• Jan 2024

What is the Glycaemic Index (GI)? An essential guide

What is the Glycaemic Index (GI)? An essential guide
  • Carbohydrates have the highest impact on glucose levels. The glycaemic index (GI) measures how foods that contain carbs affect your glucose levels and ranks them on a scale of 1 to 100. 
  • High GI foods are usually processed and are high in sugar and low in fibre, whereas low GI foods tend to be whole grains, legumes, fruits, and vegetables. Choosing low-GI foods can help you better manage your glucose levels and weight. 
  • The glycaemic index ranks individual foods based on a 50-gram serving size — your portion size, overall meal composition, and other factors ultimately affect your glucose levels, and each person's response is unique.

Out of the three macronutrients (protein, carbohydrates, and fat), carbs impact your blood glucose the most. But not all carbs are created equal. Certain carb-containing foods can send your blood sugar levels soaring and then quickly crashing down, while others can trigger a slower glucose release that provides you with sustained energy. The handy tool known as the glycaemic index can help you figure out how your favourite carb-rich foods might affect your glucose levels. 

What is the glycaemic index?

The glycaemic index is a measure of how foods that contain carbohydrates affect your glucose levels. (1) Carbohydrates have the highest impact on your glucose, and understanding how foods that contain carbs (like whole grains, fruits, and vegetables) can affect your glucose can help you optimise your diet plan.  

How is the glycaemic index calculated?

The glycaemic index uses a scale of 1 to 100 and categorises foods into three groups: High GI, medium GI, and low GI. (2) Here’s a breakdown of how they are scored:

  • High GI: 70 to 100
  • Medium GI: 56 to 69
  • Low GI: 55 and below

High GI foods tend to spike your glucose the most. They contain quick-digesting carbs (or simple sugars) that the body breaks down fast, often causing a blood sugar spike and sometimes followed by a crash. Low GI foods digest more slowly, releasing glucose into the bloodstream over a longer period of time. And steadier glucose levels means better blood sugar control. 

To get to these number ranges, scientists calculated the GI value of “test foods” by testing how quickly certain carb-rich foods affect glucose levels in people without diabetes. (1) They achieved this by measuring glucose levels of at least 10 healthy people before eating the test food and then continued testing their glucose over the next two hours after eating a serving of the test food that contains 50 grams of carbs. After plotting all these points on a graph, the researchers found the area under the curve (AUC), a measure of how glucose values change based on the food consumed.  

The second part of this process occurred at a later time and involved having the same 10 people eat 50 grams of the sugar (glucose — referred to as the “reference food”). The researchers then recorded the participants’ glucose response over the next two hours after eating the sugar. Then, they divided each person’s glucose AUC for the test food by the glucose AUC for the reference food. To land the final GI value for the test food, they averaged out all 10 participants’ GI values.  

While the glycaemic index is pretty accurate, it’s not foolproof. The way a certain food affects your blood sugar can be different from the next person’s. And many other factors can also affect the GI of a food, including its ripeness (if it’s a fruit or vegetable), the way it’s prepared or cooked, what you eat along with it, how much of it you eat, and more.  

Effect of high GI foods on your glucose levels

High GI foods usually send glucose levels soaring and often are followed by a crash — especially if they are consumed on their own, compared to a mixed meal. High GI foods tend to contain rapidly digested carbs with little to no fibre, which causes your bloodstream to become saturated with glucose. Some examples of foods that are high glycaemic include:

  • White bread (about 3.5 slices)
  • White rice (a little over a cup)
  • White pasta (a little over a cup)
  • Pretzels (about 2 ounces)
  • White potatoes (1.5 potatoes)
  • Sodas (16 ounces)
  • Sugary cereals (about 1.5 cups)
  • Sugary baked goods (¾ of a large muffin or 1.5 scones)
  • Candy (2 ounces of hard candy)

While these foods all have a GI value of 70 or higher, not everyone may have the same glycaemic response to them. Different factors can influence your glycaemic response, such as your genetics, the food’s serving size, and whether you paired the food with something else like a protein or fat. For example, eating a piece of plain white bread will produce a higher glucose response than eating a slice of white bread with a piece of cheese because the protein and fats in the cheese can help slow down digestion and blunt a glucose spike.

Testing your glycaemic response

Since each person’s glycaemic response is individualised, using a continuous glucose monitor (CGM) like Lingo is the best way to measure your body's unique response. A CGM gives you insights into how what you eat and drink throughout the day affects your glucose, so you can better tailor your diet to prioritise foods and food combinations that won’t spike your glucose too drastically.

Glycaemic index in your diet

Understanding the glycaemic index of foods and using a CGM can help you gain better control over your glucose levels. Low-GI foods are usually whole grains, fruits, vegetables, and legumes, as well as dairy, proteins, nuts, and seeds. High-GI foods tend to be more processed, refined, or full of sugar. Therefore, prioritising low-GI foods in your diet can help you gain better glucose control as well as help you lose weight and maintain that weight loss over time. It can also help you become more mindful of your food choices, especially as you continue to learn how certain foods affect your glucose levels.

However, the GI isn’t without its shortcomings. As mentioned, the GI of a food doesn’t paint the entire picture. The way a food affects your glucose levels is dependent on many factors, one of them being the food’s serving size. 

That’s where glycaemic load comes into play. While the glycaemic index relies on a standardised 50-gram-carbohydrate serving for every food, the glycaemic load is based on a specific food's standard serving size. (2) And since different foods have different serving sizes, the glycaemic load tends to be a more accurate measure of how carb-containing foods affect glucose levels. For example, watermelon has a high GI but a low glycaemic load because 1 cup of watermelon has only 12 grams of carbs, so you’d have to eat a little more than 4 cups to hit 50 grams of carbs (which is the standardised measurement for GI).

A final note from Lingo  

The glycaemic index is a handy tool for measuring how carb-containing foods affect your glucose levels. A diet rich in low GI foods — such as whole grains, high-fibre fruits, vegetables, and legumes — can help you better manage your glucose levels, helping prevent spikes that can lead to weight gain and other health issues over time. Since everyone’s glucose response is different, using a CGM like Lingo can help you monitor how certain foods affect your personal glucose levels. 

January 18, 2024


  1. Jenkins DJ, Wolever TM, Taylor RH, Barker H, Fielden H, Baldwin JM, Bowling AC, Newman HC, Jenkins AL, Goff DV. Glycemic index of foods: a physiological basis for carbohydrate exchange. Am J Clin Nutr. 1981 Mar;34(3):362-6. https://pubmed.ncbi.nlm.nih.gov/6259925/
  2. Atkinson FS, Brand-Miller JC, Foster-Powell K, Buyken AE, Goletzke J. International tables of glycemic index and glycemic load values 2021: a systematic review. Am J Clin Nutr. 2021 Nov 8;114(5):1625-1632. https://pubmed.ncbi.nlm.nih.gov/34258626/

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