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.
Why the small intestine is so good at this
Absorbing almost everything from a meal in a few hours takes an enormous amount of surface, and the small intestine is built to supply it in three nested stages.
The tube itself is several metres long. Its inner surface is thrown into folds, those folds are covered in finger-like projections called villi, and every cell on a villus carries a dense pile of even smaller projections called microvilli — the brush border. Each stage multiplies the last, turning a few square metres of plain tube into a surface measured in hundreds.
The brush border is also where the final enzymes sit. Some digestion happens right at the membrane, so a sugar is cut and absorbed almost in the same motion. Glucose is then pulled in against its concentration gradient by a transporter that moves it alongside sodium, riding the sodium gradient the cell maintains for the purpose.
Two things follow. Absorption is a transport problem, not a dissolving problem — it needs the right carrier for the right molecule. And most of it is finished early, within the first stretch of the small intestine, long before the material has anywhere near reached the colon.
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.