Digestion is a combination of mechanical and biochemical processes by which food absorbed by a person is converted into substances that can be absorbed by the body.
After the food is chewed and swallowed, it enters the stomach, where it undergoes various modifications, allowing further absorption.
The digestive process continues in the small intestine under the influence of various food enzymes. It converts carbohydrates into glucose, splitting lipids into fatty acids and monoglycerides, and proteins into amino acids. These substances, absorbed through the intestinal wall, enter the blood. . Meanwhile, despite some generally accepted views, the absorption of these macronutrients does not last for hours and does not stretch to all six and a half meters of the small intestine. It is very important to know that the absorption of carbohydrates and lipids by 80%, and proteins – by 50% – is carried out during the first 70 centimeters of the small intestine.
Some believe that carbohydrates, fats and proteins are always fully digested. Many patients think – and nutritionists do not interfere with it – that absolutely everyone present on their plate (and, of course, counted) calories will enter the blood immediately after splitting the relevant food. In fact, everything is different.
The breakdown of carbohydrates is carried out by the action of digestive enzymes, especially amylase of the salivary and pancreas. And the hydrolysis of carbohydrates, that is, the conversion into digestible glucose, is directly dependent on their glycemic index.
The glycemic index of carbohydrate determines the ability of carbohydrate to increase glycemia, that is, the amount of glucose in the blood. In other words, GI expresses the ability of carbohydrate to hydrolyze, that is, to break down to glucose.
Thus, the glycemic index (GI) measures the proportion of glucose that will be obtained from a given carbohydrate during its processing by the body and, therefore, enters the blood.
If the glycemic index (GI) of glucose is 100, it means that if it enters the small intestine, it is absorbed through the intestinal wall by 100%. If the GI of white bread is 70, this means that the carbohydrate (starch) contained in it is 70% hydrolyzed and passes through the intestinal walls in the form of glucose.
By the same principle, if the GI of lentils is 30, then it can be assumed that the starch contained in it will only be absorbed by the body as glucose by 30%. Thus, with an equal caloric index of carbohydrates absorbed by us, the amount of glucose obtained by splitting them and entering the blood can vary considerably, depending on the GI carbohydrate. In other words, the glycemic index of the carbohydrate-containing product expresses its glucose bioavailability.
To facilitate the understanding of this phenomenon, we will reveal it using the term traditional dietetics, that is, “calories”.
From this table it can be seen that after the assimilation of fried potatoes in the body, three times more calories are released than after the assimilation of lentils, with equal portions of carbohydrates. Conversely, with equal portions, after splitting, lentils release three times less “calories” than potatoes.
In addition, it was experimentally revealed that the use of sugar (within reasonable limits) at the end of a meal, if it affects the glycemic result of a meal, is very insignificant. Sugar absorption (GI 70) will be reduced depending on how diverse the food was and how much dietary fiber and protein it contained. The situation is completely different if sugar enters the organism on an empty stomach, for example, in the form of sugary carbonated beverages (Coca-Cola). In this case, the carbohydrate is absorbed almost completely.
This moment is extremely important!
It is one of the basic principles of the Montignac Method and allows you to understand how you can reduce weight without reducing the amount of food consumed, but only by learning how to choose the right products.
This point is also important because it forces us to reconsider the blind and naive conviction of traditional dietetics that all the calories consumed by us are completely absorbed by the body.
Many nutritionists, who use the concept of the glycemic index, are mistaken in believing that the GI expresses only the magnitude of the peak of the glycemia. So the whole benefit of a low GI product is, in their understanding, to the fact that it helps to avoid a sharp rise in blood sugar levels, slowing down the absorption of glucose. Thus, the principle of the glycemic index of carbohydrates is mistakenly associated with the concept of “slow” and “fast sugars”, which many authors, in particular, Professor J. Slam, consider incorrect.
According to Jenkins’ explanation given in more detail in a special section of the site, the glycemic index of the carbohydrate product corresponds to the area of the triangle, which forms on the graph the hyperglycemia curve resulting from the intake of sugar. In other words, the carbohydrate GI expresses the amount of glucose produced by its breakdown and entering the blood through the intestinal walls. The lower the GI of the product, the less glucose will be released into the blood when it is digested.
In conclusion, we say that the glycemic index of the carbohydrate product, in addition to glycemia, measures the degree of absorbability of the carbohydrate, that is, its bioavailability. So an increase in blood glucose level only indicates the proportion of carbohydrate that has entered human blood as glucose after digestion.
Lipid absorption (fat)
The topic of lipids is traditionally unloved by nutritionists. Aversion to fats due to the fact that they are high-calorie: 9 kilocalories per gram.
