Short-sightedness could be linked to childhood over-consumption of bread, rather than holding books too close, researchers suggest.

Scientists say diets high in refined starches, such as breads and cereals increase insulin levels in children.

This, they say, may then affect the development of the eyeball.

The theory put forward by the researchers from Colorado State University in Fort Collins, and the University of Sydney is that the excess hormone makes the eyeball abnormally long and causes short-sightedness.

It's an interesting theory, but it needs more evidence to support it

The condition now affects an estimated 30% of people of European descent.

Eye growth impact

Jennie Brand Miller, a nutrition scientist at the University of Sydney, said the rate of starch digestion was quicker in modern processed breads and cereals.

The body reacts to the rapid digestion by the pancreas pumping out more insulin.

The researchers said high insulin is leads to a fall in levels of insulin-like binding protein-3, which they suggest could disturb the development of the eye, and the co-ordination of eyeball lengthening and lens growth.

If the eyeball grows too long, the lens can no longer flatten itself enough to focus a sharp image on the retina, they suggest.

It has also been observed that people are more likely to develop myopia if they are overweight or have adult-onset diabetes, both of which involve elevated insulin levels.

Short-sightedness has been shown to progress more slowly in children whose protein consumption is increased.

But they point to trends seen in Inuit and Pacific Island populations.

Under 1% had myopia, or short-sightedness early in the last century.

Rates are now as high as 50%.

The increase has been linked to literacy and compulsory schooling leading to an increase in reading.

Evidence call

But the researchers putting forward the starch theory say focussing on reading does not explain why the levels of short-sightedness are low in societies that have adopted Western lifestyles but not Western diets.

Studies in animals are now planned by the team.

Loren Cordain. an evolutionary biologist at Colorado State University, said: "In the islands of Vanuatu they have eight hours of compulsory schooling a day.

"Yet the rate of myopia in these children is only 2%."

He said the difference was that Vanuatuans ate fish, yam and coconut rather than white bread and cereals.

Experts reaction to the theory was mixed.

Nick Astbury, vice-president of the Royal College of Ophthalmologists, told BBC News Online said: "It's an interesting theory, but it needs more evidence to support it."

He said the reasons for short-sightedness were "multi-factorial" so diets high in refined starches could play a part.

Bill Stell of the University of Calgary in Canada said: "It wouldn't surprise me at all. Those of us who work with local growth factors within the eye would have no problem with that -in fact we would expect it."

But James Mertz, a biochemist at the New England College of Optometry in Boston, said: "It's a very surprising idea."

The research is published in New Scientist and Acta Ophthalmologica Scandinavica.