Soy contains a number of isoflavones that exert an oestrogen-like affect, like daidzein, genistein and glycitein. They are marketed to menopausal women as a natural alternative to hormone replacement therapy (HRT), but some animal studies have indicated that high consumption of soy isoflavones could affect fertility.
The new study, published in the journal Human Reproduction, indicates that there may be a link, and if further randomised trials reach the same conclusions could lead to advice that men should avoid eating too much soy if they are planning a family.
The research team, led by Dr Jorge Chavarro of the Harvard School of Public Health in the US, analysed the diets of 99 men who attended a fertility clinic with their partners between 2000 and 2006.
The men were asked about their consumption of 15 different soy foods over the last three months, including tofu, tempeh, soy milk and other non-daily alternatives, energy bars, and vegetarian products using soy as a meat analogue.
The researchers then worked out isoflavones according to the portion size and the level of isoflavones in each food.
Those who had the highest intake - an average of half a portion of soy foods per day - were seen to have sperm concentrations of 41m less per ml. The normal sperm concentration range is 80-120m per ml.
Chavarro stressed that the half-portion figure is an average, and the biggest soy consumers in the group were eating as much as four servings a day.
Half a serving is said to be equivalent to one cup of soy milk or one portion of tofu, tempeh, or soy burgers every other day.
Large or small amounts?
While this represents a relatively large amount of isoflavones, in 2005 UK researcher Lynn Fraser, professor of reproductive biology at King's College London, presented evidence European Society of Human Reproduction and Embryology conference that even tiny doses of the natural compound can cause human sperm to 'burn out' and lose fertility.
Fraser's team investigated the effect of genistein and environmental oestrogens 8-prenylnaringenin (found in hops), and nonylphenol (in industrial products) had on capacitation, the stage when a sperm acquires the ability to fertilise an egg. They found that combinations of small quantities of these three chemicals stimulated sperm far more than when used individually.
In particular, the chemicals stimulated the sperm to undergo an acrosome reaction - when the cap on the head of the sperm ruptures and releases enzymes that enable the sperm to penetrate the coverings of the egg. If the acrosome reaction happens before a sperm reaches the egg, then fertilisation is unable to take place because the sperm has lost special 'docking' molecules that allow it to bind to the egg.
High sperm count and overweight
The effect on sperm concentrations seemed to be more pronounced in men who already had higher sperm counts.
"The implication is that men who have normal or high sperm counts may be more susceptible to soy foods than men with low sperm counts, but this remains to be evaluated," said Chavarro, who believes that further randomised trials are needed.
The researchers also found that, amongst the 72 per cent of study participants who were overweight, the effect was more pronounced.
The study methodology did not allow for a cause behind the observation to be established, but Chavarro included his speculation in the report.
He said it may be down to the oestrogenic activity of the isoflavones interfering with hormone signals, and therefore affecting sperm production.
In the overweight and obese men, this may be further accentuated by their higher levels of body fat, which produce more oestrogen than in slimmer men.
It is worth noting, however, that Asian populations have long consumed soy-rich diets without signs of reduced fertility or other health problems being traced back to the plant. Prosoy, a research and strategy consultancy, valued the European market from meat-free and tofu products at €1.2bn in 2006, a five per cent increase on the previous year.