There are four primary stages of food digestion: 1) intake through the mouth into the stomach and small intestine, 2) digestion of nutrients in the small intestine, 3) absorption of nutrients into the bloodstream through villi in the small intestine, and 4) passage of remaining waste to the large intestine and out of the body. Carbohydrates, proteins, and lipids are broken down by specific enzymes into simpler molecules that can be absorbed. For example, carbohydrates are broken into monosaccharides by enzymes in the mouth and small intestine, while lipids undergo emulsification and are broken into fatty acids and monoglycerides by pancreatic lipase.

1. DIGESTION AND
ABSORPTION OF
MACRONUTRIENTS
A.JEEVARATHINAM
ASSISTANT PROFESSOR
DEPARTMENT OF HOME SCIENCE
V.V.VANNIAPERUMAL COLLEGE FOR WOMEN
VIRUDHUNAGAR

2. INTRODUCTION
• Digestion is the process of breaking large, insoluble food molecules
into smaller molecules for absorption into the bloodstream.
• This process involves the use of many digestive fluids and enzymes
such as saliva, mucus, bile and hydrochloric acid, among others.
• Mechanical digestionbreaks large food particles into smaller ones.
• Chemical digestion involves the breaking of covalent chemical bonds
in organic mol ecules by digestive enzymes. Carbohydrates break
down into mono-saccharides, lipids break down into fatty acids and
monoglycerides, and proteins break down into amino acids

3. STAGES OF FOOD DIGESTION
There are four primary stages of food digestion in the human body that
include:
• After the intake of food through the mouth, it makes its way through the
stomach into the small intestine, where it is digested.
• The nutrients from the digested food get absorbed into the bloodstream
through small pores in the small intestine.
• The remaining undigested food is sent to the large intestine, where any
unprocessed water or nutrients are reabsorbed into the body.
• The remaining waste food product is passed out of the body in the form of
stools.
• Absorption is the process of the absorbing or assimilating substances into
the cells or across the tissues and organs through the process of diffusion
or osmosis

4. DIGESTION AND ABSORPTION OF CARBOHYDRATES
There are two types of carbohydrates that can
be digested by the human digestive system–
sugar and starch.
Sugar is broken down in the gastrointestinal
tract by the small intestine and three enzymes
present in the mouth, namely, Lactase, Sucrase,
and Maltase. In the same way, starch is broken
down with the help of the Amylase enzymes
which are present in the mouth and the
stomach. After digestion, carbohydrates are
absorbed in the small intestine with the help of
minute finger-shaped projections known as Villi
After digestion, carbohydrates are absorbed in
the small intestine with the help of minute
finger-shaped projections known as Villi.

5. DIGESTION AND ABSORPTION OF PROTEINS
Proteins play a vital role in the
growth and replenishment of
body cells and tissues. The
digestion of proteins takes place in
the stomach with the help of
protease and pepsin enzymes,
which breaks down the proteins
into amino acids. The process is
facilitated by the hydrochloric acid
present in the stomach. Amino
acids are tiny elements which get
absorbed into the blood system
through the wall of the small
intestine.

6. DIGESTION AND ABSORPTION OF LIPIDS
Lipids are organic compounds comprising fatty acids,
which are insoluble in water. Fats are the most common
examples of lipids.
The insoluble property of lipids makes the digestion and
absorption of fats a complicated process. The first step in
lipid digestion is emulsification (ē-mŭl′ si-fi-kā′ shŭn), by
which large lipid droplets are transformed into much
smaller droplets.
Emulsification is accomplished by bile salts secreted by
the liver. The enzymes that digest lipids are soluble in
water and can digest the lipids only by acting at the
surface of the droplets.
The emulsification process increases the surface area of
the lipid droplets exposed to the digestive enzymes by
increasing the number of lipid droplets and decreasing
the size of each droplet.
Lipase, secreted by the pancreas, digests lipid molecules
.The primary products of this digestive process are fatty
acids and monoglycerides.

7. DIGESTION AND ABSORPTION OF LIPIDS
In the intestine, bile salts aggregate around small droplets of digested
lipids to form micelles (mi-selz′ , mı̄-selz′ ; small morsels) (figure 16.25).
The hydrophobic (water-fearing) ends of the bile salts are directed
toward the lipid particles, and the hydrophilic (water-loving) ends are
directed outward, toward the water envi-ronment. When a micelle
comes in contact with the epithelial cells of the small intestine, the
lipids, fatty acids, and monoglyceride molecules pass, by simple
diffusion, from the micelles through the cell membranes of the
epithelial cells. Once inside the intestinal epithelial cells, the fatty acids
and monoglycerides are recombined to form triglycerides. These,
andother lipids, are packaged inside a protein coat. The packaged lipid-
protein complexes, or lipoproteins, are called chylomicrons (k -l̄o -mi′
kronz). Chylomicrons leave the epithelial cells and enter the lacteals,
lymphatic capillaries within the intestinal villi. Lymph containing large
amounts of absorbed lipid is called chyle (kı̄ l; milky lymph). The
lymphatic system carries the chyle to the bloodstream. Chylomicrons
are transported to the liver, where the lipids are stored, converted into
other molecules, or used as energy. They are also transported to
adipose tissue, where they are stored until an energy source is needed
elsewhere in the body. Other lipoproteins, called low-density
lipoproteins (LDLs) and high-density lipoproteins (HDLs), transport
cholesterol and fats in the blood.