The process of making food available for our cells to use starts with cooking. Starches in cereals, pulses and tubers need to be softened and gelatinised by a period of moist cooking before they can be digested. Cellulose cell walls need to be ruptured by the cooking process before the contents are accessible to digestion. Meat fibres also need to be softened to improve their digestibility.
Cooking also destroys the toxins that many plants contain. For example red kidney beans require over-night soaking and then boiling rapidly for ten minutes before they are safe to eat.
Food in the mouth is broken down mechanically and mixed with salivary amylase. The amylase will only start the digestion of starch in a neutral solution. If the food is acid it will not work. If the starch is coated in fat, then digestion of the starch will be limited.
The stomach produces gastric juice in response to the food entering the stomach. About three litres of juice are produced each day. The strong muscular walls mix the food with the gastric juices. The gastric juice contains much more of the enzyme pepsin and more hydrochloric acid when protein enters the stomach. This is a 0.2% to 0.4% solution of acid which is much stronger than acid foods. The acid provides the conditions needed for the digestion of protein as well as destroying most bacteria eaten with the food. A protein meal will remain between 2 - 4 hours in the stomach.
A starch and vegetable meal will pass through the stomach very quickly without stimulating the production of large amounts of acid or pepsin.
Fat can bring the digestion in the stomach to a standstill. The contents remain acid for a prolonged period and this can give rise to discomfort.
Small amounts of water, minerals such as sodium chloride, most B vitamins, vitamin C and alcohol are all absorbed directly from the stomach. Simple sugars are also absorbed and if the food contains too much sugar, this can result in a dangerous surge in blood sugar levels.
The semi-liquid partially digested food (chyme) is allowed to move slowly into the small intestine. Here, bile from the liver emulsifies the fats into tiny droplets. An alkaline liquid from the pancreas neutralises the stomach acid. In the neutral solution, the enzyme lipase splits the fats into fatty acids and glycerol, the enzyme amylase splits starch into maltose. The enzymes trypsin and chymotripsin continue to split proteins into short chain peptides and amino acids.
The final stages of digestion take place when the food is in contact with the cells that line the walls of the small intestine. The food molecules are absorbed by the cells and any remaining peptides are split into amino acids, maltose is converted to glucose, sucrose into glucose and fructose, lactose into glucose and galactose. Most of the absorption of digested food takes place through the surface of the small intestine, whose surface area is greatly increased by tiny finger-like projections called villi. Laxatives and excessive fibre interfere with the absorption of food, and phytic acid in wholemeal breakfast cereals can interfere with the uptake of calcium, iron and zinc.
The remaining cellulose and other fibre and indigestible remains now get processed by bacteria in the large intestine. These bacteria form some B-vitamins as well as useful amounts of vitamin K which are absorbed through the walls of the large intestine.
All the products of the digestion of starch and sugars form simple sugars which are carried by the bloodstream directly to the liver. The liver keeps a close control of the level of sugar in the blood, supplying the tissues with a constant level of glucose and storing the surplus as glycogen in the liver and muscles. When the glycogen reserves are full, further surplus sugar is converted into fat for storage.
Fatty acids absorbed by the small intestine do not pass directly into the bloodstream, but are converted back into fats and added to the lymph fluid. This rejoins the bloodstream but bypasses the liver. The fat circulates in the bloodstream and is available as an energy source for the body cells. Fat in the diet is more easily converted into body fat than carbohydrate in the diet.