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.