Digestion, Absorption and Assimilation of Fats

Fat digestion differs fundamentally from the digestion of carbohydrate and protein in one crucial respect: fat is not water-soluble, whereas the digestive enzymes that must act on it are dissolved in the watery fluid of the gut lumen, and an enzyme can only act efficiently at the surface where it meets its substrate. Left undisturbed, dietary fat entering the small intestine would form large globules with a comparatively small total surface area, on which pancreatic lipase could act only very slowly; the digestion of fat therefore requires a physical, non-enzymatic preparatory step — emulsification — before any significant enzymatic hydrolysis can proceed at a useful rate.

Very little happens to fat before it reaches the small intestine: a minor amount of gastric lipase in the stomach can act on already finely dispersed fat, such as that in milk, but this makes only a small contribution to overall fat digestion, and the churning of the stomach itself does not achieve true emulsification. The major work begins in the duodenum, where bile — secreted continuously by the liver and released in a concentrated form from the gall bladder under the action of cholecystokinin (Section 14.6) — supplies bile salts that act rather like a detergent: their molecules have one water-soluble and one fat-soluble end, allowing them to coat the surface of large fat globules and break them up, with the aid of the mechanical churning of segmentation, into a very much larger number of far smaller droplets. This process, emulsification, dramatically increases the total surface area of fat exposed to the surrounding fluid, without itself breaking any chemical bond in the fat molecules.

Only once fat has been emulsified in this way can pancreatic lipase, secreted by the pancreas into the duodenum, act efficiently at the surface of each droplet, hydrolysing the triglyceride molecules there into free fatty acids and monoglycerides (and, to a lesser extent, glycerol). These fatty digestion products, being still poorly soluble in the watery intestinal fluid, are then packaged by bile salts into much smaller aggregates called micelles — tiny, water-soluble spherical structures with the fatty acids and monoglycerides tucked inside a shell of bile salt molecules — which carry them through the watery layer immediately adjacent to the intestinal epithelium and deliver them directly to the brush-border surface of the absorptive cells, where the fatty acids and monoglycerides then simply diffuse across the cell membrane (being fat-soluble, they cross with no need for the active-transport carriers used for sugars and amino acids), leaving the bile salts themselves behind in the lumen to be recycled and used again. …