Introduction to the Gylcemic Index

11 Dec

What is the glycemic index?


It’s becoming increasingly clear that the secret to achieving and maintaining a healthy weight is not as simple as counting calories.  Although counting calories is a great first step in assessing what you are eating, there are many more factors which we must keep in mind.  For example, are we eating the right types of food or only food with empty calories?  Are we getting the proper macro- and micronutrients?  Are we eating too much sugar?

Today, we are going to talk about sugar.

Most healthy eating plans tell participants to avoid processed foods and sugary foods.  However, what some may not release is that any carbohydrate we consume, whether it be a doughnut or whole grain bread or a banana, will eventually be converted into glucose in our bodies.  This glucose accumulates in our blood and leads to the release of insulin. The insulin allows us to store the glucose as energy in our bodies for later.  This secretion of insulin and storage of glucose for later is what goes awry in Type I and II Diabetes.

The most desirable outcome is that the food we consume is broken down slowly into glucose.  This slow breakdown will not result in a spike of blood glucose or insulin.  Spikes in blood glucose led to the so-called “sugar highs” we most often associate with kids eating lots of candy.  This “high” is followed soon thereafter by a “crash.”  Spikes in blood glucose result in a rapid secretion of insulin.   This constant high demand of insulin can lead to Type II Diabetes.

To help us understand how the food we eat is converted to glucose and how quickly this occurs, the glycemic index (GI) was created.  Foods that break down slowly are considered to have a low GI.  The GI was introduced in the 1980s by Dr. David Jenkins as his group tried to determine the healthiest food for diabetics since diabetics need to avoid blood sugar spikes (source).  Diabetics need to avoid blood sugar spikes as they cannot process the sugar effectively and cannot release insulin to store the glucose.  Thus, the glucose stays in their blood and can lead to blindness and nerve damage.

How is the GI of a food determine?

To determine the GI of a food, 50g of the food  is ingested by volunteers and blood samples are taken at different time intervals over a 2 hour period.  These blood sugar levels are plotted on a graph and the area under the curve (AUC) is calculated (see below).  This is then compared to glucose (the standard) and multiplied by 100.  These AUC are calculated for each food in 10 volunteers and these values are averaged (source) to determine the GI of the food.

Here we have a sample blood glucose response curve from which we can calculate the AUC (source).  The AUC is simply the area under the red or blue line all the way down to the X-axis (the bottom horizontal black line).  Notice how the high GI food has a higher peak than the low GI food.  Even without calculating the AUC, we can assume the AUC is greater for the high GI food.  The next step would be to compare the AUC of the food to that of glucose and then multiply by 100.  Now we have the food’s GI.

What are some samples glycemic indexes? (glucose=standard=100)



Cherries Low 22
Grapefruit Low 25
Apricots  (dried) Low 31
Apples Low 38
Pears Low 38
Plums Low 39
Peaches Low 42
Oranges Low 44
Grapes Low 46
Kiwi fruit Low 53
Bananas Low 54
Fruit cocktail Medium 55
Mangoes Medium 56
Apricots Medium 57
Apricots  (tinned in syrup) Medium 64
Raisins Medium 64
Pineapple Medium 66
**Watermelon High 72

You can find the GI of common foods here courtesy of the South Beach Diet program.

Why the glycemic index is so confusing

This picture (Source) demonstrates all the complex factors that can influence the GI of a food.  Thus, it’s not as simple as how “sugary” a food may taste.

Just because a food has a low GI doesn’t necessarily mean it’s healthy.  Let’s consider an example.  Dates have a GI of 103 and chocolate milk has a GI of 42.  Does that mean the chocolate milk is better for you?

Not really!

The GI  of chocolate milk is low due to its fat content.  Foods with fat in them are released slower from the stomach into the  intestines.  This is why a high fat meal keeps you fuller longer than a low fat meal.  As the carbs are converted to glucose in the intestines so if the food is held in the stomach longer and released into the intestines slower, the food is converted into glucose on a slow, gradual basis.

This scenario can be compared to the digestion of dates which have no fat but are high in sugar.  They rapidly reach the intestines and are converted into glucose.  As they are dried fruit, they have natural sugars and lead to a quick blood sugar spike.  Despite their high GI, date are known to have nuermous health benefits and tons of fiber.

So, just because a food has a low GI doesn’t mean it’s healthy for you.

Talking about fat brings up another point.  Food pairing.  We just demonstrated how foods with fat in them stay in the stomach longer and slow the production of glucose in the intestines.   What if you eat a high GI food with another food that is high in fat?  The high fat food will delay the emptying of the high GI food into the intestine and slow the production of glucose.  So, if you have a burger on a white bun, the burger plus white bun will spike your glucose less rapidly than if you jsut had the white bun.  Confusing?  Yes!


How can I incorporate this into my diet?

The American Diabetic Association gave a presentation at a conference where they presented low GI meals and snacks.   For example, in the picture below, the salmon and veggies have a low GI and the potato’s GI could be further lowered by adding a pat of butter.


The take home message is to be aware of the sugar content of the food you eat and how the foods impact your glucose and insulin levels.  Our goal should be to maintain stable blood glucose and insulin levels.  If you are going to eat a high GI food, pair it with a healthy fat to slow the release of the food into your intestines.  If you would like to learn more about food pairing for a low GI diet, you can get either of these books on Amazon.


Did you know about the glycemic index?  Do you try to eat low GI foods?


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