The glycemic index (GI) is the rate at which blood sugar increases after intake of carbohydrates. The speed is expressed in percentage by taking glucose as the reference point (100%).
A glycemic index of 50 indicates that the food raises the blood sugar examined with a speed that is half that of glucose.
This value is influenced primarily by the quality of carbohydrates; in general, the more simple and refined they are, more the glycemic index increases. However, there are exceptions; for example, experimental studies have shown that some polymers of glucose are absorbed more rapidly than glucose. Simple sugars, in fact, when taken in high doses attract water into the lumen of the digestive system by slowing the emptying times and absorption.
The obtained value depends not only on the complexity of carbohydrates; for example, rice and potato starch has a glycemic index higher than that of apples and peaches. Fruits and vegetables generally have low glycemic indexes.
- HIGH INDEX: glucose, honey, white bread, potatoes, cereals, crackers, breakfast cereals, grapes, bananas, carrots, rice.
- MODERATE INDEX: wholegrain bread, pasta *, corn, oranges, whole grains for breakfast, polished rice.
- LOW INDEX: fructose, yogurt, peas, apples, peaches, beans, nuts, parboiled rice, milk.
* Between different types of pasta, the noodles are those with lower glycemic index.
The glycemic index depends on several factors: the prevalence of amylose or amylopectin (glucose polymers, components of starch), by any technological manipulations of the food, cooking, the presence of fibers and the amount of sugar present (a ripe fruit has a higher GI than an unripe fruit).
In addition, the glycemic index is influenced by interactions with fats and proteins as the presence of these two macronutrients slows the rate of intestinal absorption.
That is why it is healthier to associate with a meal rich in carbohydrates such as pasta protein foods such as meat or fish and/or high-fiber foods such as vegetables. Paradoxically, if the goal is to lose weight it is best to eat a plate of pasta with a can of tuna and tomato rather than eating the same amount of pasta without toppings. Furthermore, adding a tablespoon of olive oil will slow the subsequent rise of hunger and decrease the glycemic index of the meal.
The glucose-fructose ratio is important: a higher fructose content reduces the rate of absorption of carbohydrates.
Importance of glycemic index and glycemic load
The glycemic load (GL) is an index that in addition to representing the quality of carbohydrates (IG) also considers their amount. So 100 grams of pasta, that contain about 74 grams of carbohydrates in the medium glycemic index, will increase your blood sugar more than a banana that has a higher glycemic index but contains less carbohydrates.
And yet, fructose has a very low glycemic index (GI = 20), but 50 g of fructose cause an increase in blood glucose greater than 10 g of sugar (GI = 66).
Calculation of the glycemic load
The glycemic load is calculated by multiplying the amount of carbohydrate in grams for the glycemic index.
That’s why 50 grams of fructose increases more blood glucose compared to 10 grams of sugar.
GLYCEMIC LOAD FRUCTOSE GL = 20 * 50g = 1000
GLYCEMIC LOAD SUGAR GL = 66 * 10g = 660
where 20 and 66 are, respectively, the glycemic index of fructose and sugar.
Glycemic index and glycemic load offer information about how foods affect blood sugar and insulin. The lower a food’s glycemic index or glycemic load, the less it affects blood sugar and insulin levels. Here you can download a list of the glycemic index and glycemic load for more than 100 common foods (“International tables of glycemic index and glycemic load values: 2008”).*
*The complete list of the glycemic index and glycemic load for more than 1,000 foods can be found in the article “International tables of glycemic index and glycemic load values: 2008” by Fiona S. Atkinson, Kaye Foster-Powell, and Jennie C. Brand-Miller in the December 2008 issue of Diabetes Care, Vol. 31, number 12, pages 2281-2283.