TRANSFORMATION INTO NUTRIENTS
Imagine an experiment with a tube perforated with several small holes. If we pour water into the tube, it easily comes out of the holes. The same thing can be done with sand.
Once again, the grains of sand pass through the holes easily. If we use pebbles instead, they will go from one end of the tube to the other, without passing through any of the lateral holes. These pebbles must be made smaller if they are to pass through the holes.
The digestive tract does the same with food, without us even realizing it. It transforms the food we eat into simple nutrients, so that these nutrients can pass through the intestinal wall. The body can absorb some nutrients directly. This means that they can pass through the wall of the digestive tract, just like water and sand passed through the perforated tube. Here we are obviously not talking about sand, but about vitamins, minerals and some simple carbohydrates.
Other nutrients are more complex and must be transformed, just like our pebbles. For example, lipids must be transformed into fatty acids.
Proteins must be transformed into small peptides and amino acids.
Complex carbohydrates, such as starch, must be transformed into simple carbohydrates such as glucose.