Glycemic Index is Not for Everyone

This month, Dr. Mark Anthony warns us of a new twist on the old low-carb nutrition fad. Handy as it is for diabetics, the Glycemic Index isn't the "be-all and end-all" of carbohydrate nutrition for everyone.

By Mark Anthony, Ph.D., Contributing Editor

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We’ve gone off the wagon again. Hard as we tried, we just couldn’t stop eating carbohydrates. They taste too good and our beleaguered brains needed the sugar. But just to make sure that we continued to feel guilty about our comfort-food fixation, someone came up with at great idea: Label the carbs “good” and “bad.”

It’s this labeling of carbs as “good” and “bad” that was at the core of the low-carb movement and similar fads preceding it. But as a nation, we didn’t pay attention to the failure of this approach until we sobered up from meat and cheese binge. Sensing the change in consumer awareness, the faddists — lest we abandoned low-carb diets entirely — “gave us permission” to indulge: “OK, you can eat some carbs, but first we’ve got to label them.”

Heaven forbid nutrition should be simple, or (gasp!) we should enjoy our food. The popular nutrition arbiters decided we needed another number to bog us down and that’s what we got. It’s called “Glycemic Index.”

The originators of the Glycemic Index weren’t seeking to spoil our dinner, just to give persons with diabetes and other metabolic disorders some comparative and quantitative data. But something got lost in translation. Suddenly, glycemic index ended up the “be-all and end-all” of carbohydrate nutrition for all people, everywhere, healthy or not.

How Sweet It Is

Glycemic index (GI) is the rate at which the food you eat becomes the sugar in your blood. Developed in the 1970s, its first application was in diabetes — particularly in insulin-dependant diabetes — where it was intended as a guide to closely monitor blood sugar. The theory behind measuring GI seemed sound. As blood sugar rises after eating carbohydrates, insulin, the hormone secreted by the pancreas to help cells take up sugar, rises in response.

A person dependant on an outside source of insulin has a difficult balancing act, matching food with a dose of insulin, keeping blood sugar stable and within normal limits. It’s the sustained elevated blood sugar that leads to the debilitating conditions associated with diabetes.

Thus, the concept is if you know which carbohydrates to avoid, insulin (and thus blood sugar) can be kept under control. Determining GI goes something like this:

First, gather about 10 people and measure their baseline blood sugar. Have each one consume a known quantity of glucose in solution. Measure blood sugar again, then periodically over a couple of hours. Enter this data on a graph and measure the area under the curve. Assign it a value of 100. That’s the measuring stick — supposedly the quickest any food is likely to enter the bloodstream.

Next, feed the volunteers the same measure of different foods, take the same blood sugar data, average it and compare the data with the measuring stick. Run all the foods out on a scale 0 – 100. For example, if the average blood sugar rises 72 percent as fast with one particular food as it does when ingesting pure glucose, the GI is 72 (and so on).

We now have an index of the rate at which food turns into blood sugar, a glycemic index. A score of 70 or above means that the food gets into the blood stream at a rate similar to glucose (a high glycemic index); 55-69 is medium; and 54 and below is considered low.

GI and Obesity

The more recent application of GI has been for weight control where on the surface, it seemed to have relevance. The belief was, if sugar enters the blood stream too quickly, insulin will spike, and your cells will pull up glucose too fast to be used for immediate energy. That will force more glucose to be deposited in cells as fat. Blood sugar will drop dramatically because insulin is still hanging around, leaving you hungry for more carbohydrates. This will make overeating and obesity inevitable.

The theory seemed to make good sense. Many natural carbohydrates like beans and most whole grains have a low GI. Lots of refined carbohydrates like white bread have a high GI. Thus refined foods were blamed for making us fat. It was revolutionary.

So the GI-weight loss connection was embraced by everyone and we all lived happily (and thinly) ever after, right? Not quite. Logical as this idea sounded on the surface, in practice it fails to impress.

Curbed Enthusiasm

The American Diabetes Association concluded in a September, 2004 statement published in Diabetes Care that observing GI along with total carbohydrates can provide additional benefit for blood sugar control. They also concluded the role of low-GI foods in preventing development of type-2 diabetes (non-insulin dependant) was unclear, and that preventing obesity was the most important factor.

So how useful is GI as a tool to combat or prevent obesity? It depends on whose research you read. Proponents point to studies suggesting the benefits of eating certain low-GI foods, such as beans and whole grains. Detractors point to inconsistencies in the research, studies showing no effect and the mixed messages which can be gleaned from reading GI charts.

Yet the GI premise is selling diet books by scores of millions. In books on the South Beach, Zone, Atkins, Glucose Revolution and other diets, GI is treated as the Holy Grail. The only good carb is a low-GI carb, unless of course you’re an athlete.

Studies on athletic performance and carbohydrates show high-intensity workouts deplete glycogen stores, the reserve of carbohydrate that we carry in our muscles and liver for quick energy. We need high-GI foods to replenish this vital reserve. Still, from a dieter’s perspective, a high-GI food translates to “bad carb.” This overpowering guilt by association, with a number on a chart, is what led to the incredibly mixed messages associated with GI.

Ironically, the “good carbs” and “bad carbs” labeling runs contrary to the most important revelation that came from GI research. “Complex carbohydrates” (starches) did not all have a low GI as predicted. We had previously assumed that since starches were aggregate chains of glucose molecules, they would have the lowest GI, but this was only partially true. For example, white bread had a GI nearly equal to that of pure glucose, while some sugars had a GI much lower than predicted.
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