It’s a wonder that any of us are here at all! Human sperm is some of the least effective in the animal kingdom – and the situation seems to be getting worse. Writing in the London Independent, Steve Connor picked up a story last month about how researchers believe that a man's fertility as an adult may be largely laid down in the few months before and after his birth.
Around 20% of healthy men between the ages of 18 and 25 have abnormally low sperm counts, and the sperm they do produce is often of very poor quality (with only 5% to 15% of sperm cells being ‘normal’).
Almost 20 years ago now Professor Niels Skakkebaek of the University of Copenhagen presented research to the World Health Organisation showing that human sperm counts had fallen on average by about 50% since the end of World War II. The speed of this drop suggested that the explanation might relate to lifestyle changes rather than genetic factors, and various explanations were put forward: from exposure to chemical pollutants to the (then) modern fashion for tight underpants.
The latest research suggests that it is not the lifestyles of young men so much as that of their mothers which lie at the root of the problem. Although sperm manufacture does not begin until adolescence, the initial preparation in the testes takes place in the few months before birth and up to 6 months afterwards. If the developing testes are affected during this time it seems that a man will never become optimally fertile. Professor Richard Sharpe of the Medical Research Council suggests that several different factors may come together to have a combined effect on the early development of Sertoli cells in the testes.
|Does your Mum like beef?
Men whose pregnant mothers were exposed to high levels of toxic dioxins as a result of the 1976 industrial accident in Seveso, Italy have been found to have lower-than-average sperm counts, but men exposed to dioxins in adulthood showed no such effect. Another study found women who ate large amounts of beef during pregnancy, a diet rich in potentially damaging chemicals called polycyclic aromatic hydrocarbons (PAHs), had sons with relatively low sperm counts. But eating beef as an adult man shows no similar impact. A man who smokes typically reduces his sperm count by around 15 %, and this can be rectified by giving up smoking, however if a man’s mother smoked during pregnancy he is likely to have a decrease in sperm count of up to 40% - for life. One study has indicated that overweight pregnant women tend to produce sons with poor semen quality – but it is unclear whether it is being fat which causes the problem or the environmental chemicals stored in the fat. Such chemicals may well include those which mimic female sex hormones or block male ones, and are found in some types of plastics, pesticides and in traffic fumes.