The digestibility of food and the ease with which it can be absorbed in the intestine are the most important facts to be considered in connection with any food. The time required for food to be digested in the stomach can be gauged by the time which elapses before it passes into the intestine. During this time the food is being converted into a fluid condition, and the more rapidly this is done the greater the digestibility of the food. The following table gives the results of some standard observations on the rate of digestion of some common foods: -

(A) One To Two Hours

7 oz. boiled milk.

7 oz. beef-tea.

Whites of 3 eggs.

7 oz. water, tea, coffee, and cocoa.

5 oz. white fish.

7 oz. cauliflower or asparagus.

2 1/2 oz. toast, rusk, biscuits.

(B) Two To Three Hours

5 raw or poached eggs. 3 1/2 oz. raw meat. 9 oz. sweetbread.

2 1/2 oz. oysters. 5 1/2, oz. bread or biscuits. 5 1/2 oz. rice, spinach, apples, or carrots.

(C) Three To Four Hours

8 oz. stewed or roast chicken.

3 1/2 oz. beefsteak.

(D) Four To Five Hours

Roast beef, goose, or duck.

Mashed lentils, peas, French beans.

The most easily digested foods are not necessarily the most easily absorbed. Information as to the ease and completeness of absorption of different foods is obtained by a comparison of the results of a joint analysis of the food and faeces. The figures gained by analysis of the faecal matters are, strictly speaking, not entirely reliable, since some of the nitrogen in the faeces is an excretion from the epithelial lining of the bowel. Proteins are less completely absorbed than fats and carbohydrates. Carbohydrates are the most completely absorbed. Experiments on the absorption of fats have shown that as much as 1/4 lb. of butter can be absorbed daily by individuals in health, but when taken in excess of that amount the balance appears in the faeces. The percentage of protein absorbed is Greater when taken in a mixed diet. On a diet of beef, bread and butter, milk, oatmeal, potatoes, and fruit the percentage absorbed has been found as follows: -

The proteins of vegetable food are less easily absorbed than the proteins of animal foods. This is well brought out in the following table: -

Table showing percentage of Proteins, Fats, and Carbohydrates absorbed for different Foods.

The bowels require a certain amount of residue (ballast) to act as a stimulus to peristalsis. In "invalid" feeding it is sometimes imperative to take advantage of the different absorptive powers of the intestine for various foodstuffs. For example, in cases of diarrhoea where peristalsis is excessive, foods like rice, milk, white of egg, pounded meat, which leave little residue, are indicated.

On the other hand, constipated subjects require a certain proportion of food to act as ballast for the intestine - these being taken from wholemeal bread, coarse oatmeal porridge, green vegetables, and the like. These points are further considered in the sections dealing with the treatment of disease.

In health, diet is usually prescribed in one of two ways, either by using tables of percentages by weight of proteins, fats, and carbohydrates - known as the weight per cent. method; or by using tables with the number of calories in form of proteins, fats, and carbohydrates per ounce of food - known as the calories per ounce method. These methods are necessary for the formation of dietaries for healthy people in large institutions, such as the army, navy, prisons, and workhouses; they are also of some value in the framing of hospital dietaries. Their value in clinical work is, however, much less than is commonly supposed. In diseased conditions the diet has to be adjusted in each instance from a knowledge of the digestive and assimilative power of the patient, special reference being paid to the influence of the food on fermentative and putrefactive processes in the intestine, little clinical importance attaching to the caloric value of the food.

Before passing to the more detailed consideration of the nutritive constituents of food, it is advisable to give a resume of our present knowledge of the processes of digestion, absorption, and assimilation of food. This is given in the following chapter.