more soy was eaten, the less sperm there were).

They say that the apparently greater association with soy for men with higher sperm counts suggests that the foodstuff has a greater effect at this end of the spectrum, and less for men with low sperm counts.

What does the NHS Knowledge Service make of this study?

This small cross-sectional study suggests a link between soy and sperm quality. The idea that soy affects male fertility is not new, and there is a growing body of research on it. However, so far there is little consensus from the studies as to the true relationship here. The findings of this study should be interpreted carefully for the following reasons:

• Most importantly, the cross-sectional study design means that it is impossible to establish a causal link between sperm quality and diet. This type of study is not able to determine what is cause and what is effect, i.e. whether the soy intake necessarily preceded problems with sperm count and quality and whether it is the only or major factor responsible.
• The majority of the participants were overweight or obese, and the researchers acknowledge that it is possible that ‘excess body weight’ modifies the relationship between intake of phytoestrogens and semen quality. Therefore, it is not possible to assume that the results apply to men who are of average weight.
• The researchers relied on retrospective data 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. As it has not been tested before, it is not clear how accurately the tool assessed actual intake of soy products.
• In addition, they also were not able to determine isoflavone intake from other sources (i.e. only quantified isoflavones that are consumed through soy products). If they had managed to record this, most likely the measures of association between isoflavones intake and sperm count would be less.

There are contradictory results from studies in this area (both human and animal studies), including the argument that the Asian diet (high in phytoestrogens from soy foods) has no apparent effect on fertility. Others support the view that soy has a positive or null effect on sperm quality. As the researchers state, this lack of consistency – particularly between animal and human studies – "highlights the importance of conducting further studies in humans".

Until then, there is no harm in men who have low sperm counts who are trying to conceive and who are worried about their sperm counts falling further, limiting their intake of foods containing soya. This should be in context of other factors that may affect fertility including lifestyle factors (smoking, alcohol), sexual history, general health and other aspects of a healthy diet.