Glycemic Index (GI) provides only an estimate of how quickly a food raises the blood sugar levels in your body. The glycemic index ranges from zero to one hundred. Foods with a low GI value are digested slowly and raise blood sugar slowly. Foods with a high GI value are digested quickly and raise blood sugar quickly.

Glycemic Load (GL) combines glycemic index with carbohydrates per serving, fiber content, and water. To give you a more complete picture the glycemic load ranges from zero to sixty.

Foods with a low GI value raise blood sugar slowly and are low in carbohydrates. Foods with a high GI value raise blood sugar quickly and are high in carbohydrates. While carrots may have a GI value of 35, their fiber content brings the GL value down to 2. While black beans may have a GI value of 30, their fiber content brings the GL value down to 7. While watermelon may have a GI value of 72, its water and fiber content brings the GL value down to 4. A low glycemic load is good for blood sugar levels. It is crucial for you to select whole foods such as carrots, black beans and watermelon to keep your glycemic load low.

The Glycemic Index (GI) classifies hundreds of foods according to their impact on blood sugar. The GI indicates how intensely and rapidly a given food will influence glucose and insulin levels when consumed. Originally developed as an aid to diabetics, it can be referenced​
effectively by anyone to maintain a healthy weight and reduce the
risk of chronic conditions.

How It Works

Indexed foods are ranked from 0 to 100. Glucose, as the sugar that cells can immediately use as food, is the GI’s measuring stick and ​
tops the index at a rating of 100. Foods with a low Glycemic Index​
value are digested more slowly than those with a higher value. When low-GI foods are consumed, carbohydrates are released into the body slowly, blood glucose spikes are limited, cravings are reduced, and ​
hunger pangs are delayed.

GI values provide only a comparison of how quickly the body converts ​
a specific food into glucose. The GI does not account for portion size ​
or for fiber—both important factors in measuring how fast a food raises blood sugar levels.​


A numerical system of measuring the rate of BLOOD GLUCOSE generation from a particular food item as compared to a reference item, such as glucose = 100. Foods with higher glycemic index numbers create greater blood sugar swings.

Emerging research shows that not all calories are processed in the same way. In particular, the quantity and quality of carbohydrates consumed affects insulin release, hunger, and fat storage. Glycemic index (GI) is an objective way of measuring this effect. This Whole Health tool defines glycemic index and glycemic load and explains how they can be used. Resources for gathering more information are provided below.

What is Glycemic Index?

The glycemic index of a food refers to the effect the food has on the body's blood sugar levels. Blood sugar levels are raised after a person eats foods containing carbohydrates (sugars and starches). Various carbohydrate-containing foods affect blood sugar levels differently. This is a function of the type and quantity of carbohydrate, as well as the method of preparation and the presence of other substances in the food, such as soluble fiber, fat, and protein. For example, your body absorbs glucose from whole fruit differently from extracted fruit juice due to the fiber content.

The glycemic index compares the rise in blood sugar level after eating a particular food to a reference food, often glucose. Glucose has a GI of 100. The GI of fructose is 25 and sucrose, which is a blend of the previous two, has a GI of 65. Most natural sweeteners are a combination of these three carbohydrates. Figure 1 shows the difference between a low GI and a high GI food in terms of effects on blood glucose levels over time.

The glycemic load (GL) is a more accurate tool for assessing the impact of eating carbohydrates. It gives a more complete picture than does glycemic index alone, because it accounts for the amount of carbohydrate in a serving. A GI value indicates only how rapidly a particular carbohydrate turns into sugar, whereas GL accounts for how much of that carbohydrate a person would typically eat. The carbohydrate in watermelon, for example, has a high GI. However, there is not a lot of sugar in a serving of watermelon, since most of it is fiber and water. Thus watermelon’s glycemic load is relatively low.

Foods that have a low GL almost always have a low GI. In contrast, foods with an intermediate or high GL can have a GI that ranges from very low to very high.

Why Are These Numbers Important?

A low glycemic index diet has been associated with improvements in:

• Obesity
• Type 2 Diabetes
• Gestational Diabetes
• Metabolic Syndrome
• PCOS
• NAFLD
• Dyslipidemia
• Hypoglycemia

Conversely, diets with high glycemic index or glycemic load are strongly associated with type 2 diabetes incidence. Dietary glycemic index is also positively associated with the prevalence of metabolic syndrome, though according to one review, a significant association between dietary glycemic load and metabolic syndrome has not yet been found.

