It is estimated that up to 4% of sperm in a healthy man carry either too many or too few chromosomes - a condition known as aneuploidy.

Aneuploidy is linked to failure to conceive, miscarriages, and children born with conditions such as Down's syndrome, Turner's syndrome and Klinefelter's syndrome.

However, the reason why sperm become mutated in this way is poorly understood.

The Berkeley team analyzed sperm samples from 89 healthy, non-smoking men, and quizzed them about their intake of zinc, folate, vitamin C, vitamin E and beta carotene.

Folate either came from the men's diet, or in the synthetic, folic acid form, which is found in dietary supplements.

The researchers found a statistically significant association between folate intake and lower sperm aneuploidy.

Dose effect

Men who consumed the most folate - between 722 and 1150 micrograms a day - had 20-30% lower levels of several types of aneuploidy than men with low folate intake.

The researchers stressed they had not proved that folate had a direct impact on sperm quality, merely that there was an association between the two. They say more work is needed to investigate cause and effect.

Researcher Professor Brenda Eskenazi said while the importance of maternal diet on reproduction was well known, the research suggested paternal diet might also be important.

She said: "In previous studies, we and others have shown that paternal micronutrient intake may contribute to successful conceptions by improving the quality of the sperm.

"This study is the first to suggest that paternal diet may play a role after conception in the development of healthy offspring."

Professor Eskenazi said that if the findings were confirmed the current recommended daily folate intake for men of 400 micrograms might have to be revised upwards for men trying for a baby in order to reduce the risk of chromosomal abnormalities in their children.

Mounting evidence

Dr Allan Pacey, an expert in male reproduction at the University of Sheffield, said evidence was mounting that diet affected sperm quality.

"The story so far suggests that dietary factors won't help you make any more sperm, but good diet might just improve the quality of the ones that are made."

But he added: "Before couples run out to the chemist and stock up on supplements, I would suggest that they just lead sensible lives and stop smoking, moderate alcohol intake and eat sensibly, making sure they get their five portions of fresh fruit and veg each day.

"And because it takes three months to produce sperm, any lifestyle changes should take place well in advance of any attempt to conceive."

A decision on whether folic acid should be added to bread and flour in the UK is due next year.

Campaigners say it would significantly cut the risk of neural tube defects such as spina bifida, but those against the move say some research has raised concerns that folic acid may be linked to an increased risk of cancer.