Defining Glycemic Index (GI)

What is Glycemic Index (GI)?

Carbohydrates are found in all forms of food.  We find them in fruit, vegetables, legumes, beans, whole grains, and dairy.  We also find them in white bread, pastries, candy, cake and soda.  The extent to which these foods are 1., absorbed into the bloodstream and 2., raise our blood sugar levels, varies.  Glycemic Index (GI) is the measurement of the blood glucose response of a given food, as compared to the control (glucose or white bread).

The latter group of foods mentioned above (white bread, pastries, etc) are considered to have a high GI; the former group (fruit, vegetables, etc) is considered to have a low GI.   The high GI foods are purported to cause a greater spike in insulin, and thus hunger.  This is the concept behind why people have been interested in looking at the Glycemic Index (GI) of foods.

Is GI an effective measure to determine my food choices?


GI is influenced by a couple of factors: fiber content, processing of the food, physical structure (e.g., more simple sugars in a ripe banana than green one) and other macronutrients eaten with the meal (fat and protein).  A better reflection of a food’s effect on blood glucose is a measure called, Glycemic Load (GL).  GL is the amount of carbohydrate in a food multiplied by the GI of that food.  The reason why this is a better determinant than GI is that it measures the quantity of the food consumed. The most classic example is carrots.  Carrots have a high GI since it has a relatively higher amount of sugar content, but because carrots are so low in calories for the amount consumed, it has a low GL.

How do I measure GI and GL?   

GI uses a scale of 0-100, with white bread and glucose as the 100 reference point.  GL uses a scale of 0-10 to be “low,” 11-19 “medium” and 20+ to be “high.”  To measure GL, we need to first get the GI of a food:

1. First look up the GI of a given food on the GI database:

2. Look at how many carbs your food offers (look at the package) and subtract the fiber content.  Then multiply that amount by the GI of the food. Divide by 100.  Using the formula: GL = (GI x the amount of “non fiber” carbohydrate) divided by 100, here is an example:

  • In Thomas’ Whole wheat bagel thins, 1 bagel has 24 grams carbohydrates and 5 grams fiber
  • Total carbs = 24 minus 5.
  • The GI database whole wheat bread foods with ~19g carbs shows a GI of 85.  85 x 19 = 1615, divided by 100 = 16.15.
  • 16 is Thomas’ Whole wheat bagel thins’ Glycemic Load.
  • Using the scale, 16 is considered to have a medium GL

Should I use GL when determining what to eat?

There is some merit in using GL, but don’t rely on it entirely.

Long term controlled studies are lacking (+6 months) since it appears that from most metabolic studies, any decrease in hunger (and thus decrease in food intake) is associated with the FIBER content in the foods–not the GI or GL of the diet or grams of carbohydrate.  This is because fiber causes fullness feelings and slows gastric emptying time.  It also releases glucose into the bloodstream more slowly.

Bottom line?

Low GI and/or GL foods tend to be higher in fiber and lower in calories.  The fiber effects are what results in decreased hunger and thus, a lowered overall food intake.  These combined effects are more important to note than GI or GL, alone.

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