The glycemic index shows how long the body needs to obtain glucose from a food compared with pure glucose, which is set at 100. A lower glycemic index means slower digestion, a slower release of glucose into the blood, and usually more fiber and nutrients in the food.
The glycemic index is useful, but the glycemic index does not define whether a food is simply “good” or “bad.” Glycemic load and the amounts of sugars and fiber in a product also matter.
The short version
- The glycemic index compares a food with pure glucose, which has a reference value of 100.
- A lower glycemic index usually means slower digestion, a gentler rise in blood glucose, and longer satiety.
- A higher glycemic index often means a food is more refined, more processed, and lower in fiber.
- The glycemic index is not an absolute measure of usefulness, because the amount of carbohydrates in a portion also matters.
- Glycemic load helps assess a real portion, and low-GI syrups are not automatically a better choice.
What does the glycemic index mean?
The glycemic index is a relative measure of how long the body needs to obtain glucose from a food. The comparison uses pure glucose as the reference point, with a value of 100.
Carbohydrates are a large group of foods and the main sources of energy and fiber for the body. Under the action of enzymes, carbohydrates are broken down into simple glucose molecules, and glucose is then used to produce energy.
The part of carbohydrates that the body does not digest, namely fiber or dietary fiber, becomes food for microorganisms in the large intestine. The source page says modern research has shown that these microorganisms protect people from a number of diseases, so carbohydrates that contain both glucose and fiber are considered the most beneficial for health.
The body breaks large food molecules into smaller ones until it finally gets a glucose molecule, and glucose is small enough to be absorbed from the intestine directly into the bloodstream. Pure glucose requires no breakdown, while a food such as celery takes longer to break down because the body has to work through complex polysaccharides and separate them from fiber.
Why is a lower glycemic index considered useful?
A lower glycemic index means the body digests a food longer and releases glucose into the blood more slowly. A slower release causes a gradual and relatively small rise in blood glucose and helps a food keep a person full for longer.
A lower glycemic index also usually means the body receives fiber along with energy. The source page says vitamins, minerals, and other useful substances are generally found in the fibrous part of food, so a higher glycemic index usually means a product has been more thoroughly stripped of fiber and contains fewer useful substances.
Glycemic index categories are low at 55 or less, medium at 56 to 69, and high at 70 or more. The source page also notes that there is no upper limit, so 100 is not the maximum, and sweetened instant breakfasts with sugar or syrups may reach a glycemic index of 132.
What changes the glycemic index of a food?
The glycemic index depends not only on the presence or absence of fiber, but also on how finely a product is ground and how it is heat-treated. A more finely milled product and a more thoroughly cooked product are easier for the body to break down.
The source page gives simple examples of lowering the glycemic index through preparation. Pasta can have a lower glycemic index when cooked al dente, and porridge can have a lower glycemic index when larger grains are used.
Why the glycemic index does not make a food good or bad
The glycemic index is not an absolute measure of whether a food is useful. The source page gives the example that mayonnaise has a glycemic index of about 50, while a banana has a glycemic index of 65, and that comparison does not mean bananas should be replaced with mayonnaise.
The same food or dish can also produce different glycemic reactions in the same person at different times. The source page therefore advises against dividing foods into “good” and “bad” based only on the glycemic index.
The glycemic index is more useful for understanding whether a food provides only energy or also vitamins, minerals, and food for gut bacteria. The glycemic index also helps show whether a food gives a quick burst of energy or more even satiety over a longer period.
How to use the glycemic index when reading food labels
The glycemic index is useful when the total amount of carbohydrates does not reveal the quality of those carbohydrates. Two products can show the same total carbohydrates on the label and still differ greatly in fiber, sugar, and the speed at which glucose enters the bloodstream.
The source page uses cookies as an example. One cookie may be made from refined flour and sugar, with almost no fiber or nutrients and with a large amount of glucose entering the blood quickly, while another cookie may be made from coarse wholegrain flour, providing fiber, a more gradual release of glucose, and longer satiety.
An alternative to the glycemic index is to look at the carbohydrate breakdown on the label and compare sugars with dietary fiber. A product with a lot of sugar is worth limiting, while a product with a lot of fiber is a welcome part of a healthy diet, even when the total carbohydrates are the same.
What glycemic load means
Glycemic load takes into account not only the quality of carbohydrates, but also the quantity of carbohydrates in a portion. Glycemic load is calculated by multiplying the glycemic index of a product by the grams of carbohydrates in a portion and dividing the result by 100.
The source page gives two examples. One apple has a glycemic index of 38 and contains 13 grams of carbohydrates, so the glycemic load is 38 × 13 / 100 = 5, while potatoes have a glycemic index of 85 and contain 14 grams of carbohydrates, so the glycemic load is 85 × 14 / 100 = 12.
Glycemic load categories are low at 10 or less, medium at 11 to 19, and high at 20 or more. Glycemic load helps assess the effect of a specific amount of food.
The source page gives watermelon as an example of why glycemic load matters. Watermelon has a glycemic index of 75, but 100 grams of watermelon contain only 9 grams of carbohydrates, so the glycemic load is only 7, which is low.
Are low-glycemic syrups a better choice?
Low-glycemic syrups are not automatically a better choice. The source page says such syrups are largely made of fructose, and fructose has a low glycemic index, which can make those syrups seem like a healthier alternative to sugar.
The source page explains that the body needs glucose to survive and is well adapted to using glucose, while fructose is not essential and is absorbed less well. The source page also says scientific studies link excessive fructose intake with higher risks of metabolic syndrome, type 2 diabetes, non-alcoholic fatty liver disease, and gene damage in the brain that may lead to a range of mental illnesses.
The source page also says fruit should not be treated as forbidden because fruit contains fructose. The source page gives a practical comparison: 1 teaspoon of agave syrup contains 6 grams of fructose, while 1 large peach contains 1.5 grams of fructose, and eating 40 peaches at once is unlikely while 10 teaspoons of syrup is much easier.
Common questions
What is the glycemic index in simple terms?
The glycemic index shows how quickly the body turns a food into glucose compared with pure glucose. Pure glucose is the reference point with a value of 100.
Is a low glycemic index always better?
A low glycemic index is often useful because it usually means slower digestion and more fiber. A low glycemic index does not automatically make a food the best choice, because the full composition and the portion size also matter.
What is the difference between glycemic index and glycemic load?
The glycemic index describes how quickly carbohydrates from a food can raise blood glucose. Glycemic load also includes how many grams of carbohydrates are in the portion you actually eat.
Why can watermelon have a high glycemic index but a low glycemic load?
Watermelon can have a high glycemic index because the carbohydrates in watermelon are absorbed relatively quickly. Watermelon can still have a low glycemic load because 100 grams of watermelon contain only 9 grams of carbohydrates.
Are low-GI syrups healthier than sugar?
A low glycemic index does not make syrup automatically healthier. The source page says low-GI syrups are often rich in fructose, and excessive fructose intake is linked in research to several health risks.