The digestion is begun by the saliva in the mouth and is continued in the stomach, by the saliva, until the gastric secretions begin to act.
Starch is not acted on by the gastric juice but passes unchanged into the intestines, where it is converted, by the pancreatic juice, into dextrin, maltose, and glucose. It is thus absorbed into the blood.
After the digested starch passes into the blood it is taken to the liver and is there changed into glycogen and is stored in reserve. When the system needs to produce energy it is first furnished by the glycogen. When this is exhausted the fats and proteins are used.
The starches and sugars then are really the energy “reserves” of the body, any excess over the daily needs being stored until required.
Starchy foods should not be given to any one in whom, from disease or derangement, the starch-converting ferments, ptyalin in the saliva and pancreatin in the pancreatic juice, are lacking.
Because the child has not developed the ferment in sufficient quantities necessary for starch digestion, starchy food must not be given to a child under twelve to eighteen months; at least not until he has teeth and chews his food. Then he should be given starchy food in the form of a crust or hard cracker which he chews thoroughly and mixes with saliva.
Potatoes or bananas which the child does not masticate, should not be given him under the age of two years.
Fat
Fat is the most concentrated form of fuel and is readily oxidized. It has about twice the fuel value of carbohydrates. It is almost pure carbon, hence less chemical work is required to convert it into fuel, but more oxygen is needed.
The average fat person does not breathe deeply and does not take in sufficient oxygen to cause a combustion of the fat and produce energy. He is thus inclined to be lethargic.