The Glycemic Index of foods or GI is a value given to a specific food based on how it impacts your blood sugar levels after you eat it.  This concept was first developed in the 1980’s as a nutritional therapy for diabetics with the goal of maintaining and lowering blood sugar by substituting high-glycemic foods for lower-glycemic foods.  This article discusses blood glucose in the body and how the GI is used to predict the blood sugar changes caused by food consumption.

what is the glycemic index

How Do I Figure Out the Glycemic Index of a Food?

Glycemic Index Values and Classifications

The glycemic index of foods is measured on a scale ranging from 0-100.  You can think of this scale as a type of speed indicator – a lower score (or glycemic index) means these foods raise your blood sugar at a slower rate, while a higher GI score means your blood sugar goes higher more quickly.

According to the American Diabetes Association, low GI foods have a value below 55, medium GI foods have a value of 56-69, and high GI foods have values above 70.   

Determining Glycemic Index Values: The Hard Way

So, how do you know what the GI value of a food is?  Well, determining a food’s GI is a complicated process.

To figure this out for yourself, you’d need to compare how your blood sugar reacted over two hours when you ate 50g of pure sugar and when you ate 50g of your chosen food.  So you’d have to carry out two separate tests by eating 50g of pure sugar (on an empty stomach) and measuring your blood sugar at specific intervals over two hours, and charting them so you can see the curve of your blood sugar rise and fall.  Then you’d copy the process after eating 50g of the other food (again on any empty stomach), again measuring and charting your blood sugar trend.  Next, you need to determine the differences in the area under these curves and you can do so using a few different equations, but they’re all fairly complicated.

Then, to make sure you have the right number, you have repeat this process at least 3 times.  For every type of food!  And then, for good measure, to make sure you’ve really, really got the right GI value you’d want to have 9 friends or strangers go through this process with you!

Determining Glycemic Index Values: The Easy Way

Now, obviously, that is really not practical for you to actually do yourself!  It’s totally unsafe if you’re not trained to draw blood.  You’d need access to a lab where you can test blood sugar levels and the know-how to do it!  Not to mention the hours and hours and hours of spare time you’d have to have laying around… for each food!

Luckily, scientists have those things at their disposal and they have gone through and completed these tests on all kinds of foods for you already.  All you have to do to determine the GI of a food is check out one of the lists they’ve put together of the values they calculated (like we’ve got for you below).

What Does It Mean to have a “High Glycemic Load”?

The glycemic load (GL) is a measurement similar to the glycemic index of foods but is not quite the same thing.  Rather than measuring just the way a food changes blood sugar, the glycemic load also takes into account how much digestible carbohydrate, total, is in a food.   

To calculate the glycemic load, start by finding the amount of digestible (or net) carbohydrate is in your food (total carbohydrate – dietary fiber).  Then, use the following equation to determine the GL:

GL = GI/100 x net carbs

If you calculate this through for a food and end up with a very large number — a “high glycemic load” — it means that the food contains a lot of total carbohydrates and these carbohydrates make it into your bloodstream very quickly.  This value may offer you more accurate information about how a food will impact your blood sugar than a GI value alone.

Why is the Glycemic Index of a Food Important to Know?

To understand why GI is important and what it means for your metabolic health, we first need to take a step back and look at how carbohydrates influence your blood sugar to begin with.

Blood Glucose Control in the Body

Getting Glucose into the Body

When you eat food that contains carbohydrates, your body breaks them down into glucose.  This glucose is then absorbed into your body through your intestinal wall, into your blood.  It is this glucose in your blood that is measured as “blood sugar”.

There are a few different categories of carbohydrates in food that your body uses in slightly different ways: sugars (simple carbohydrates), starches (complex carbohydrates), and fiber.  