Examples of GIs for common foods are listed in Table 1.

Table 1. Average Glycemic Index of Common Foods

You’ve probably heard of glycemic index and glycemic load. Some studies suggest that sticking to foods with a low glycemic index may help prevent diabetes, cardiovascular disease and cancer. Some claim it helps with weight loss. The truth is, we don’t know all the answers yet. Here’s what you need to know.

The glycemic index and load concern carbohydrates, or carbs—one of the main types of nutrients in our diets. Carbs with a simple chemical structure are called sugars. Sugars are found naturally in foods like fruits, vegetables and milk products. They’re also added to many foods and drinks. Complex carbs, like starches and fiber, are found in whole-grain breads and cereals, starchy vegetables and legumes.

Your digestive system changes the carbs you eat into glucose, a type of sugar that your body uses for energy. Simple carbs are more quickly digested and absorbed than complex ones, so simple carbs can raise your blood glucose levels faster and higher.

People with diabetes need to manage their blood glucose levels. High blood glucose can damage tissues and organs. In time, it can lead to heart disease, blindness, kidney failure and other problems. If you have diabetes, controlling your blood glucose will prevent or delay these health complications. So it’s important to understand how foods and drinks affect your blood sugar.

“The evidence seems to support the concept that the more complex carbohydrates will lead to better blood sugar control than the more simple sugars,” says Dr. Myrlene Staten, an NIH diabetes expert.

Researchers developed the glycemic index to measure the quality of carbs in foods. It shows how the carbs in different foods raise blood sugar. White rice, for example, has a higher glycemic index than brown rice, which has more complex carbs.

But it’s not just the types of carbs that matter. The more carbs you eat, the more your blood sugar rises. “The glycemic index really doesn’t take into consideration how much you eat,” explains Dr. Somdat Mahabir, who studies cancer risk at NIH.

That’s why researchers came up with the concept of glycemic load. It captures both the types of carbs in a food and the amount of carbs in a serving. Essentially, it shows how a portion of food affects your blood sugar. Many things affect the glycemic load, including food processing, how ripe a fruit is, how a food is prepared and how long it’s been stored.

Studies of people who use these concepts to guide their diets have found mixed results. “There’s evidence to show that glycemic index and glycemic load are not associated with body weight,” says Dr. Catherine Loria, an NIH expert on nutrition and heart health. “There’s really not enough evidence to show if glycemic index is related to heart disease.” A possible link to cancer is also being explored.

Glycemic index and glycemic load aren’t things you’ll see on a label, so they’re not easy to use. But labels do show helpful information: calories, total carbohydrates, sugars and fiber.

“It makes sense for everybody, not only diabetics, to eat the more complex carbohydrates because they will be more gradually absorbed, and blood sugar highs and lows will be smaller,” Staten says. Whole foods with complex carbs will give you more minerals and vitamins, too, and are usually good sources of fiber.

Choose More Complex Carbs

• Choose cereals high in fiber (5 or more grams per serving).
• Switch to whole grains. Look for whole-grain bread, whole-wheat pasta, brown or wild rice, barley, quinoa and bulgur.
• Eat more fresh fruits and vegetables and drink less juice.
• Don’t forget legumes. Try different kinds of beans and lentils.
• Snack on fruit and small portions of nuts and seeds. Limit candy and other junk food.

Summary

• The glycaemic index (GI) rates carbohydrates according to how quickly they raise the glucose level of the blood.
• The glycaemic load (GL) rates carbohydrates according to the glycaemic index and the amount of carbohydrate in the food.
• A low GI rating of a food does not mean you can eat a larger serve of that food – the total amount of carbohydrate and kilojoules eaten is still important.

About the glycaemic index (GI)

Foods and drinks provide our body with energy in the form of carbohydrates, fat, protein and alcohol.

Carbohydrates are the body’s preferred energy source.

The glycaemic index (GI) is a way that carbohydrates in foods and drinks are ranked according to how quickly they raise the glucose level of the blood (also known as ‘blood sugar level’). It has replaced classifying carbohydrates as either ‘simple’ or ‘complex’.

