Page 87 - Nutrition Essentials for Nursing Practice

C H A P T E R 4 Lipids 75

Mouth Mouth Ingested fat—Triglycerides
Tongue Stomach
Lingual lipase
(limited roles)

Gastric lipase

Gallbladder Stomach Small intestine Bile
Gallbladder
Common
bile duct Emulsified fat (triglycerides)

Intestinal and Lipases
Pancreas pancreatic lipases

End-products for Monoglycerides,

absorption fatty acids, and glycerol

Large intestine Small intestine

Rectum
Anus

■ F I G U R E 4 . 5 Fat digestion. A minimal amount of chemical digestion of fat occurs
in the mouth and stomach through the action of lingual lipase and gastric lipases, respec-
tively. As fat enters the duodenum, it stimulates the release of the hormone cholecystoki-
nin, which in turn stimulates the gallbladder to release bile. Bile prepares fat for digestion
by suspending the hydrophobic molecules in the watery intestinal fluid. Most fat diges-
tion occurs in the small intestine. Pancreatic lipase splits off one fatty acid at a time from
the triglyceride molecule, working from the outside in until two free fatty acids and a
monoglyceride remain. Usually, the process stops at this point, but sometimes digestion
continues and the monoglyceride splits into a free fatty acid and glyceride molecule.
The end products of digestion—mostly monoglycerides with free fatty acids and little
glycerol—are absorbed into intestinal cells. It is normal for a small amount of fat (4–5 g)
to escape digestion and be excreted in the feces.

Their job done, most of the released bile salts are reabsorbed in the terminal ileum,
transported back to the liver, and recycled (enterohepatic circulation). Some bile salts
become bound to fiber in the intestine and are excreted in the feces.

Fat Catabolism

Whether from the most recent meal or from storage, triglycerides that are needed for
energy are split into glycerol and fatty acids by lipoprotein lipase and are released into the
bloodstream to be picked up by cells.

The catabolism of fatty acids increases when carbohydrate intake is inadequate (e.g.,
while on a very-low-calorie diet) or unavailable (e.g., in the case of uncontrolled diabetes).
Without adequate glucose, the breakdown of fatty acids is incomplete, and ketones are
formed. Eventually ketosis and acidosis may result.

Because fatty acids break down into two-carbon molecules, not three-carbon molecules,
they cannot be reassembled to make glucose. Only the glycerol component of triglycerides can
be used to make glucose, making fat an inefficient choice of fuel for glucose-dependent brain
cells, nerve cells, and red blood cells. Fortunately, most body cells can use fatty acids for energy.