We recently saw what the glycemic index of food (GI) is and how it can be used in meal planning.
In this article we are going to focus on carbohydrates, as they are our main source of energy.
So that the starches of the carbohydrates pass into the blood, they must first transform into glucose. This work is done by digestive enzymes, mainly alpha-amylases.
Many different elements, such as the food, its manipulation and the person influence the process of digestion and absorption.
Keeping them in mind is important because they can help us to keep the glucose levels more stable.
WHAT INFLUENCES THE GI OF FOOD?
Types of sugars
Simple sugars (monosaccharides) have different GIs. Although many things happen in our body so that what we eat is transformed into more or less high blood sugar values. In general we could say that the more we have to “transform” or “decompose” a food, the longer it will take to absorb that food.
For example, fructose is not only absorbed at a rate lower than glucose or sucrose, but also once it is assimilated at the intestinal level; it has to be transformed into the liver to become glucose. Therefore, foods rich in fructose have a GI lower than those containing glucose.
For its part, sucrose is the common sugar or “table sugar”. It is composed of a glucose and a fructose molecule, hence it has a GI lower than that of pure glucose, but higher than that of fructose.
Foods that contain maltose deserve a special mention. Although it is a type of sugar composed of two glucose molecules (whose index is 100), it has a GI of 110.
In summary, if we want to introduce something “sweet” into the diet, it is better to choose those foods that contain sugars with a lower GI. In this way, we can “help” moderate the sudden increases in blood sugar.
The composition of starch
The starch consists mainly of two types of components: amylose and amylopectin.
Amylose is less accessible to digestive enzymes and, therefore, more resistant to digestion. Instead, amylopectin is easier to digest, so it has a faster absorption.
The proportion of one and the other in a starch will determine the GI of the food.
Gelatinization of starch
When a starch is heated in too much water (for example, by boiling) its structure is modified. In the first phase, the grains that compose it are swollen progressively and a fraction of amylopectin is released into the water. If the heating is prolonged, an amylose fraction is also released. This phenomenon is known as gelatinization of starch.
The lower the proportion of amylose, the greater the gelatinization and alpha-amylase enzymes absorb the starch more easily. That is, that starch is transformed faster into glucose and, therefore, has a higher GI.
Conversely, if a food has a higher proportion of amylose, it will gelatinize less, it will not be converted so easily into glucose and its GI will be lower. As a result, blood sugar will rise less.
Practical examples:
- A potato has a very low amylose rate, which is why it has a high GI.
- Legumes contain between 33-66% amylose, which is why they have a low GI.
- Cereal starches usually contain between 15-28% of amylose, that is why absorption varies depending on the type of cereal consumed.
An important point: We must give special attention to industrial food labels. The waxy maize (1) almost contains no amylose (approximately 1-2%), so it has a very high GI (close to 100, almost like pure glucose).
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The thermal or mechanical process of foods
Any process that reduces the size of the particles of the foods will also cause those of starch to transform into smaller molecules. That will make them more accessible to the intestinal enzymes and, therefore, they will be absorbed faster (they will have a higher GI).
These modifications can be through thermal processes (for example, prolonged cooking) or mechanical processes (for example, the conversion of grain from wheat to flour).
Both hydration and heat increase the GI of a food. Thus, raw carrots have a GI of 20, but cooked carrots have a GI up to 50 by the effect of gelatinization of starch.
Some industrial processes also cause an increase in gelatinization by the use of binders such as modified starches, dextrinated starches (2) or the manufacture of flakes (instant potato puree, breakfast cereals, etc.). These processes produce, by extension, a greater GI. So you have an idea: cornflakes have a GI of 85 and the mashed potato flakes have a GI of 95.
The inflation of cereals (for example, popcorn or inflated rice) also increases between 15% and 20% the initial GI.
In summary, every process that makes the food molecules smaller makes them easier to absorb and, therefore, blood sugar rises faster.
The “pastification” of coarse wheat
“Pastification” is the result of exposing some industrial pastas composed only of durum wheat semolina, such as spaghetti or macaroni, to a high pressure extrusion process.
The extrusion causes a warming that forms a protective layer in these types of pasta, which is known as the “pastification” of coarse wheat. It is a natural technical process that stops the hydration of the starch and, consequently, its gelatinization. Therefore the GI of macaroni and spaghetti does not raise so much.
Laminated pasta, such as lasagna, is not extruded; hence its GI is greater. The same thing happens with fresh pasta, which has a mixture of durum wheat semolina and soft wheat flour.
Thus, depending on the production process and the flour used, similar foods can be found whose GI vary substantially, such as, for example, ravioli (70) and spaghetti (40).
Manipulation at home
Our way of cooking food also modifies the final GI. Thus, pasta al dente (3) is able to keep its GI at a lower value. However, prolonged boiling make the pasta to be soft and its GI will increase due to gelatinization of starch.
