What Is The Glycemic Index?

This nutrition data tool can help you better manage your blood sugar levels.

It’s the 40th anniversary of the glycemic index (GI), which was first introduced in 1981. Despite its age, you may not know what it is and how it contributes to a healthy diet. Here, we review all you need to know about the glycemic index — including why you should pay attention to it, how to use it and how to determine what food items have a preferably low glycemic index number.

What is the glycemic index?

The glycemic index (GI) is one tool used to help manage blood sugar (glucose) levels. The purpose of the GI index is to assign carbohydrate foods a number so you can select foods that are less likely to cause spikes in your blood glucose levels, giving you better blood glucose control. The GI pre-assigns each food item a number based on several factors, including nutrient composition, cooking methods, and manufacturing process. Generally speaking, the lower the GI number, the metabolically healthier the food item. Glycemic index diets provide guidelines on what foods are best to choose. 

GI is related to, but different from, another important nutrition barometer — glycemic load (GL) — that tells you how quickly a food causes glucose to enter the bloodstream and how much glucose you’ll get per serving. Ideally, both GI and GL should be analyzed together. See our report on glycemic index vs glycemic load to learn more about how GI and GL can be analyzed concurrently for optimal nutritional guidance.

Why does the glycemic index matter?

Since the glycemic index measures each carbohydrate’s impact on blood glucose levels, it is an informative and useful tool to help you watch what you eat. For all of us, and especially people at risk of diabetes or living with diabetes, having an increase in blood glucose can lead to health complications. Repeated spikes in blood glucose can contribute to microvascular complications, including:

  • Diabetic nephropathy (deterioration of kidney health and function)
  • Neuropathy (nerve damage)
  • Retinopathy (damage to the back of the eyes)

These complications are often silent, but deadly — and in combination with insulin resistance, they contribute to the pathogenesis of diabetes and diabetes-related complications/comorbidities such as kidney failure, nerve damage, cardiovascular disease, eye disease, foot damage, hearing impairment and skin disorders. The scary part is that these underlying complications often begin to cause damage years before we can detect any significant malfunction. (Learn more: The 7 Top Risk Factors For Prediabetes and Type 2 Diabetes.)

Therefore, paying attention to your blood glucose levels and taking steps to keep them at healthy levels — through lifestyle modifications such as a healthier diet and more frequent exercise — can have a major impact on your long-term health and longevity. And the glycemic index tool is one way to measure the health of foods you consume. See also: 7 Simple Ways To Control Blood Sugar Spikes

How is the glycemic index calculated?

Carbohydrates are a type of nutrient in foods. There are three basic forms of carbs: sugars, starches, and fiber. When you eat or drink sugars and starches, your body breaks them down into glucose, which is the preferred source of energy for cells. Only a small amount of fiber is metabolized; the rest passes through your gastrointestinal system and bulks up your stool. Thus, fibers cause minimal increases in blood glucose levels. 

The glycemic index of a specific food is determined in a lab and research setting. A food’s GI is determined by feeding individuals food with 50 grams of carbohydrates. Then, their blood glucose response is checked for the next two hours and graphed accordingly. The GI value is assigned based on the average response across all participants. 

For consumers, there are tables, lists, and resources containing GI values. The smaller the number, the less impact it has on blood glucose. The categories are: 

55 or less = low GI food

56 – 69 = medium GI food 

70+ = high GI food

Reduce your intake of these high-GI foods

Foods that have a high GI score can cause a sharp rise in insulin levels. Thus, you are better off if you can reduce your intake of these high-GI carbohydrates.

Examples of high-GI foods: 

  • white potatoes 
  • white rice 
  • white bread 
  • cake and cookies 
  • watermelon 

One limitation of the glycemic impact ratings is that you may have a different response based on how much you eat. It also does not account for other nutrients in the food item.

A special note: Pay attention to the glycemic index of fruits and fruit juices

Eating well-balanced meals is part of a healthy diet. Fruits and vegetables are part of a healthy diet, even for those with diabetes or looking to manage their blood glucose levels. However, some fruits will have a higher GI than other fruits. Importantly, fruit juices consumed in quantity are associated with increased risk of type 2 diabetes, whereas most whole fruits are associated with decreased risk. Several factors affect the glycemic rating of fruit, including: 

Acidity: Foods that are highly acidic have a lower GI rating than those with low acid. Grapefruit has a GI score of 25 and oranges have a score of 40. Both fruits are more acidic. 

Fiber Load: Fruits that are higher in fiber generally have lower glycemic ratings. One medium apple has 4.5 grams of dietary fiber and a glycemic index score of 39. Fruits that have edible skin typically have more fiber, as the fiber in the skin helps protect the fruit. 

Ripeness: Since the amount of sugar increases as a fruit ripens, the more ripe it is, the higher it is on the GI scale. Green bananas contain less sugar and are lower on the glycemic index than yellow or brown spotted bananas. 

What foods have a low glycemic index?

The most accurate way to confirm the glycemic index for food is to review its score on a database such as this one from The University of Sydney. There are plenty of healthy and nutritious options to choose from if you’re following a low-GI diet. 

Examples of low-GI foods: 

  • Fruit: apples, strawberries, peaches, pears, kiwi, oranges, cranberries, tomatoes, blueberries
  • Vegetables: broccoli, celery, zucchini, carrots, asparagus, artichokes, mushrooms, Brussels sprouts, bok choy 
  • Beans and legumes: black-eyed peas, butter beans, chickpeas, green beans, snow peas, lentils, kidney beans 
  • Grains: barley, whole wheat, oats, wild rice, bulgar, brown rice, quinoa, wheat bread
  • Dairy: milk, cheese, yogurt, non-dairy milks

What’s the best low GI diet?

Essentially, the best low glycemic index diet involves choosing:

  • whole grains over refined ones 
  • fruits instead of sugary snacks 
  • lean proteins such as chicken breast rather than fatty meats
  • vegetables with lower amounts of added sugars

You may find some foods that list their GI index on their food label. More commonly, you’ll need to look it up

Key takeaways

While carbohydrate-rich foods are part of a balanced diet and provide our bodies with essential nutrients, they affect blood glucose levels. If blood glucose levels spike repeatedly, indicating poor long-term glycemic control, health complications (such as diabetes, coronary artery disease and several other conditions mentioned above) can ensue. Therefore, it is advantageous to choose those carbohydrates that do not cause a sharp rise in blood glucose. Low glycemic index foods are a part of that solution, especially if you are at risk of or are living with diabetes. Thus, paying attention to which carbohydrates score low on the GI index — especially in conjunction with foods also scoring low for glycemic load (GL) — can contribute to better nutrition and lower your risk of becoming metabolically unhealthy.

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