Foods with carbohydrates include bread, breakfast cereals, rice, pasta, legumes, corn, potato, fruit, milk, yoghurt, sugar, biscuits, cakes and lollies.

Digesting and absorbing carbohydrates

The digestive system breaks down carbohydrates in foods and drinks into simple sugars, mainly glucose. For example, both rice and soft drink will be broken down to simple sugars in your digestive system. This simple sugar is then carried to your body’s cells through the bloodstream.

The pancreas secretes a hormone called insulin, which helps the glucose to move from your blood into the cells. Once inside a cell, the glucose is ‘burned’ along with oxygen to produce energy. Our brain, muscles and nervous system all rely on glucose as their main fuel to make energy.

The body converts excess glucose from food into glycogen. Glycogen acts as a storage form of glucose within the muscle tissue and the liver. Its role is to supplement blood glucose levels if they drop between meals (especially overnight) or during physical activity.

The glycaemic index (GI)

The glycaemic index (GI) is a way of ranking carbohydrate-containing foods based on how slowly or quickly they are digested and increase blood glucose levels over a period of time – usually 2 hours.

The GI uses glucose or white bread as a reference food – it has a GI score of 100. Carbohydrate-containing foods are then compared with this reference to assign their GI. This ensures all foods compared have the same amount of carbohydrate, gram for gram.

Carbohydrates that break down quickly during digestion have a higher glycaemic index. These high GI carbohydrates, such as a baked potato, release their glucose into the blood quickly.

Carbohydrates that break down slowly, such as oats, release glucose gradually into the bloodstream. They have low glycaemic indexes. The blood glucose response is slower and flatter. Low GI foods prolong digestion due to their slow breakdown and may help with feeling full.

GI scale examples

The terms ‘low GI’, ‘medium GI’ and ‘high GI’ are given to foods that fall within different ranges of the GI.

These ranges, along with some example foods, include:

• low GI (less than 55) – examples include soy products, beans, fruit, milk, pasta, grainy bread, porridge (oats) and lentils
• medium GI (55 to 70) – examples include orange juice, honey, basmati rice and wholemeal bread
• high GI (greater than 70) – examples include potatoes, white bread and short-grain rice.

Factors that affect the GI of a food

Factors such as the size, texture, viscosity (internal friction or ‘thickness’) and ripeness of a food affect its GI. For instance, although both ripe and unripe bananas have a low GI (less than 55), an unripe banana may have a GI of 30, while a ripe banana has a GI of 51.

Fat, protein, soluble fibre, fructose (a carbohydrate found in fruit and honey) and lactose (the carbohydrate in milk) also generally lower a food’s glycaemic response. Fat and acid foods (like vinegar, lemon juice or acidic fruit) slow the rate at which the stomach empties and slow the rate of digestion, resulting in a lower GI.

Other factors present in food, such as phytates (used to store phosphorus in plants) in wholegrain breads and cereals, may also delay a food’s absorption and lower the GI.

Cooking and processing can also affect the GI – food that is broken down into fine or smaller particles will be more easily absorbed and so has a higher GI. Foods that have been cooked and allowed to cool (potatoes, for example) can have a lower GI when eaten cold than when hot (for example, potato salad compared with hot baked potato).

High GI foods are influenced by low GI foods

Generally, eating low GI foods and high GI foods at the same time has the effect of ‘averaging’ the GI. This is important, as most foods are eaten as part of a meal and this affects the GI value of foods. For example, eating cornflakes (a higher GI food) with milk (a lower GI food) will reduce the overall effect of the cornflakes and milk meal on blood glucose levels.

GI symbol and claims on packaged foods

You might have noticed that some packaged food products have a GI symbol or make claims about the food’s GI and its health effects (for example, ‘low GI to help you stay fuller for longer’).

The Low GI Symbol and claims about the relationship of a low GI product and its effect on health is only available to packaged food products that meet strict nutritional and testing criteria.

This labelling is not compulsory for food companies to follow, so not all products that are eligible will display the symbol or make a claim. This is often the case for smaller companies who may not have the money to go through the necessary processes to be given the label. These claims also won’t generally be used on food products that would be eligible but are not typically packaged (for example, fresh fruit and vegetables).

