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    考博英语阅读篇章复习材料精选 (28)　　　　　　　　　　　

　　Blinded by Bread

　　Don't hold the book too close to your eyes or you'll 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??? shortsightedness.

　　Diets high in refined starches such as breads and cereals in- creased insulin levels. This affects the development of the eyeball, making it abnormally long and causing shortsightedness, 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 de- veloped countries over the past 200 years, it now affects 30 per cent of people of European descent, for example.

　　“The rate of starch digestion is faster with modem processed breads and cereals,” 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.

　　“It’s a very surprising idea,” says James Mertz, a biochemist at the New England College of Optometry in Boston. But it's plausible, says Bill Stell of the University of Calgary in Canada. “It wouldn't surprise me at all. Those of us who work with local growth factors??? within the eye would have no problems with that—in fact we would expect it.”

　　Mertz’s 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 “overnight epidemics???” have usually been blamed on the increase in reading following the sudden advent of literacy and compulsory schooling[2] in these societies.

　　But while reading may play a role, it doesn't explain why the incidence of myopia has remained low in societies that have adopted Western lifestyles but not Western diets, says Cordain. “In the islands of Vanuatu [3] they have eight hours of compulsory schooling a day,” he says, “yet the rate of myopia in these children is only 2 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 slo- wer in children whose protein consumption is increased.