The food children eat might play as big a role as books and computer screens when it comes to causing short-sightedness, reports UK science journal the New Scientist.
According to the report, 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.
"The rate of starch digestion is faster with modern processed breads and cereals," said 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 pointed out.
That could disturb the delicate choreography that normally co-ordinates 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 suggested.
The researchers are now planning studies in animals. But there is already evidence to support the theory, the New Scientist reports. While fewer than one 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 "overnight epidemics" 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 does not explain why the incidence of myopia has remained low in societies that have adopted Western lifestyles but not Western diets, said Cordain.
"In the islands of Vanuatu they have eight hours of compulsory schooling a day," he added, "yet the rate of myopia in these children is only two per cent." 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.