Lipids - Adequate Food for All - page 92

enhanced through emulsification by bile salts and fat breakdown products, produc-
ing small fat droplets with greater surface area and an increased accessibility to
fat-splitting enzymes (lipases). Fat digestion may begin in the stomach through the
action of lingual lipase, secreted by glands lying underneath the tongue. However,
most fat digestion occurs in the lumen of the upper small intestine, where pancreatic
lipase splits fatty acids from triacylglycerol to produce diacylglycerol, then monoa-
cylglycerol. Only a small percentage of triacylglycerol is completely hydrolyzed to
glycerol and free fatty acids.
The products of digestion combine with bile salts to form small aggregates
called micelles (which also may contain fat-soluble vitamins and cholesterol). These
micelles interact with the luminal membrane of the enterocyte (intestinal mucosal
cell), releasing their contents to the enterocyte interior. Short-chain fatty acids (fewer
than 10 to 12 carbon atoms) may pass through the cell and the basolateral mem-
brane into the portal blood, where they combine with albumin and are transported
to the liver. Longer-chain fatty acids and monoacylglycerols are reassembled into
triacylglycerols within the interior of the enterocyte, are coated with protein to which
carbohydrate is attached, and are formed into chylomicrons (which also carry fat-
soluble vitamins, cholesterol, and cholesterol esters [cholesterol combined with a
fatty acid]), which pass through the basolateral membrane into the lymphatics and
ultimately to the blood.
fAtty AcIds
C L a s s i f i C at i o n
saturated
Fatty acids with carbon chain lengths of 4 to 24 carbon atoms are found in foods
and body tissues. When connected by single bonds and with hydrogen atoms occu-
pying all available positions, the fatty acids are designated as saturated. Examples
include myristic acid (with a 14-carbon chain and no double bonds, 14:0), palmitic
acid (16:0), stearic acid (18:0), arachidic acid (20:0), and lignoceric acid (24:0).
monounsaturated
Fatty acids with one unsaturated bond include palmitoleic acid (a 16-carbon chain
with 1 double bond, 16:1) and oleic acid (18:1).
Polyunsaturated
Fatty acids with two or more double bonds include linoleic acid (18:2), α-linolenic
acid (18:3), arachidonic acid (20:4), eicosapentaenoic acid (20:5), and docosa-
hexaenoic acid (22:6).
trans Isomers
Fatty acids with double bonds can exist in either a cis (folded) or a trans (linear)
configuration. Natural fats contain mostly cis fatty acids, but when vegetable oils are
partially hydrogenated (by catalytically adding hydrogen at double bonds) to make
them solid at room temperature (as in production of margarines), electronic shifts