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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.
Acta
Ophthalmologica Scandinavica March 2002 vol 80, p 125
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