Exploring the sense and nonsense of food and health

Is the Glycemic Index Accurate?


Illustration of the changes in blood glucose o...

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Whenever a food or beverage is ingested, our bodies respond with a routine chain of events. Complex nutrients get broken down into the simplest form of energy the body can use (glucose or “blood sugar“). Though digestion is routine, what varies is how severely and how quickly the ingested item causes blood sugar to rise.

For the average person deciding what to cook for dinner tonight, considering the effect of a particular food on blood sugar is too time consuming. Well, insert the concept of glycemic index (GI) and glycemic load (GL) and the guesswork is gone…right?  Not so fast.

Our bodies react uniquely to different sources of carbohydrates, such that a serving of a high-fiber food such as brown rice results in lower blood glucose levels than the same size serving of mashed potatoes.

There are two tools that are useful in predicting the blood glucose response to various foods.  The first of these tools is glycemic index (GI) which is a ratio of the blood glucose response to a given food compared to a standard (typically glucose or white bread).  It is influenced by starch structure, food temperature and other macronutrients in the meal, such as fat.  So it is only entirely accurate when a carbohydrate food is eaten alone.  A shortcoming of the GI is that the number is based on a serving of food that would provide 50 grams of carbohydrate.  So obviously, this amount may not reflect the actual amount typically consumed.

Another way and probably better way to deal with how foods affect blood glucose (and thus insulin) levels is the glycemic load (GL).   The GL takes into account the glycemic index and the amount of carbohydrate consumed, and in doing so actually better reflects a food’s effect on one’s blood glucose than GI alone.

To calculate the glycemic load of a food, the grams of carbohydrate in a serving of food are multiplied by the glycemic index of that food, and then divided by 100 (since GI is actually a percentage).  For example, vanilla wafers have a GI of 77, and a small serving contains 15 g. of carbohydrate.  This yields a GL of 12.  (77X15) divided by 100 = 12.  So even though the GI of vanilla wafers is considered high, the GL indicates that the impact of this food on blood glucose is fairly low 


Glycemic Index (GI)-Measures the body’s response to a particular food in terms of what effect the food has on blood sugar. In general, “high GI” foods are less desirable because they spike blood sugar while “low GI” foods have a more moderate effect on blood sugar, causing a lower and slower rise overall.

  • Scores of 70 and above are considered high glycemic index
  • Scores of 55 or below have a low glycemic index
  • Check out Harvard’s GI page for more info

Glycemic Load (GL)-A measure of how much of a certain food it takes to have the “high” or “low” effect on the body. GL is determined by multiplying a food’s glycemic index by the amount of carbohydrate it contains. A glycemic load of:

  • 20 or more is high,
  • 11 to 19 is medium
  • 10 or under is low

Glycemic index and glycemic load became a popular concept to serve as a foundation for meal plans, cookbooks, and diets. However, is there evidence to support it as a deciding factor in what makes the grocery list and what doesn’t? Probably not.

There are problems in applying the glycemic index.  Responses vary widely due to body size and weight, blood volume and metabolic rate.  Even within the same person, results vary with the time of day. Many food factors also change GI results, including plant variety, food ripeness, processing and preparation, and other foods eaten at the same time.  Even a cup of coffee can alter glucose absorption after a meal.

Many sellers of diet books tout the glycemic index as a guide to “good carbs” and “bad carbs”, but this is an oversimplification.  True, nutritious whole foods such as legumes often rank low on the glycemic index.  But cola beverages and pure table sugar rank only moderate on the scale- and no one would suggest these foods as sound carbohydrate choices.  Conversely, two nutritious foods as whole-grain brown rice and pumpkin, rank fairly high.

Associating a food with a number takes the pleasure out of eating. The goal of eating is to fuel the body, not require critical thinking.

That said, there are valuable lessons to be learned if you’re interested in hopping on the GI train. The concept can be used as a learning tool to understand complex versus simpler carbohydrates. Use it as a guide for understanding how certain foods typically affect the body.

Interestingly, when one looks at the glycemic index and glycemic load of white bread and whole-wheat bread, they are very similar.  The GL of both is 9 and 7 respectively.  The GI is 70 vs. 69, respectively.  This is because whole-wheat flour is typically so finely ground that it is quickly digested.  Thus, the effect of fiber in slowing down digestion and related absorption of glucose is no longer present.  Some experts suggest we focus more on minimally processed grains, such as coarsely ground whole-wheat flour and steel-cut oats, to get the benefits of these fiber sources.  So don’t always believe the hype of products that plaster WHOLE GRAINS on their boxes.  Read the ingredient list to get a better picture.

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2 thoughts on “Is the Glycemic Index Accurate?

  1. Oranges and orange juice are low on the Glycemic Load and I can’t understand how that can be since as a diabetic, those would shoot my blood sugar to the sky. Same with black beans, potatoes, rice, bananas, corn, and milk to name a few that come up low on GL but that would spike my blood sugar.


  2. Pingback: Carbohydrates |

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