Despite the deep-rooted stereotypes, not all fats that fall on our plate are fully absorbed in the digestive process. Their absorption depends on the following parameters.
The absorption of fatty acids is influenced by their origin and chemical composition:
- Saturated fatty acids (butter, beef tallow, lamb, pork, palm oil …), as well as trans fats (hydrogenated margarine …) tend to be deposited in fat reserves, and not immediately burned in the process of energy metabolism.
- Monounsaturated fatty acids (olive oil, duck or goose fat) are mainly used immediately after absorption. In addition, they contribute to the reduction of glycemia, which reduces insulin production and thereby limits the formation of fat reserves.
- Polyunsaturated fatty acids, in particular Omega-3 (fish oil, rep oil, linseed oil …), are always consumed directly after absorption, in particular, due to an increase in food thermogenesis – energy consumption of the body for food digestion. In addition, they stimulate lipolysis, (splitting and burning fat deposits), thereby contributing to weight loss.
Therefore, with an equal caloric composition, different types of fatty acids have a different, sometimes even opposite, effect on metabolism.
Fat absorption depends on the location of fatty acids relative to the glycerol molecule:
95 – 98% of the fats absorbed from food have a structure triglycerides. Their daily rate for a person is on average 100-150 grams.
In terms of chemistry, triglycerides are trihydric alcohol esters of glycerol and higher fatty acids. There are three possible options for the location of fatty acids relative to the glycerin molecule.
The proportion of fatty acid absorption depends on what position it occupies. It is important to know that only those fatty acids that occupy the P2 position are well absorbed .. This is due to the fact that food enzymes that break down lipids (lipases) have different degrees of impact on fatty acids, depending on the location of the latter.
This means that not all fatty acids from food are absorbed completely in the body, as many nutritionists mistakenly believe. They can be partially or completely not absorbed in the small intestine and be removed from the body.
- For example, in butter, 80% of fatty acids (saturated) are in position P2, that is, they are completely absorbed. The same applies to the fats that make up the milk and all non-fermented dairy products.
- On the other hand, the fatty acids present in mature cheeses (especially long-lasting cheeses), although they are saturated, are still in the P1 and P3 positions, which makes them less absorbable.
In addition, most cheeses are rich in calcium (especially hard cheeses, for example, Swiss gruyere …). Calcium combines with fatty acids to form “soaps” that are not absorbed and removed from the body.
From the above, we can conclude that the degree of absorption of fatty acids by the body, which are part of dairy products, is determined by the chemical factors of these products (fermentation, calcium content …). Not only the amount of energy released during digestion depends on these factors, but also the degree of risk for the cardiovascular system.
This observation was confirmed by specialized studies that revealed the relationship between eating dairy products that are not fermented (milk, butter, cream …), and the occurrence of coronary diseases. It was also found that with quantitatively equal consumption of dairy products that have undergone fermentation (cheese), the risk of developing cardiovascular diseases varies from country to country. A rather interesting comparison between residents of Finland and Switzerland. It was noted that the death rate from cardiovascular diseases in Switzerland is two times lower than in Finland, with approximately equal consumption of dairy products per person. One of the main explanations for this is that the Swiss, unlike the Finns, consume most of the dairy products in the form of fermented cheeses. Even more striking is the comparison between Finland and France. With the fact that the French eat two times more dairy products, the death rate from cardiovascular diseases in France is two and a half times lower. There are several explanations for this, one of which is the following: the French eat cheeses, which are not just fermented, but also seasoned. The ripening of cheese contributes to the transition of its fatty acids to the position P1 and P3, which indicates their low absorbability.
The amount of dietary fiber also affects lipid absorption.
The presence in food simultaneously with the fats of dietary fibers, in particular, soluble, affects the absorption of fatty acids. For example, eating pectin-rich apples and legumes, a source of gum, can lower hypercholesterolemia and also help prevent excess weight, reducing the amount of calories absorbed by the body.
Different parameters affect protein absorption:
- Protein originAnimal proteins are almost 100% absorbed in the intestine. Thus, they are completely released for use by the body. The percentage of absorption of vegetable proteins, with the exception of soy, is much lower:
– lentils – 52% – chick peas (chickpeas) – 70% – wheat – 36%
- Protein compositionIt is known that proteins are composed of different amino acids. The lack of one or several amino acids can become the limiting factor interfering the correct use of the others. So sometimes absorbed proteins after absorption are either inoperable, or have a weak activity that does not correspond to their number.
- Conclusion: Nutrients from food do not have full one hundred percent digestibility. The degree of their absorption can vary significantly, depending on the physico-chemical composition of the product itself and other products absorbed simultaneously with it. It is important to take this into account when taking measures to reduce weight or prevent cardiovascular diseases.