The Glycemic Index has its roots in a 1981 study published in the American Journal of Clinical Nutrition that measured the effects on blood sugar of certain starches. While originally these studies were done to determine the proper amount of carbohydrates for diabetics, they have now become the basis for many of the popular low-carbohydrate diets.

The Glycemic Index, from here on GI, ranks various foods according to how much of an increase of blood glucose and subsequently of the hormone insulin, they cause. The tests are conducted on samples of individuals by giving them 50 gram servings of such foods and 2 to 3 hours later, checking glucose and insulin levels. Pure glucose is the reference point of the GI, with a value of 100. Generally, a high GI has a value of 70+, medium is 50-69, and low is 0-55, although some low-carb diet advocates would say that 50 is high. Whether the the food is a simple carbohydrate or complex carbohydrate does not necessarily determine where it ranks on the GI. A case in point: a mashed potato measures 92, and a Snickers bar measures 55! Generally, though, the more processed a food is from its natural state and the smaller the particles of the food matter, the higher GI.

Some low-carbohydrate diets consider carrots and corn to be off-limits because of their relatively high GIs. This is why understanding the Glycemic Load is crucial. It takes into account the distinction of availible carbohydrates. This simply means that the grams of fiber are considered when evaluating the glucose impact of a food. Fiber is not considered an availible carbohydrate because it is not "availible" to the body for fuel purposes. In the Atkins Diet, the grams of carbs after fiber is subtracted is referred to as "net carbs." The formula for determining the GL is:(GI times # of grams of availible carbs)/100. So a high GI food could still have a low GL if it contains more fiber and thus less availible carbs. For example, a medium carrot has only 6 availible carbs, because many of its carbs are from fiber. So, using the GL formula, we see that it has a value of only 6, despite its GI value of 47. So, if you hear something to the effect of "carrots impact blood sugar as much as a candy bar", know how GL works!