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There's no sweet escape from myopia

Myopia or near-sightedness is extremely prevalent. It affects more than 25 - 35% of European descent populations and up to 50% or more of Asian descent populations.

If we did not have compensatory mechanisms for myopia and were left to mere Paleolithic resources, it is likely myopic individuals would not survive very long as clear distance vision is required for escape from predators, location of food, recognition of other species members and awareness of environmental dangers and benefits.

Studies carried out in hunter-gatherer societies and in recently westernized hunter-gatherer groups indicate that the prevalence of myopia normally occurs in 0-2% of the population, and most refractive errors are less than minor. Moderate to high myopia is either non-existent or occurs in about one person out of a thousand.

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.

When these hunter-gatherer societies change their lifestyles and introduce grains and carbohydrates, they rapidly develop (within a single generation) myopia rates that equal or exceed those in western societies.

In Dr. Cordainís study of 229 hunter-gatherer societies, he found that although refined cereals and sugars were rarely if ever consumed by groups living in their traditional manner, these foods quickly became dietary staples following western contact.

Hunter-gatherer diets are typically characterized by high levels of protein, moderate levels of fat and low levels of carbohydrate when compared to modern western diets.

The carbohydrates present in hunter-gatherer diets are of a low glycemic index: they are slowly absorbed and produce a gradual and minimal rise in plasma glucose and insulin levels when compared to the sugars and refined starches in western diets.

Studies of recently acculturated hunter-gatherer populations that have adopted western dietary patterns frequently show high levels of hyperglycemia, insulin resistance, hyperinsulinemia and type II diabetes. Conversely, hunter-gatherer populations in their native environments rarely exhibit these symptoms.

In industrialized countries, this dietary shift from hunter-gatherers occurred more slowly over the 200 or so years since the advent of the industrial revolution as more and more refined sugars were gradually included in the diet along with increasingly greater levels of refined cereals.

Although highly refined sugars and cereals are common elements of the modern urban diet, these carbohydrates were eaten sparingly or not at all by the average citizen in 17th and 18th century Europe and only started to become available to the masses after the industrial revolution.

Only with the widespread introduction of steel roller mills in the late 19th century did fiber-depleted wheat flour of a low extraction become widely available.

Hence, over the last 200-250 years the average glycemic load of foods in urban areas of industrialized countries has risen steadily, primarily because of increasing consumption of refined cereals and sugars. This increase in sugars is clearly related to increased levels of insulin.

"Overnight Epidemics"

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, says Cordain.

"In the islands of Vanuatu they have eight hours of compulsory schooling a day," he says, "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.

High Carbohydrate Intake Increase Insulin Levels.

This elevated level of insulin from consumption of excess grains and sugars will serve to increase free insulin like growth factor (IGF-1) which can then accelerate scleral tissue growth during critical developmental stages thus leading to myopia.

A variety of studies also suggest that high carbohydrate diets may cause permanent changes in the development and progression of refractive errors, particularly during periods of early growth and development.

High insulin levels from the carbohydrate loads 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,

Population studies have demonstrated that people of Asian and

Chinese descent tend to be more insulin resistant than people of European descent. The prevalence of myopia is also higher in Asian populations than it is in European populations; it is possible that the higher rates of myopia in Asian populations may, in part, be due to their increased genetic susceptibility to insulin resistance.


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