What actually gets absorbed?
Almost everything you eat is absorbed in the small intestine. It is worth knowing what is on that list, because the interesting part of this course is what is not.
Protein becomes amino acids
Protein is a chain. Acid in the stomach starts unfolding it, enzymes in the small intestine cut the chain into single amino acids and short fragments, and those cross the lining into your blood. Your body then rebuilds them into whatever it needs — muscle, enzymes, antibodies.
Fat becomes fatty acids, with help
Fat does not dissolve in water, which is a problem in a watery gut. The solution is bile, made in the liver and released into the small intestine, which breaks fat into droplets small enough for enzymes to work on. The products are absorbed and repackaged for transport.
Remember bile. It comes back in a later chapter, because certain fibres bind it, and what happens next is one of the clearest effects fibre has on cholesterol.
Carbohydrate becomes sugar — if your enzymes can reach it
Starch is a chain of glucose units. You make an enzyme, amylase, that cuts those chains, and the glucose is absorbed. Table sugar, lactose and other simple carbohydrates are handled by their own specific enzymes.
The qualifier matters. Your enzymes recognise particular bonds. Present them with a carbohydrate chain built from different bonds, or wrapped in a structure they cannot get into, and nothing happens.
Vitamins and minerals
Water-soluble vitamins are absorbed directly. Fat-soluble ones travel with dietary fat, which is why they are better absorbed from a meal containing some. Minerals have their own transporters, and absorption varies considerably with what else is in the meal.
Go deeper: absorption is not the same as intake
What you swallow and what enters your bloodstream are different numbers. Absorption depends on the form a nutrient is in, what it is eaten with, how fast the meal moves through, and in some cases the state of the person eating it. This is why a label tells you what is in the food, not what you will end up with — and why claims built on absorption rather than composition need to be read carefully.
By the time material leaves the small intestine, the digestible part of your meal is gone. What continues into the colon is the part your own biology had no tool for.
So what happens to fibre?
Fibre is carbohydrate. It is built from sugar units, sometimes the very same glucose that makes up starch. And yet it arrives in the colon largely untouched.
That is not an accident of quantity or timing. It is structural, and it is the subject of the next chapter.