Men who have a diet composed of fruit and vegetables risk their fertility and chances of having children, according to researchers.

A study has found that males who don't eat meat have reduced sperm counts. Scientists at Loma Linda University Medical School in California conducted a four-year survey on how diet can affect sperm count.

The findings concluded that vegetarians and vegans had significantly lower sperm counts compared with meat eaters, 50 million sperm per ml compared with 70 million per ml.

They also had lower average sperm motility – the number of sperm which are active. Only one third of sperm were active for vegetarians and vegans compared with nearly 60% for meat eaters, according to a Telegraph report.

The team suggested that vitamin deficiencies could be partly to blame but also believe that replacing meat with soy has an effect.

Soy contains phyto-oestrogens which have similar properties to the female hormone oestrogen. "The theory that we have come up with is that vegetarians are replacing meat with soy, which contains phytooestrogens and could be affecting fertility," added Dr Eliza Orzylowska an obstetrician at Loma Linda University Medical Centre in California.

"It's hard to tell people not to be vegetarians if they are trying to conceive, but I would caution against using soy, at least for 74 days beforehand, which is the time it takes for sperm to be replaced."

Sperm quality has been in decline in recent years, and scientists are undecided as to the cause, suggesting a range of factors from smoking to an increased exposure to oestrogen.

A report published by Ohio's Cleveland Clinic found that consumption of lycopene improved the quality, mobility and volume of sperm dramatically, increasing sperm count by up to 70%. Lycopene is found in red fruit and vegetables such as tomatoes, strawberries, cherries and peppers.

Having a shower rather than a hot bath can have a dramatic effect. A University of California study in 2007 found that five out of 11 men who stopped taking hot baths experienced a sperm count rise of almost 500%.