| 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. Cordain’s 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. |
