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DON鈥橳 hold the book too close to your eyes or you鈥檒l need glasses, parents often warn their children. But the food kids eat might play just as big a role as books and computer screens when it comes to short-sightedness.

Diets high in refined starches such as breads and cereals increase insulin levels. This affects the development of the eyeball, making it abnormally long and causing short-sightedness, suggests a team led by Loren Cordain, an evolutionary biologist at Colorado State University in Fort Collins, and Jennie Brand Miller, a nutrition scientist at the University of Sydney. The theory could help explain the dramatic increase in myopia in developed countries over the past 200 years. It now affects 30 per cent of people of European descent, for example.

鈥淭he rate of starch digestion is faster with modern processed breads and cereals,鈥� says Brand Miller. In response to this rapid digestion, the pancreas pumps out more insulin. High insulin is known to lead to a fall in levels of insulin-like binding protein-3, the team points out. That could disturb the delicate choreography that normally coordinates eyeball lengthening and lens growth. And if the eyeball grows too long, the lens can no longer flatten itself enough to focus a sharp image on the retina, they suggest.

鈥淚t鈥檚 a very surprising idea,鈥� says James Mertz, a biochemist at the New England College of Optometry in Boston. But it鈥檚 plausible, says Bill Stell of the University of Calgary in Canada. 鈥淚t wouldn鈥檛 surprise me at all. Those of us who work with local growth factors within the eye would have no problem with that鈥攊n fact we would expect it.鈥�

Metz鈥檚 institution is now planning studies in animals. But there is already evidence to support the theory. While fewer than 1 per cent of the Inuit and Pacific islanders had myopia early in the last century, these rates have since skyrocketed to as high as 50 per cent. These 鈥渙vernight epidemics鈥� have usually been blamed on the increase in reading following the sudden advent of literacy and compulsory schooling in these societies.

But while reading may play a role, it doesn鈥檛 explain why the incidence of myopia has remained low in societies that have adopted Western lifestyles but not Western diets, says Cordain. 鈥淚n the islands of Vanuatu they have eight hours of compulsory schooling a day,鈥� he says, 鈥測et the rate of myopia in these children is only 2 per cent.鈥� The difference is that Vanuatuans eat fish, yam and coconut rather than white bread and cereals.

The theory is also consistent with observations that people are more likely to develop myopia if they are overweight or have adult-onset diabetes, both of which involve elevated insulin levels. The progression of myopia has also been shown to be slower in children whose protein consumption is increased.