By Judy Foreman, Globe Staff

If you haven't heard of it yet, get ready to grapple with the ''glycemic index,'' the latest
wrinkle in America's endless diet debate. On the surface, the glycemic index is a simple
concept -- a way to measure how much blood sugar rises two hours after eating
carbohydrates. Carbohydrates with a high glycemic rating, like cake with icing, trigger
huge, rapid spikes in blood sugar, followed by steep spikes in insulin, the hormone that
escorts sugar into cells. Carbohydrates with low glycemic ratings, like whole grains and
fresh fruits and vegetables, trigger more modest, slower rises.

The basic idea is that high-glycemic carbohydrates are bad because they leave the stomach
quickly and trigger the rapid rise in blood sugar and insulin, which is soon followed by a crash
in blood sugar that prompts renewed hunger. The result can be more calories consumed
and more weight gained.

Using the index in real life is dicey. For instance, carrots have a high glycemic rating, but
they're good for you. French-fried potatoes could be interpreted as having a better (lower)
rating than a naked baked potato, because they contain fat, which causes slower emptying
of the stomach and, hence, a more modest rise in blood sugar and insulin, according to
Karen Chalmers, a registered dietician and director of nutrition services at the Joslin Diabetes
Center in Boston.

But what is important about the glycemic index is its symbolic value as a reminder to
ever-fatter, ever-more-confused Americans that carbohydrates, the big no-no of current fad
diets, are not a monolithic entity. There are ''good'' carbs -- found in whole grains, fruits and
vegetables -- and ''bad'' ones, found in highly refined baked goods, candy and many processed
foods, said Dr. Walter Willett, chairman of the department of nutrition at the Harvard School
of Public Health.

In the last several years, Dr. David Ludwig, director of the obesity program at Children's
Hospital in Boston, and others have published studies suggesting that obese people put
on low-glycemic diets lose more weight than those on reduced-fat diets.

In one such study, Ludwig gave some obese youngsters high-glycemic-index meals and
others low-glycemic ones. The number of calories given to both groups was the same. But
the children on the high-glycemic diet ate almost twice as much as the others when allowed
free access to food. Obviously, more snacks mean more calories and more weight gain.

That said, there are limits to the logic behind the glycemic index. It is incorrect to infer, as
some do, that sugar is ''addictive.'' Nor is it correct, as some diet gurus say, that
carbohydrates turn into body fat more easily than fat. It's fat that's most efficiently converted
into body fat; that is, it takes the fewest calories for the conversion. Carbohydrate is the
next most efficient, then protein.

More important, a calorie is still a calorie is still a calorie. No matter what combination of
protein, fat, and carbohydrate you eat, if you take in more calories than you burn, you'll
get fat.

And getting fat triggers a dangerous cascade of biochemical events. Excess body weight
causes the pancreas to work overtime to produce enough insulin, a phenomenon called
insulin resistance, noted Dr. Edward Horton, director of clinical research at the Joslin
Diabetes Center in Boston.

Insulin resistance, in turn, is associated with a whole cluster of metabolic problems,
including elevated triglycerides (fats), low HDL (''good'' cholesterol), high blood pressure,
changes in blood clotting patterns and a build-up of plaque in artery walls. High insulin
can also raise the level of PAI-1, or plasminogen activator inhibitor, which prevents the breakdown of potentially dangerous clots. Insulin resistance is also associated with high levels of C-reactive protein, a marker for heart disease. Eating too many calories -- from any source, protein, fat or carbohydrate -- can lead to obesity. But part of the problem today -- two-thirds of Americans are overweight and one third are obese -- is that for years we were all told to cut down on fats. There's still some truth in this message; saturated fats and trans-fats (like those found in some margarines) are dangerous because
they boost cholesterol and the risk of heart disease. Some fats, though, are good, notably
the monounsaturated and polyunsaturated fats found in olive, canola or peanut oils; nuts;
other plant products; and fish. These fats can lower the bad kind of cholesterol (LDL)
without lowering the good (HDL).

The lesson that most of us have absorbed over the years, however, was not this mixed
message, but a starker one: All fat is bad. That, not surprisingly, prompted consumer
demand for ''low-fat'' foods, and the food industry responded, with a vengeance. In recent
years, ''the food industry has screwed around with fat-free foods to keep the taste up,''
said Dr. David Heber, director of the UCLA Center for Human Nutrition. ''They lowered the
fat content but raised the sugar,'' he said, noting that many processed foods are now
loaded with high-fructose corn syrup, in part because ''corn is subsidized by the federal
government.'' Indeed, one reason for the current obesity epidemic is all the high-fructose
corn syrup added to processed foods.

So, where does this leave us, besides fat and frustrated? It's pretty straightforward,
actually. Don't take the glycemic index itself too literally. It's too complicated and you
could spend hours trying to calculate the glycemic rating for each food on your plate.

But do use the concept as a valid, potent way of remembering that not all carbohydrates
are created equal. After all, as Ludwig noted, lumping all carbohydrates together as a
diet rationale has clearly not worked to help people lose weight.

Bottom line? Stick to whole grains and fresh fruits and vegetables, and avoid refined
carbohydrates as much as you can. Chances are, you'll be less hungry and much
healthier.

Source: Boston Globe

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