Digestion breaks large food molecules into units small enough to cross the intestinal wall. Absorption moves those units into the blood or the lymph.
Carbohydrates
Amylase from saliva and the pancreas breaks starch into small sugars, and brush border enzymes finish the job. Glucose, galactose and fructose are absorbed and travel to the liver through the portal vein.
Proteins
Pepsin in the stomach and pancreatic proteases cut proteins into peptides. Brush border enzymes release amino acids, which enter the blood.
Fats
Bile salts emulsify fat and lipase breaks it into fatty acids and monoglycerides. Intestinal cells repackage them as chylomicrons, which enter the lacteals and reach the blood through the lymph.
Vitamins, minerals and water
Iron and calcium are absorbed mainly in the duodenum, and vitamin B12 in the last part of the ileum. The small intestine absorbs most of the water, and the colon takes up much of the rest.
| Nutrient | Broken into | Absorbed into |
|---|---|---|
| Carbohydrates | Glucose, galactose and fructose | Blood (portal vein) |
| Proteins | Amino acids and small peptides | Blood (portal vein) |
| Fats | Fatty acids and monoglycerides | Lymph (lacteals) |
At the bedside
A nutrition assessment looks for signs of poor absorption such as weight loss, diarrhea, anemia and low albumin. A patient who cannot eat but has a working gut can be fed through a tube.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).