Carrots have long been said to be good for your vision. Now, it has emerged, they can also improve the fertility of men.

Researchers investigating the effect of fruit and vegetables on the health of sperm discovered that carrots produced the best all-round results.

They had the greatest effect on ‘motility’ – a term used to describe the ability of sperm to swim towards an egg.

Researchers at Harvard University in the US asked almost 200 young men to follow diets containing a variety of fruit and veg before testing to see what effect it had on sperm.

Yellow and orange foods were found to help make the sperm stronger.

The boost was attributed to pigments called carotenoids because the body converts some of these into health-boosting antioxidants.

These include beta-carotene, which the body can make into the antioxidant vitamin A.

Antioxidants help to neutralise free radicals, destructive groups of atoms that are a by-product of metabolism and can damage cell membranes and DNA.

Sweet potato and melon can enhance the quantity and quality of sperm, but carrots were found to improve sperm performance by between 6.5 and 8 per cent, according to a report in the journal Fertility and Sterility.

Red fruit and veg, particularly tomatoes, which contain the anti-cancer chemical lycopene, were associated with fewer abnormally shaped sperm.

They contributed to between 8 and 10 per cent more ‘normal’ sperm, said the research, which could make a significant difference for couples having problems conceiving.

The report said: ‘In a population of healthy young men, carotenoid intake was associated with higher sperm motility and, in the case of lycopene, better sperm morphology.

‘Our data suggest that dietary carotenoids may have a positive impact on semen quality.’

It comes amid reports that the quantity and quality of male sperm is declining in Western countries, with some studies showing that average sperm counts have fallen by over half.

A previous study from Harvard showed that men eating diets containing most saturated fat had the lowest sperm counts and poorer quality sperm.

However, men who ate more ‘good’ fats – including omega-3 fatty acids found in fish and some plants – had better quality sperm than those eating less.

Carrots have long been prized for their proven ability to help maintain eye health.

They are a prime source of vitamin A, which the retina of the eye needs to function. The vitamin’s antioxidant properties may help prevent cataracts and a deficiency causes night blindness.