Food fat's digestive journey (how body digests fats)
Food fat's digestive journey
Some definitions
Triglycerides, phospholipids and cholestero l - see
Triglycerides,
phospholipids and cholesterol
Esterification - a chemical reaction between an alcohol and an acid.
Emulsification - Process of mixing fats with water to help solubilize hydrophobic
molecules - E.g.
Phospholipids are emulsifiers.
Lipolysis -
break down of
fats.
Exogenous lipid metabolism (Lipid
Sources from outside the body )
Lipids
(Triglycerides, Phospholipids,
Cholesterol [and other
sterols])
are absorbed from digested food
- 95% of dietary
lipids are Triglycerides (TGs) -
glycerol esterified to 3 fatty
acids; primary source of cellular energy production, or can be stored for later
use. Contain more than twice the energy of carbohydrates or protein.
- The rest are
Phospholipids (PLs) -
(glycerol esterified to two fatty
acids and a phosphate group), free fatty acids
(FFAs), esterified cholesterol (food form of cholesterol is
esterified), and fat-soluble
vitamins
The Fat Journey - From food ==> fat cells,
muscle cells and liver
Large
water-insoluble fats must be transformed into a water soluble form for their
journey in the circulatory system to the liver:
(1) Stomach Peristalsis - after partial
breakdown in the mouth by lingual lipase enzymes in saliva, all food fats are
first vigorously churned in the stomach .
(2)
Emulsification/Micellarization - The insoluble lipid food globules are then emulsified and made smaller
by the action of bile salts * in the small intestines-this results in
bile-enveloped drops (micelles) with increased accessible surface area for
further break down by the pancreatic enzymes.(* Bile salts are made from cholesterol in the liver and
stored in the gallbladder).
Gallbladder and bile
(3) Fats are freed from their packages
Fatty acids are split
from triglycerides
(Hydrolyzation) - Pancreatic lipase enzymes catalyze the hydrolization of
TGs into free fatty acids (FFAs) and monoglycerides (MGs):
TGs
(triglycerides) ➜
MGs (monoglycerides) and
FFAs (free fatty acids)
( pancreatic lipase )
Cholesterol De-esterified
Cholesterol esters are de-esterified
➜ free cholesterol
(Pancreatic
cholesterol ester hydrolase )
Fatty
acids split from phospholipids
Phospholipids ➜
Phospholipid intermediates and FFAs
(Phospholipase A2 )
(4)
MGs, FFAs, Free
cholesterol shuttled to intestinal villi for absorption - Generally
however, FFAs with < 14 carbons in their chain enter the liver directly via the
portal vein. This is of value to those with (1) impaired bile secretion (e.g
with biliary obstruction, liver diseases, gall-bladder removal) or (2) poor
pancreatic lipase enzyme secretion (as in cystic fibrosis). Short/Medium-length
carbon chain FFAs (E.g.in coconut oil and butter) are thus better
utilized as energy sources in individuals with fat malabsorption.
(5)
MGs and FFAs are
reassembled into TGs, and packaged together with cholesterol into chylomicrons - this takes place
in the intestinal lining cells (enterocytes). Chylomicrons are the largest of
the lipoproteins, whose purpose is to transport fat from food in the "watery" blood.
(6)
Chylomicrons transport
dietary TGs and cholesterol - through the lymphatics into the circulation.
(7)
ADIPOSE and MUSCLE CELLS
take up FREE FATTY ACIDS and GLYCEROL for energy use or storage - obtained by
hydrolizing 90% of the TGs contained in the chylomicron.
(8)
CHOLESTEROL and the FAT-SOLUBLE VITAMINS (A,
D, E, and K)
are delivered directly to the LIVER in chylomicron remnants - where these
remnants are cleared.
Endogenous lipid metabolism (Lipid
Sources from inside the body )
Primarily, the liver synthesizes TGs, cholesterol and their carriers (called lipoproteins)
to transport them in the blood - These carriers circulate until their contents are taken up by
peripheral tissue cells or the lipoprotein carrier itself is cleared by the
liver. As they lose TGs, the lipoproteins become more cholesterol rich.
Cells can also produce cholesterol