Sugars, or simple carbohydrates, are easily broken down and absorbed in the bloodstream, resulting in a quick spike of blood glucose levels after you consume them.  Since your body doesn’t have to work very hard to break down or absorb sugars, foods rich in sugars usually result in a pretty quick rise in your blood sugar levels.  In terms of glycemic index, simple-sugar-rich foods are usually in the mid- to high-range on the scale.
Starches, or complex carbohydrates, on the other hand, need time to be broken down into smaller parts by digestive enzymes in your gut.  This slows down the rate at which your blood sugar rises.  Foods rich in starches fall in the mid-range of the glycemic index.
Fibers, unlike the other types of carbohydrates, aren’t broken down into glucose.  In fact, your body can’t digest fibers at all (not just not make them into glucose), and they simply travel through your body completely undigested.  By staying undigested and hanging around in your gut, fiber gets between sugars and starches in your food and your digestive enzymes, making it take longer for the energy-producing carbohydrates to make it into your blood.  This hindering effect of fiber means that most high-fiber foods fall into the mid- to the low-GI range.

Below are some examples of foods rich in the three different types of carbohydrates:

Sugars:

  • Milk
  • Fruits
  • Honey
  • Sodas and Juices
  • Cakes, Candies, Cookies

Starches:

  • Oats, grains, whole-wheat pasta, barley, rice
  • Black beans, garbanzo beans, lentils, pinto beans, kidney beans
  • Starchy vegetables: peas, corn, potatoes

Fiber:

  • Beans, legumes
  • Fruits and vegetables
  • Nuts
  • Whole grain breads and cereals

Keeping Blood Glucose Levels Steady

Now you know how blood sugars rise after you eat.  But how do they come back down?

Blood sugars come down when the cells of your body take glucose out of your blood and use it for energy.  To coordinate this flow of glucose out of your blood and into your cells, your body uses a hormone called insulin.  You can think of insulin as the key that opens up the door to your glucose.

How much insulin is in your blood to “unlock” your cells is determined by your pancreas.  Your pancreas detects when your blood sugar raises after you eat and releases just enough insulin to let just the right number of cells take glucose out of your blood so that your levels go back to normal.

Figure 1 shows the natural insulin secretion throughout the day in response to meals.  

Remember, insulin is a hormone that allows cells to take in the glucose from the blood, so high insulin levels after a meal mean lower, healthier blood sugar levels.

Figure 1. Normal Blood Glucose Levels and Natural Insulin Secretion. Modified from Glucose-Insulin-Day by Mcstrother, which is licensed under CC 3.0.

The Consequences of Regular or Chronic High Blood Glucose

Sometimes the delicate balance between your blood sugar levels, your insulin levels and your cells’ ability to take glucose out of your blood break down, leading to very high blood sugars.  This can happen for one or both of the following reasons:

  • Your body can’t release enough insulin to bring your blood sugar down normally.  This can happen if you consume so many simple sugars that your pancreatic cells simply do not have enough insulin.  It can also happen if your pancreatic cells start malfunctioning and dying (as is the case in type 1 diabetes or the very end stages of type 2 diabetes).  
  • Your insulin receptors stop responding to insulin normally.  This can happen if your cells detect inflammation or too much fat around.  Inflammatory chemicals and free fats tell the insulin receptor not to open up for glucose.  It can also happen if your pancreas releases so much insulin, so often, that the insulin receptors get confused.  They try to “open” when they are already open and get “stuck”.

What happens to the rest of your body if malfunctions in your pancreas or insulin receptor lead to very high blood glucose levels?

Sometimes, a lot of very serious things.

Common Symptoms of Chronic High Blood Sugar

Have you heard of Dana Hill?  She was an actress in the ‘80s who suffered from Type 1 diabetes (a condition where all the pancreatic cells that make insulin die.)  Unfortunately, she had extremely out of control blood sugars, despite an early diagnosis and regular medical advice and attention.  Her chronic high blood sugar levels led to:

  • stunted growth
  • kidney damage
  • gastroparesis (slow emptying of the stomach)
  • coma
  • death  

Though Dana is an extreme example, these kinds of complications are very common in diabeticsAnd, even if you don’t have diabetes, high blood sugar levels can be dangerous for you.  Chronic or extremely high blood glucose levels can lead to:

  • obesity
  • high blood pressure
  • diabetes
  • ketoacidosis (a severe form of poor insulin signaling that is life-threatening)
  • elevated levels of triglycerides and cholesterol in your blood
  • cardiovascular disease
  • kidney damage
  • blindness
  • impaired feeling in hands and/or feet
  • cancer

Related: Ketogenic Diets and Fat-Loss: The Role of Insulin in Weight Management

Benefits of the Glycemic Index for Controlling Blood Glucose

Figure 2: Blood Sugar Response for High vs. Low GI Foods.  Glycemic by Studio34 at English Wikipedia is in the public domain.

