GLYCEMIC INDEX vs. GLYCEMIC LOAD

"You shouldn't be eating those carrots, they have a really high glycemic index."


"Oh I can't eat watermelon, the sugar content is just way too high."


Those two comments are from a coworker of mine who happens to be at least 75 lbs. overweight (and I'm being conservative).  She's actually telling me NOT to eat fruits and vegetables??  Well, rather than get into an argument with her (because she clearly knows what she is talking about), I decided to just put together a blog post...maybe she'll read it.


Wikipedia defines glycemic index as follows:


The glycemic index (GI) is a measure of the effects of carbohydrates on blood sugar levels.  Carbohydrates that break down quickly during digestion, releasing glucose rapidly into the bloodstream, have a high GI; carbohydrates that break down more slowly, releasing glucose more gradually into the bloodstream, have a low GI.


So basically the glycemic index tells us how rapidly the sugar content of a particular food will enter our bloodstream. Sounds pretty good, right?


Actually what the glycemic index fails to do is consider how much sugar will be entering our bloodstream.  This is where the glycemic load comes into play.  Wikipedia gives us the following definition:

The glycemic load (GL) is a ranking system for carbohydrate content in food portions based on their glycemic index (GI) and portion size.  The usefulness of the glycemic load is based on the idea that a high glycemic index food consumed in small quantities would give the same effect as larger quantities of a low glycemic index food on blood sugar.


The benefit of considering the glycemic load instead of the glycemic index can be easily illustrated using the watermelon example. According to Wikipedia, a high GI is anything greater than or equal to 70, and watermelon weighs in at 72! Now consider the fact that watermelon is composed almost entirely of...drum roll please...water!! The carbohydrate content of watermelon is a mer 5%. So given a 100g serving of watermelon, that means it only contains 5g of carbohydrates. Using those number we can calculate the glycemic load:

(Carbohydrates (grams) * Glycemic Index) / 100 = Glycemic Load

(5g * 72) / 100 = 3.6

The glycemic load of watermelon is 3.6.  A glycemic load of 10 or less is considered low, so watermelon is clearly well under that threshold.  While it may be true that the sugar contained in watermelon is quickly released into the bloodstream, the actual amount of sugar released is very low and therefore has a minimal impact on blood sugar.


Carrots, by the way, have a glycemic load of 3.5.