Digestion: Breakdown of Food
How We Manage to Digest a Cow
Nearly everything we eat and drink (except
water) comes from living things and is
composed of carbohydrates, proteins and
fats that were produced during the life of
those organisms. Much of digestion involves
taking the complex molecules and structures
of life and breaking them down into smaller
and smaller pieces, first mechanically by
cutting and chopping, then often using heat,
then by chewing and finally by the actions of
acids and enzymes produced by the human
digestive tract that convert "food" into a
complex mixture of sugars, amino acids,
glycerols and free fatty acids that the cells
lining our small intestine can then carry from
the inside of our gut to our circulatory
system so they they can be transported
throughout the body to supply energy for
metabolism and movement, amino acids for
growth and excess energy for storage.
Why Does Food Have to be Broken Down?
Perhaps the most obvious reason is so it can
fit in our mouths. Many animals accomplish
this by tearing at food with their teeth. Most
humans use tools.
Once in our mouths, food has to be soft
enough to chew and swallow. Techniques like
heating and fermenting help accomplish this.
Even softened, chewed and swallowed, most food could never nourish us if
further breakdown did not occur. The cells lining our small intestine can
generally only absorb simple molecules like individual sugars, amino acids and
free fatty acids. By the time food hits our stomach, it may be softened and
chewed, but it's still mostly complex carbohydrates, proteins and triglycerides.
The invisible magic of digestion are the processes that take food from the
stomach to the bloodstream.
Enzymes are protein molecules that fold--at a molecular level and by virtue of
their amino acid sequence-- in very precise ways that allow parts of their
molecular structure to act as catalysts for chemical reactions. Catalysts are
compounds that greatly accelerate the rate of a chemical reaction--in some
cases allowing a reaction to occur in a fraction of a second that would
otherwise take thousands of years. Enzymes are among the most distinctive
and critical molecules of life. They control nearly every chemical reaction in the
body and digestion is no exception.
There are a number of enzymes involved in human digestion. Some break
starches down into sugars, some break proteins down into amino acids and
others cleave triglycerides in food into free fatty acids and glycerol. Because
these enzymes are crucial to digestion and because some of them are targets
for weight loss drugs, lets examine them in greater detail.
Amylases are enzymes that digest starch. One
amylase, Salivary Amylase is produced by
salivary glands in the mouth so that starch
digestion asctually begins even before food
enters the stomach. Starches are very long
chemical chains of individual sugar molecules.
Amylases will randomly break bonds between
sugars in the chain so that starches get
gradually and randomly chopped up into ever
smaller chains until eventually there is nothing
left but "links". These "links" are sugar molecules
and once a starch is fully converted into sugars,
starch digestion is complete and those sugars
can now be absorbed into the blood stream.
"Starch Blockers" are a class of chemicals that
interfere with amylases. They bind chemically to
the amylase enzyme and "poison" it-or render it
innactive and therefore unable to break-down
starch. If taken in large doses, starch blockers
can effectively cause starches to become
indigestable so that they remain stuck inside the
intestine where eventually they get passed into
the colon and digested by bacteria. Starch
blockers will be covered in greater detail in a
Lipases are digestive enzymes that break-down
fat. More specifically they break triglycerides
down into free fatty acids and glycerol which
can then be absorbed.
The weight loss drug called "Ally" or Orlistat is a "fat blocker" and it works by binding to and
innactivating pancreatic lipase so that about 30% of triglycerides in a meal remain
un-cleaved and therefore unabsorbable so that they pass unchanged into the large intestine
where (as every Ally user can attest) they cause some significant gastrointestinal distress.
Ally is covered in more detail in another section.
Proteases are enzymes that break proteins down into amino acids. There are a number of
different protease enzymes involved in human digestion. Because protease enzymes destroy
protein and because all enzymes including proteases are made of protein, they tend to
"attack" each other causing them to auto-digest. For this reason, many proteases like
trypsin have an innactive form and only get converted to the active enzyme when they are
Nucleases break down nucleic acids (DNA and RNA) and
they are also found in the human digestive tract because,
not surprisingly, much of the food we eat contains DNA and
RNA. Most foods do not contain a very large proportion of
nucleic acids but some, like liver, do.
Acids & Bile
Some digestion does not require enzymes. The stomach
produces strong acid that helps digest protein directly. The
gallbladder secretes bile which is a substance that helps
In order to absorb nutrients from food, we have to break it down into very tiny pieces and
then use acid and enzymes in our gut to convert starches, fats, proteins and nucleic acids
into small nutrient molecules that we can absorb.
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