HOW GLUCOSE SHOCK RATINGS ARE CALCULATED

   

Carbohydrates are foods the body breaks down to glucose during digestion.  In the past, nutritionists thought that all carbohydrates had similar effects on blood glucose levels.  Scientists have since discovered that some carbohydrates release glucose into the bloodstream faster than others, and the more rapidly glucose enters the bloodstream the more it raises blood glucose levels. 

 

Researchers found that the best way to measure the effects of different carbohydrates on blood glucose concentrations was to give standard amounts to experimental subjects and measure blood levels afterwards.  Now they rate the effects of various carbohydrates on a scale called the glycemic index.  The size of glucose surges produced by various foods is expressed as a proportion of the surge of a standard food, usually white bread, arbitrarily assigned a value of 100. 

 

Several diet books have published glycemic index values ostensibly to help dieters.  However, the scientists who developed these measurements warned against using these ratings without correcting for the sizes of servings people typically eat.  This may seem like a mundane technicality, but neglecting it is a fatal mistake.  Here’s why:

 

You can be sure that a typical helping of spaghetti will raise your blood glucose levels more than a typical helping of carrots, but many diet books, including The South Beach Diet and Sugarbusters, would have you believe otherwise.  They list carrots as raising glucose levels more than spaghetti.  How did this happen?  To measure the glycemic index of a food, researchers have to give subjects enough of it to provide 30 grams of carbohydrate available for absorption into the bloodstream.  Carrots are high in indigestible fiber but low in available carbohydrate, so to provide 30 grams of available carbohydrate, researchers had to feed subjects seven full-sized carrots each, a lot more than most people usually eat in one sitting.  To provide 30 grams of available carbohydrate in spaghetti, they only had to give subjects one cupful, about half as much as a typical American serving.

 

Scientists have attempted to correct this problem by calculating something called glycemic loads, which are glycemic indexes corrected for typical serving size.  The glucose shock ratings used in The New Low Carb Way of Life are glycemic loads multiplied by a constant that makes the value of white bread turn out to be 100.  This makes the scale intuitively more understandable. 

 

The glycemic load values published in the tables most doctors use were based on studies conducted in several different countries.  The serving sizes listed often vary significantly from typical American sizes--sometimes larger, sometimes smaller.  A serving of orange juice, for example, is listed as nine ounces; American restaurants commonly serve four.  The glucose shock ratings used in The New Low Carb Way of LIfe reflect usual, moderate American serving sizes.  You should check the serving sizes listed in the book to see if yours are similar.  If they are not, you should take into account any variation.   

 

Correcting glycemic indexes for the amounts people typically eat changes everything.  For one thing, it takes the mystery out of these measurements.  A hierarchy emerges with starchy carbohydrates like bread, potatoes and rice on one end of the scale and fruits, vegetables and dairy products on the other.  The pattern becomes so predicable you no longer need a list to tell you what to eat.  More important, correcting the glycemic indexes in this way markedly reduces the number of foods you need to avoid.