Food digestion physiology varies between
individuals and with the composition and size of a meal. A primer on what is
most common and universal will be useful to understanding the various processes
and their effect on our biological systems. This digest (note the pun), from Wikipedia, should provide some useful
information.
Digestion, the
hormonal process, has three phases. The
cephalic phase begins when receptors in the head are stimulated by the thought,
sight or smell of food, triggering various enzymatic and hormonal activities. The gastric phase starts with
distension, acidity, and the presence of amino acids and peptides in the
stomach. The intestinal phase is
initiated by distension, acidity, and osmolarity of digestive products in the
intestine, as we’ll explain.
Digestion, the
mechanical and chemical process, begins with chewing. Chewing breaks food down
to smaller units that are in turn broken down by enzymes to the smallest units,
permitting them to be absorbed into the blood, mostly through the wall of the
small intestine. Saliva containing mucus and the enzyme amylase is secreted
from salivary glands located in the mouth. Mucus moistens the food and amylase partially
digests polysaccharides (starches) into a disaccharide called maltose. About 30%
of starch is broken down in the mouth (optimum pH: 6.8).
In addition, papillae on the surface of the
tongue secrete a small amount of the enzyme lingual lipase which begins the
process of breaking down certain fats from triglycerides into diglycerides.
Long chain triglycerides cannot be absorbed unless completely broken down to
monoglycerides, so the process starts in the mouth and continues in the
stomach. As much as 30% of fat is broken down in 1 to 20 minutes by lingual
lipase, according to Wikipedia.
Food then passes through the pharynx and descends
the esophagus to the stomach, a sac that stores, mixes and processes food into
a milky solution called chyme. Glands lining the stomach secrete hydrochloric
acid, which is necessary for protein digestion. The stomach’s high acidity
(optimum pH 1.8) inhibits breakdown of carbohydrates within it. However, a
small amount of the enzyme lipase is secreted to continue the digestion of fats.
So far, although the breakdown of food particles
by enzymatic and mechanical action is continuing apace, virtually no absorption of nutrients into the
bloodstream has occurred. The exceptions are water, some simple sugars, and other
small molecules like alcohol that are
absorbed in the stomach, entering the circulatory system directly.
The final stages of digestion and most of the
nutrient absorption occur in the next portion of the tract: the small
intestine. The small intestine is divided into three segments – duodenum,
jejunum and ileum. The duodenum in turn is connected to the hepato-pancreatic
duct which connects to the liver and to the gall bladder and pancreas, providing
digestive enzymes and an alkaline fluid (pH 8.5) to neutralize acid emptied
from the stomach.
In the duodenum, dipeptides from partially
broken-down protein from the stomach are broken down to amino acids by these
enzymes. Other enzymes break down the disaccharides (maltose, lactose and
sucrose) into the monosaccharides glucose, fructose and galactose. The major portion of fat digestion, also happens
here.
Absorption of nutrients occurs mostly in the
jejunum and ileum. Amino acids (from protein) and the monosaccharides (glucose,
fructose and galactose, all from carbohydrates) are water soluble and enter the
blood directly through the small intestine wall. However, the products of fat
digestion, fatty acids and glycerol, are not water soluble and therefore enter
the circulation through the lymph system by a process called passive diffusion.
Passive diffusion requires no energy input from
the body because it is driven by concentration. The small intestine is full of
nutrients and the blood is not, so they cross over. This process is also called
osmosis. Monosaccharides (“sugars”) and amino acids from protein, are
transported across the membrane barrier by a process called “facilitated
diffusion,” meaning they need a little help because they occur against a
concentration gradient.
Finally, gastric emptying –the rate that food
leaves the stomach to enter the small intestine – is tightly controlled.
Liquids are emptied more quickly than solids; then carbohydrates are emptied,
followed by protein, fat and fiber.
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