Retrogradation: the reverse process to gelatinization.
Starches that have undergone a baking process and have been gelatinized are modified again when they are cooled. This phenomenon is known as retrogradation. Thanks to it, amylose and amylopectin recover, greater or lesser, their original molecular structure. The richer the starch is in amylose at the start of the process, the more effective the phenomenon of retrogradation.
Keep in mind that retrogradation does not reverse 100 percent gelatinization, but it does allow the resulting GI to decrease.
In addition, it is important to know that retro-gradation increases with time and with the decrease in temperature. That is, the longer the cooling process; the lower the GI.
Also, when we add fat (lipids) to a starch that has been gelatinized, the retrograde process becomes slower.
And lastly: a starch that has undergone a retrogradation process loses some of its gelatinization capacity when it is reheated. This happens because approximately 10% of the starch becomes temperature-resistant. For example, the reheated pasta will not reach the same GI that it had when it was just boiled.
Practical examples:
With baking, the potato suffers a gelatinization process that increases its GI. That GI is maintained when it is hot, but when cooling, the starches are partly restructured, that is, their GI descends.
The same goes for macaroni: if we cook them al dente and we cool them to make a pasta salad, they will have a much lower GI than if macaroni are soft and hot.
And it also happens with the bread, which varies its GI depending on whether it is freshly cooked and hot, or hard, or toasted.
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The content in fibres, fats and proteins
There are some carbohydrates that have a natural protein content that could be the cause of a lower digestion of starches, which translates into a lower GI.
We see it, for example, in cereals. The presence of gluten causes the action of the digestive alpha-amylases to be slower, which translates into a more moderate increase in blood sugar levels.
Practical examples:
The GI of gluten-free spaghetti is 51, while that of spaghetti with gluten is 42.
Gluten-free multicereal bread has an IG of 79 and wheat base, 62.
Gluten free white bread has an IG of 80 and the wheat version, with an IG of 70.
In addition to the proteins, the presence of fibre also contributes to lower GI.
The food fibre content contained in a starch can be a barrier to the action of digestive alpha-amylases. That is, the fibre causes the absorption of carbohydrates to be slower.
Practical example:
Whole grains that are rich in fibre have a GI lower than the same “white” version, thus causing a gradual increase in blood sugar levels.
On the other hand, foods containing fat also slow down the digestion process.
The degree of maturation and aging
Amylaceous fruits such as bananas increase their GI depending on the degree of ripeness because the starches they contain are naturally transformed.
For example, a green banana starts from a relatively low IG: 40. As it goes on maturing, it can reach 65.
Although it is not a fruit, the same thing happens with potato: one that has been stored for several months will have a GI higher than the GI of a new potato.
The size of the particles
When an amylaceous is crushed, its starch particles become thinner, which facilitates chewing, absorption and, thereby, increases its GI.
For example: rice flour, chick pea’s flour and oats flour have a GI higher than the rice grain, whole chickpeas and oatmeal in flakes, respectively.
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LIMITATIONS ON THE USE OF THE GI AS A REFERENCE VALUE
Many people consider the GI as a mere theoretical concept because its usefulness depends on the multitude of factors that influence the final value.
Thus, some of the aspects that impede its application are:
- The GI varies according to the variety of the same food.
For example, in the case of potato there are many varieties (Ontario, Sebajo, Nardine …) and all have different GI.
- The way to manipulate foods can modify your GI.
For example, the GI of potato is different from that of a roast potato or fried potato. It even varies depending on whether we cook it with its skin or without it!
- The GI is affected by other foods.
For example, the GI of a potato omelette changes according to the quantity of potatoes, onions, egg, oil, and so on. In the same way, it will also be different depending on whether we eat the omelette with a salad, if we then take dessert, the accompanying drink, and so on.
- There are no GIs tables for all foods.
To make the GI more useful it would be necessary to calculate the index of all the foods that exist in the market, because there are similar products from different brands that can have a significant variation in their GI for its composition. As you will understand, it is quite unworkable to calculate the GI of everything.
- There are individual differences in the glycemic response.
Sugar levels after ingestion of one food vary considerably from one person to another … and even in one person according to the time of the day in which the test is performed!
For all this, and by way of conclusion: the GI of food can serve as a reference for organizing our meals, but not to predict what the exact response to a given food.
Updated: 24/06/2017
Vocabulary:
(1)
The waxy corn is also known as waxy maize, corn amylopectin or corn starch and is used to give texture to preserved or frozen foods, to thicken fruit jellies, and so on.
(2)
Dextrinization or hydrolysis is a digestive-like process, whereby large starch chains are broken into smaller pieces so they can be digested more easily.
(3)
Al dente is when the pasta is a little “hard”, usually after about 8 minutes of baking (depends on the type of pasta).
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If you have any doubts or questions about the treatment to follow, you should consult qualified medical personnel.