Low Down on Carbohydrates
really care that they digest potatoes faster than carrots?
Macaroni faster than spaghetti?
Rice Krispies faster than Special K? A greenish banana faster
than a freckled one? A Snickers bar faster than a Twix?
Yes, say some of the country's
top-tier nutritional experts. They are convinced that carbohydrates
should be labeled good or bad, just the way fats are, and that
some of the carbs people love most velvety puddles of
mashed potatoes, lighter-than-air white bread are dietary
evil, to be avoided like the nastiest artery-choking trans-fats.
No, contend other equally respected
nutritional experts. Potatoes and other starchy standbys are
perfectly respectable. A carb is a carb is a carb.
The debate involves an idea called
the glycemic index. It is a way of rating how quickly carbohydrates
are digested and rush into the bloodstream as sugar. Fast, in
this case, is bad. In theory, a blast of sugar makes insulin
levels go up, and this, strangely, leaves people quickly feeling
The debate over whether every
person who puts food in his mouth should know about this is
fervid even for the field of dietary wisdom, where fierce opinions
based on ironclad beliefs and sparse data are standard.
Despite its detractors, the idea
seems to be gaining momentum, in part because it is offered
as scientific underpinning by the authors of a variety of popular
diet schemes, mostly of the low-carb variety. However, some
painstakingly argue that the glycemic index is just as important
for the carbohydrate-loving brown rice aficionado as it is for
the most carbo-phobic, double-bacon-cheeseburger-hold-the-bun
To believers, the glycemic index
is a kind of nutritional Rosetta stone that explains much of
what has gone wrong with the world's health and girth over the
past two decades: Why diets so often fail. Why diabetes is becoming
epidemic. Why mankind is growing so fat.
We overeat because we are hungry,
the theory goes, and we are hungry because of what we have been
told to eat, which is too much fast-burning food that plays
havoc with metabolism by quickly raising blood sugar levels.
All of that starch at the base of the food pyramid has had the
unintended effect of making us ravenous.
"It's almost unethical to tell
people to eat a low-fat, high-carbohydrate diet with no regard
to glycemic index," says Janette Brand-Miller of the University
of Sydney, one of the field's pioneers.
The idea has already entered
the scientific mainstream in much of the world and is endorsed
by the World Health Organization, but it remains deeply controversial
in the United States. It is dismissed by some of the country's
weightiest private health societies, including the American
Heart Association and the American Diabetes Association.
To some of the skeptics, this
is just another half-baked mishmash of dietary arm-waving, cobbled
together to justify the high-fat, low-carb schemes that dietitians
love to hate.
The fact that carbohydrates break
down at different rates has been suspected for a long time.
It is why diabetics were once (but no longer) told to studiously
avoid sweets, since presumably sugary foods would quickly turn
into sugar in the blood stream. About 20 years ago, scientists
came up with the glycemic index, or GI, as a way to compare
The body converts all carbohydrates
from starches to table sugar into sugar molecules
that are burned or stored. The faster carbs are broken down
by the digestive system, the quicker blood sugar goes up and
the higher their GI.
The GI of at least 1,000 different
foods has been measured, in the process knocking down many common-sense
dietary beliefs. For instance, some complex carbohydrates are
digested faster than the long demonized simple carbs. Foods
such as white bread and some breakfast cereals break down in
a flash, while some sweet things, like apples and pears, take
In general, starchy foods like
refined grain products and potatoes have a high GI 50
percent higher than table sugar. Unprocessed grains, peas and
beans have a moderate GI. Nonstarchy vegetables and most fruits
While it seems reasonable that
chewy, whole-grain bread is digested more slowly than a French
baguette, some of the results are less obvious. For instance,
overcooking can raise the GI. Ripe fruit is lower than green.
A diced potato is lower than mashed, and thick linguini is lower
To make matters even more confusing,
the glycemic index measures only the carbohydrate in food. Some
vegetables, such as carrots, have quite high GIs, but they don't
contain much carb, so they have little effect on blood sugar.
Therefore, some experts prefer
to speak of food's glycemic load, which is its glycemic index
multiplied by the amount of carb in a serving. Considered this
way, a serving of carrots has a modest glycemic load of 3, compared
with 26 for an unadorned baked potato.
Blood sugar levels may shoot
twice as high after a high-GI meal as after a low one, and that
unleashes metabolic havoc: The body responds with a surge of
insulin, which prompts it to quickly store the sugar in muscle
and fat cells. The high sugar also inhibits another hormone,
glucagon, which ordinarily tells the body to burn its stored
Blood sugar plunges. So much
is stored so fast that within two or three hours, levels may
be lower than they were before the meal. Suddenly, the body
needs more fuel. But because glucagon is still in short supply,
the body does not tap into its fat supply for energy. The inevitable
That, at least, is the theory.
Experiments to prove this are difficult and time-consuming.
Among those trying is Dr. David Ludwig of Boston's Children's
Hospital, who has done several studies on overweight teenagers.
In one, he tested the idea that
a high-GI breakfast makes people hungrier at lunch. A dozen
obese boys were fed three different breakfasts, all with the
same calories a low-GI vegetable omelet and fruit, medium-GI
steel-cut oats or high-GI instant oatmeal.
At noon, they could eat as much
as they wanted. Those who started the day with instant oatmeal
wolfed down nearly twice as much as those getting the veggie
Ludwig says overweight people
do not need to starve themselves. On a low-GI diet, they can
eat enough to feel satisfied and still lose weight.
In a pilot study, he tested this
on 14 overweight adolescents. They were put on two different
regimens a standard low-cal, low-fat, high-carb diet
and a low-GI plan that let them eat all they wanted. After one
year, the low-GI volunteers had dropped seven pounds of pure
fat. The others had put on four. Now he is repeating the study
on 100 heavy teenagers.
Even such small experiments have
been rare. Most support for the idea comes from big surveys
that follow people's health and diets over time. Some of these
show that those who consistently favor low-GI fare are less
likely to become overweight or to get diabetes and heart disease.
The evidence is strong enough
for authors of some popular diet books, who use the glycemic
index as one of their primary rationales. "It's a new unifying
concept that brings nutritional habits out of the dark ages
and says it's all about the numbers," says Barry Sears, author
of the Zone series of diet books. "It says diet does not have
to be based on philosophy. It can be based on hard science."
Major U.S. health organizations
are less impressed. Ludwig expects this to change, in part because
paying attention to the glycemic index can help everyone choose
healthier carbs, whether they go low-fat or high.
But that seems unlikely any time
soon at the heart association. The head of its nutrition committee,
Dr. Robert Eckel of the University of Colorado, says the theory
that high-GI foods make people hungry is "ridiculous" and argues
that a scientific case can be made for just the opposite.
Dietitians generally encourage
a balanced, varied diet emphasizing unadulterated whole foods,
and they cringe at a classification that puts ordinary baked
potatoes and white rice on a taboo list.
"It's an artificial system of
classifying foods as good and bad," says JoAnn Carson, a nutritionist
at the University of Texas Southwestern Medical Center.
Others worry that the whole business
is just too hard to keep straight.
"We are putting before the public
an extraordinarily complicated message, which I don't think
they will follow or be very happy with," says Dr. Xavier Pi-Sunyer
of St. Luke's Roosevelt Hospital Center in New York City.
Not necessarily, responds Harvard's
Dr. Walter Willett. "I do think this is an important concept
for people to understand, but I don't think they need to worry
about specific numbers."
His advice: Go light on the white
bread, white rice, potatoes pasta and sugary foods.
Reference Source 102