Digestion, Absorption and Assimilation are the three
processes by which the body incorporates food. In digestion, food is softened
and converted into a form soluble in the watery fluids of the body; or, in the
case of fat, into minute globules. The substances formed are then absorbed from
the bowels and carried throughout the body by the blood. In assimilation, these
substances, deposited from the blood, are used by the various tissues for their
growth and repair. Transit time through the digestive tract in young and
middle-aged people averages two to four days (slightly longer in the elderly,
and dependent on the amount of fibre in the diet). All but 12 hours of this is
spent in the colon.
Salivary Digestion occurs in the mouth with chewing, to break up the food,
and thoroughly to mix the starchy ingredients - such as bread and potatoes -
with saliva containing ptyalin that changes them into sugar. After swallowing
the salivary action is soon checked by the gastric acid in the stomach.
Gastric Digestion is facilitated by slow churning movements mixing the food
with the gastric juice and rendering it soluble. The final product, chyme,
passes through the pylorus into the intestine. Gastric digestion of a simple
meal takes about one hour, and a heavy dinner up to seven hours.
Intestinal Digestion. The softened food leaves the stomach and enters the
intestine where four factors act on it: (a) bile; (b) pancreatic juice (c)
intestinal juice, and (d) bacteria. Intestinal juice contains enzymes which
complete the breakdown of proteins into their constituent amino acids, act on
disaccharides, such as maltose, sucrose and lactose, converting them into the
monosaccharide glucose and split fats into fatty acids and glycerin. Bacteria
are normal occupants of both small and large intestines. In the former they have
a fermentive action, acting on carbohydrate to produce acetic, butyric and
lactic acids. In the latter they have a putrefactive action, decomposing protein
into its constituent parts. The intestinal bacteria have a key part in the
manufacture of certain components of the vitamin B complex.
Absorption. The only substance absorbed from the stomach to any extent is
Alcohol. Water quickly passes through the stomach into the intestine, but it is
only after several hours of intestinal digestion that the bulk of the food is
taken into the system. Fats produced from the chyle leaving the stomach are
taken up by lymph vessels called lacteals, ultimately reaching the blood, while
sugars, salts and amino acids formed from proteins pass directly into the small
blood vessels of the intestine. The process is facilitated by the large number
of microscopic villi which line the intestine. Food materials are mainly
absorbed by the small intestine, water and salts by the large intestine or
colon. The food is passed down the intestine by contractions of its muscular
wall until finally the indigestible residue, together with various wastes
excreted from the liver and intestinal walls, is expelled from the anus as
stools or faeces.
Assimilation takes place more slowly as the blood circulates through the
body and each organ extracts what is necessary for its own growth and repair.
When the food supply exceeds immediate bodily requirements, it is stored up for
future use, with fat being deposited in various sites, sugar being converted
into glycogen in the liver. Muscles have the greatest requirements with sugar
and amino acids being assimilated for heat production and work. To function
satisfactorily the body requires an adequate daily water intake of about 1.5 litres (3 pints), with a similar amount being discharged from the body in the
urine, perspiration and other excretions such as saliva and tears.