Too much tofu may affect a man's fertility, according to a study linking soya and low sperm count.
Scientists found that even modest consumption of soya products, such as meat and dairy substitutes and bean curd, can have a significant impact on sperm count.
Men who ate an average of half a serving of soya food a day had lower concentrations of sperm than those who did not.
Low sperm count is known to make it harder for a man to conceive.
Soya compounds called isoflavones, which mimic the female sex hormone oestrogen, are thought to be behind the effect. Animal studies have linked high consumption of isoflavones with infertility, but until now there has been little evidence of their impact on human reproduction.
Sperm count ranges between 80 and 120 million sperm per millilitre (million/ml) of semen for normal healthy men.
But researchers in the US found that among the men they studied, those with the highest soya intake produced much less sperm. On average their counts were 41 million/ml lower than those of men who did not consume soya products.
The scientists, led by Dr Jorge Chavarro, from the Harvard School of Public Health in Boston, questioned 99 men seeking help for fertility problems about their consumption of 15 soya-based foods.
Each man was asked about his diet in the previous three months. The foods included tofu, tempeh, soy sausages, bacon, burgers and mince, soy milk, cheese, yoghurt and ice cream, and other soya products such as roasted nuts, drinks, powders and energy bars.
The scientists varied serving sizes to take account of the fact that different foods contained different levels of isoflavones. A standard serving of tofu was said to be 115 grams, and for soya milk, one 240 millilitre cup. Factors such as age, body mass index (BMI) which relates weight and height, smoking, alcohol and caffeine intake, and the length of time since last producing semen, were adjusted for.