Page 86 - Nutrition Essentials for Nursing Practice

74 U N I T 1 Principles of Nutrition

Cholesterol is found exclusively in animals, with organ meats and egg yolks the rich-
est sources. The cholesterol in food is just cholesterol; descriptions of “good” and “bad”
cholesterol refer to the lipoprotein packages that move cholesterol through the blood (see
Chapter 20). You cannot eat more “good” cholesterol, but you can make lifestyle changes,
such as quitting smoking, exercising, and losing weight if overweight, that increase the
amount of “good” cholesterol in the blood.

Because all body cells are capable of making enough cholesterol to meet their needs,
cholesterol is not an essential nutrient. In fact, daily endogenous cholesterol synthesis is
approximately two to three times more than average cholesterol intake. When dietary cho-
lesterol decreases, endogenous cholesterol production increases to maintain an adequate
supply. The body makes cholesterol from acetyl-CoA, which can originate from carbohy-
drates, protein, fat, or alcohol. Thus, eating an excess of calories, regardless of the source,
can increase cholesterol synthesis.

Dietary cholesterol increases total and LDL cholesterol, but the effect is reduced when
saturated fat intake is low. Dietary cholesterol may have an independent effect on heart
disease risk beyond its effect on serum cholesterol.

HOW THE BODY HANDLES 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, respectively (Fig. 4.5).
Monoglyceride: a
glyceride molecule Fat entering the duodenum stimulates the release of the hormone cholecystokinin,
with only one fatty which in turn stimulates the gallbladder to release bile. Bile, an emulsifier produced in the
acid attached. liver from bile salts, cholesterol, phospholipids, bilirubin, and electrolytes, prepares fat for
digestion by suspending the hydrophobic molecules in the watery intestinal fluid. Emulsi-
fied fat particles have enlarged surface areas on which digestive enzymes can work.

Most fat digestion occurs in the small intestine. Pancreatic lipase, the most important
and powerful lipase, splits off one fatty acid at a time from the triglyceride molecule, work-
ing 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 a 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.

The digestion of phospholipids is similar, with the end products being two free fatty acids
and a phospholipid fragment. Cholesterol does not undergo digestion; it is absorbed as is.

Absorption About 95% of consumed fat is absorbed, mostly in the duodenum and jejunum. Small fat
particles, such as short- and medium-chain fatty acids and glycerol, are absorbed directly
Micelles: fat particles through the mucosal cells into capillaries. They bind with albumin and are transported to
encircled by bile salts to the liver via the portal vein.
facilitate their diffusion
into intestinal cells. The absorption of larger fat particles—namely, monoglycerides and long-chain fatty
acids—is more complex. Although they are insoluble in water, monoglycerides and long-
Chylomicrons: lipo- chain fatty acids dissolve into micelles, which deliver fat to the intestinal cells. Once inside the
proteins that transport intestinal cells, the monoglycerides and long-chain fatty acids combine to form triglycerides.
absorbed lipids from The reformed triglycerides, along with phospholipids and cholesterol, become encased in
intestinal cells through protein to form chylomicrons. Chylomicrons distribute dietary lipids throughout the body.
the lymph and
eventually into the
bloodstream.