Chapter 3_Macronutrient Digestion
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Date
Dec 6, 2023
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26
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From
The Kansas State University Human Nutrition (HN 400) Flexbook
by Brian Lindshield is in the public
domain in the United States.
3 Macronutrient Digestion
You probably do not think too much about what actually happens to the food you eat. This
section will describe in depth how what you eat is digested. The desired end result for the
learner will be an integrated understanding of the process. This will require higher levels of
thinking, but will prove to be well worth it in the end.
Sections:
3.1 Digestion at a Glance
3.2 Mouth to the Stomach
3.3 Stomach
3.4 Small Intestine
3.6 Large Intestine
No References
3.1 Digestion at a Glance
Digestion is the process of breaking down food to be absorbed or excreted. The gastrointestinal
(GI, digestive) tract, the passage through which our food travels, is a "tube within a tube." The
trunk of our body is the outer tube and the GI tract is the interior tube. Thus, even though the
GI tract is within the body, the actual interior of the tract is technically outside of the body. This
is because the contents have to be absorbed into the body. If it's not absorbed, it will be
excreted and never enter the body itself.
A number of organs are involved in digestion, which collectively are referred to as the digestive
system.
Figure 3.12 The digestive system
1
The organs that form the gastrointestinal tract (mouth, esophagus, stomach, small intestine,
large intestine (aka colon), rectum, and anus) come into direct contact with the food or
digestive content.
Figure 3.13 The gastrointestinal or digestive tract
2
The journey through the gastrointestinal tract starts in the mouth and ends in the anus as
shown below:
Mouth --> Esophagus --> Stomach --> Small Intestine --> Large Intestine --> Rectum --> Anus
In addition to the GI tract, there are digestion accessory organs (salivary glands, pancreas,
gallbladder, and liver) that play an integral role in digestion. The accessory organs do not come
directly in contact with food or digestive content.
Figure 3.14 Digestion accessory organs
1
There are a number of
enzymes
that are involved in digestion. We will go through each one in
detail, but this table should help give an overview of which enzymes are active at each location
of the GI tract.
Location
Enzyme/Coenzyme
Mouth
Salivary amylase
Lingual lipase
Stomach
Pepsin
Gastric lipase
Small Intestine
Pancreatic alpha-amylase
Brush border disaccharidases
Pancreatic lipase
Colipase
Phospholipase-A2
Cholesterol esterase
Proteases
Brush border peptidases
Table 3.11 Digestive enzymes
References & Links
1. http://www.wpclipart.com/medical/anatomy/digestive/Digestive_system_diagram_page.png.html
2. http://commons.wikimedia.org/wiki/File:Digestivetract.gif
Video
Enzymes and Digestion
(http://www.youtube.com/watch?v=bNMsNHqxszc)
3.2 Mouth to the Stomach
Digestion begins in the mouth, both mechanically and chemically. Mechanical digestion is called
mastication, or the chewing and grinding of food into smaller pieces. The salivary glands release
saliva, mucus, and the enzymes, salivary amylase and lysozyme.
Figure 3.21 The mouth
1
Salivary amylase cleaves the alpha 1-4 glycosidic bonds in the starch molecules, amylose and
amylopectin. However, salivary amylase cannot cleave the branch points in amylopectin where
there are alpha 1-6 glycosidic bonds, as shown in the figure below. Overall this enzyme
accounts for a minor amount of carbohydrate digestion.
Figure 3.22 Enzymatic action of salivary amylase. Purple arrows point to alpha 1-4 glycosidic
bonds that can be cleaved. The yellow arrows point to the alpha 1-6 glycosidic bonds that
cannot be cleaved
Lysozyme helps break down bacteria cell walls to prevent a possible infection. Another enzyme,
lingual lipase, is also released in the mouth. Although it is released in the mouth, it is most
active in the stomach where it preferentially cleaves short-chain fatty acids in the sn-3 position.
Lingual lipase has a small role in digestion in adults, but may be important for infants to help
break down triglycerides in breast milk
2
.
Swallowing
Now that the food has been thoroughly chewed and formed into a bolus, it can proceed down
the throat to the next stop in digestion. It will move down the pharynx where it reaches a "fork
in the road", with the larynx as one road and the esophagus as the other. The esophagus road
leads to the stomach; this is the direction that food should go. The other road, through the
larynx, leads to the trachea and ultimately the lungs. This is definitely not where you want your
food or drink going, as this is the pathway for the air you breathe.
Figure 3.23 Cross section of face. The epiglottis covers larynx to prevent food and drink from
entering the lungs
3
Fortunately, our body was designed in such a way that a small tissue, called the epiglottis,
covers the opening to the trachea. It directs the food down the correct road as shown below.
Figure 3.24 Epiglottis is like a traffic cop guiding food down the correct digestion road.
Esophagus
Before being correctly guided into the esophagus, the bolus of food will travel through the
upper esophageal sphincter. Sphincters are circular muscles that are found throughout the
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