Glycaemic load (GL)

The amount of the carbohydrate-containing food you eat affects your blood glucose levels. For example, even though pasta has a low GI, a large serving can still cause the blood glucose levels to rise more rapidly than a smaller serving. This is what is called the glycaemic load (GL).

The GL builds on GI, as it considers both the GI of the food and the amount of carbohydrate in a portion. GL is based on the idea that a high GI food consumed in small quantities would give the same effect on blood glucose levels as larger quantities of a low GI food.

GL can be calculated easily if you know what the food’s GI is and how much carbohydrate is present in the serving.

Calculating glycaemic load (GL)

The GL calculation is: GI x the amount of carbohydrates (in grams) in a serving of food) ÷ 100.

Using a pasta example:

• GI of a standard white wheat pasta, boiled to al dente texture = 43.
• The carbohydrate content of a standard 180g serve = 44g.
• GL = 43 x 44/100 = 19g.

However, if a half portion of pasta was eaten, the GL would also halve:

• GI of a standard white wheat pasta, boiled to al dente texture = 43.
• The carbohydrate content of a half portion 90g serve = 22g.
• GL = 43 x 22/100 = 9.5g.

Here is another example, where both foods contain the same amount of carbohydrate but their GIs are different:

• A small baked potato (GI = 80, carbohydrate = 15g).
• GL = 80 x 15/100 = 12g.
• An apple (GI = 40, carbohydrate = 15g).
• GL = 40 x 15/100 = 6g.

Both the small baked potato and the apple have the same amount of carbohydrate (15g). However, because their GIs differ (the apple is low while the baked potato is high), their GLs also differ, which means the baked potato will cause the blood glucose level of the person eating it to rise more quickly than the apple.

GI and exercise

Eating low GI foods 2 hours before endurance events, such as long-distance running, may improve exercise capacity. It’s thought that the meal will have left your stomach before you start the event but remains in your small intestine releasing energy for a few hours afterwards.

Moderate to high GI foods may be most beneficial during the first 24 hours of recovery after an event to rapidly replenish muscle fuel stores (glycogen).

Using the GI as a guide to healthy eating

The GI can be considered when choosing foods and drinks, but there are limitations. For example, the GI of some everyday foods such as fruits, vegetables and cereals can be higher than foods to be eaten occasionally (discretionary) like biscuits and cakes. This does not mean we should replace fruit, vegetables and cereals with discretionary choices, because the first are rich in important nutrients and antioxidants and the discretionary foods are not.

GI can be a useful concept in making good food substitution choices, such as having oats instead of cornflakes, or eating grainy bread instead of white bread. Usually, choosing the wholegrain or higher fibre option will also mean you are choosing the lower GI option.

It’s not always possible or necessary to choose all low GI foods. There is room in a healthy diet for moderate to high GI foods, and many of these foods can provide important sources of nutrients. Remember, by combining a low GI food with a high GI food, you will get an intermediate GI for that meal.

Choosing between high and low GI foods

The best carbohydrate food to eat varies depending on the person and situation. For example, people with type 2 diabetes or impaired glucose tolerance have become resistant to the action of insulin or cannot produce insulin rapidly enough to match the release of glucose into the blood after eating carbohydrate-containing foods. This means their blood glucose levels may rise above the level considered optimal.

Now consider 2 common breakfast foods – cornflakes and porridge made from wholegrain oats. The rate at which porridge and cornflakes are broken down to glucose is different. Porridge is digested to simple sugars much more slowly than cornflakes, so the body has a chance to respond with production of insulin, and the rise in blood glucose levels is less.

For this reason, porridge is a better choice of breakfast cereal than cornflakes for people with type 2 diabetes. It will also provide more sustained energy for people without diabetes.

On the other hand, high GI foods can be beneficial at replenishing glycogen in the muscles after strenuous exercise. High GI can also quickly restore blood glucose levels to normal when someone with diabetes is experiencing a ‘hypo’, which is when their blood glucose levels fall below the normal range of 4 to 8mmol/L. For example, eating 5 jellybeans will help to raise blood glucose levels quickly. A person with diabetes is only at risk of a ‘hypo’ if taking certain medications or injecting insulin.

If you have a medical condition, such as diabetes, it’s important to get advice from your doctor or specialist before making any changes to your diet.