The GI is a great resource for planning out meals that promote blood sugar control!  It helps you determine which foods might make your blood sugars go crazy (high GI foods) and which ones will keep them more stable (mid-range or low GI foods).  

Certainly, this means you can only eat foods from the mid- to the low end of the scale, right?  Those high GI foods have got to be off limits forever.  

If that were the case you’d probably already want to throw in the towel with this plan (some of those high GI foods are just plain tasty)!  Fortunately, you can also use the GI to incorporate foods you love that sit on the higher end of the scale into your healthy, blood sugar-balancing GI plan!  Simply combine your high GI food choice with another low GI option (think something high fiber).  This will help balance out the overall GI of your meal.

By choosing mostly low GI foods, you can prevent blood sugar spikes throughout the day.  The more days you have like that, the lower your risk of developing those complications of high blood sugar we just talked about.

Limitations of Glycemic Index

The downside of using the glycemic index alone to regulate your blood sugar is that it assumes you’re going to eat 50g worth of food.  No more.  No less.  If you’re not — say you’re going to eat 10g or 100g — the effect of whatever you’re eating may be more or less dramatic on your blood sugar.

It is also important to note that GI does not take food combinations into account.  For example, both proteins and fats in combination with carbs can help reduce a spike of blood sugar. (Fats and meats don’t have a glycemic index score because they contain essentially no carbohydrates).  

GI also doesn’t necessarily take ripeness or processing into account either.  For instance, oatmeal can elicit extremely different blood glucose levels depending on how finely ground it is.  Many GI charts, however, only list one value for oatmeal.  

These kinds of variables make it difficult to accurately rank some foods with a static value.

Using the glycemic index score for foods is a good reference when planning out meals to keep your blood sugar level from spiking too high.  But hopefully, these limitations make it evident that the glycemic index of foods alone is not all you need to know about planning a healthy diet to prevent diabetes and other health complications associated with high blood sugars.

Strategies that Complement the GI Index

In addition to using the GI to help plan your meals, you can help regulate your blood sugar by:

  • Practicing portion control, especially with high GI foods
  • Aim for most meals and snacks to have a balance of low- to medium- GI foods, protein and fat
  • Getting moving!  Physical activity lowers blood sugar and helps regulate your blood sugar long-term!

Recent Research

Although the World Health Organization, the American Diabetes Association, Diabetes UK, and the Canadian Diabetes Association all support the Gl-approach to nutritional therapy, many health professions believe GI is too complex and variable for use in clinical practice.  Nutrition is individual and blood sugar levels are only a small part of our body’s physiological response to eating.  Many elements come into play based on one’s current health status or activity level!  GI lists should not be considered the be-all-end-all of picking foods for healthy meals.

Using the GI: Practical Food Lists

If you’re interested in using the GI to help plan your meals and snacks to aid in regulating your blood sugar here’s a list of some commonly consumed high and low GI foods to get you started!  Remember, aim to eat as many low glycemic index foods as possible.  And don’t forget that exactly how much your blood sugar will rise after eating these foods will depend on how much you eat and what else you’re eating at the same meal!

High Glycemic Index Foods List

  • Instant mashed potatoes (87)
  • Rice milk (86)
  • Cornflakes (81)
  • Boiled potato (78)
  • Watermelon (76)
  • White bread (75)
  • Wheat bread (74)
  • White rice (73)

Low Glycemic Index Foods List

  • Rice noodles (53)
  • Oranges (43)
  • Dates (42)
  • Carrots (39)
  • Milk (38)
  • Lentils (32)
  • Chickpeas (28)
  • Soybeans (16)

Take-Away Message

The glycemic index of foods serves as a good reference for predicting blood sugar changes after food consumption; however, the GI concept is not an end-all-be-all approach to maintaining blood sugar.  Most food is consumed in combination with different foods rather than in isolation, so it is important to understand that blood sugar levels can never be completely accurately predicted.  Ultimately, consuming real food, in moderate portion sizes, is most beneficial to maintaining steady blood sugar levels and balance out energy levels through the day.

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