Latte drinking is the latest threat to human sperm counts, giving the rodent the edge

Drinking just three cups of coffee a day decreases men's ability to father normal children, new research suggests.

The research is the latest of a series of studies showing how modern life cuts both the quantity and quality of sperm. One, by the US government, shows that men now produce only about a third as much sperm as hamsters.

Scientists at Bradford University and the University of California, Berkeley, have found increased genetic damage in the sperm of men who consume average amounts of caffeine. This could lead to unsuccessful pregnancies or to children being born with deformities or genetic disease.

Their research, published in the journal Human Reproduction this week, also found that the quality of sperm worsens as men age. But it concludes: "Independent of age, men with substantial daily caffeine consumption have increased sperm DNA damage."

They say that growing evidence links the damage with the risks of defects and mutations in offspring, including childhood cancer and infertility. One of the directors of the research, Dr Andrew Wyrobek of the University of California, says: "We found that men who had consumed the caffeine equivalent of three cups of coffee a day had the kind of damage that leads to chromosome rearrangements in the embryo.

"When you have these rearrangements, one possibility is that the embryo doesn't have a gene when it needs it during development. This can lead to the death of the embryo. Birth defects can also be associated with rearrangements, and genetic disease among affected children."

Just how caffeine could have such an effect is not known, he says. "We don't have a good idea of the mechanism, and we need to look it more carefully. It could be that the problem is not caffeine, but something else in the lifestyle of people who drink a lot of coffee."

Modern lifestyles, and exposure to chemical pollution, are blamed for a precipitous drop in sperm counts in men worldwide. A host of studies show that, on average, they have dropped by two and a half times over the past 50 years from about 150 million per millilitre of sperm fluid to about 60 million. Hamsters produce 160 million per millilitre.

And the decline is continuing. Studies by the Medical Research Council, for example,found that the fertility of Scottish men is declining by about 2 per cent a year, and that younger men are less fertile than their fathers: men in their mid thirties or younger produce a quarter less sperm than those born between 1950 and 1970.

Warming the testicles - for example by taking hot baths, wearing tight jeans, having a sedentary job or using laptops on the lap - is known to cause temporary losses in fertility, but these can be quickly reversed.

The real puzzle is what is causing the rapid long-term fall around the world. Research reported at the annual meeting of the American Society for Reproductive Medicine last week produced two other possible causes: taking anti-depressants and the prolonged use of mobile phones.

Most concern, however, focuses on "gender-bender" chemicals such as PCBs, dioxins and phthalates, which are widely found in food. Scientists believe that the greatest danger is to babies in the womb, when their reproductive systems are being formed and developing. Damage done then is unlikely to be reversed.

Yet the Government has inisisted on removing most gender-benders from new Europe-wide controls on chemicals now being finalised, even when safe substitutes exist.