The implications of cooking foods and methods used

This paper was adapted from an article in 'The Household Physician' by J McGregor-Roberton MA, MB, CM (Hons), FFPSG, FRS (Ed). Lecturer in Physiology at the University of Glasgow. Published by The Gresham Publishing Co, Ltd, London, 1932.

I find that the earlier works such as this, published before commercial interests started to ruin our lives for their profit, are very useful when looking at the healthy ways of eating.

Part One

General Considerations

For the nourishment of the body, proteins (substances containing nitrogen), such as albumin or the white of egg, the casein or curd of milk, the myosin of flesh, the gluten of wheat, and the legumin of peas and beans, plus carbon in the form of starch, sugar, and/or fat, are necessary, and their nutritive value depends upon the quantities of such substances various foods contain.

But these food-stuffs only become valuable when they have passed into and become part of the circulating blood in our bodies. They cannot do this in the condition in which we eat them; this is where the elaborate apparatus and process of digestion comes in, to convert foods into their constituent parts to be absorbed into the bloodstream.

It is plain that if one substance takes a longer time, and more work of the bodily organs, to undergo digestion, more energy will have been expended in the process, and that another substance, containing less nutritive material, might be actually more profitable to the body, because more easily and rapidly digested.

The time, therefore, the various food-stuffs occupy in order to undergo digestion requires to be taken into account in estimating their relative values.

Next we must consider how much of each nutritive material, which chemical analysis shows a food to contain, can be made available to the body and whether it is in the right form as to make it suitable for the action of the digestive fluids rendering it fit to enter the circulation. If the nutritive material it contains cannot readily be extracted from it by the process of digestion, much of it will never really enter the body, but will simply pass through the alimentary canal unchanged, and be finally excreted as waste from the bowels. If the quantity of such waste is measured, a fairly accurate idea of the actual amount of nourishment each diet has supplied to the body will be obtained.

Digestion

In determining the digestibility of foods, then, these two things require to be taken into account: the time required for their digestion, and the actual amount of each rendered available for purposes of nutrition. When this knowledge is added to that of the chemical composition of the various foods, we have the information necessary for estimating the values of the different foods.

Time Required for Digestion

There are several ways of acquiring this information. The active enzymes of digestion can be separated out from the organs of the body in which they are prepared. We can take a solution of egg white, a solution of boiled starch, and so on, place some in a test-tube, add to it a small quantity of the digestive enzymes, pepsin, pancreatin, etc, and then set it aside in a place kept at the temperature of the body, and note what time elapses before the food-stuff has been completely acted upon by the enzymes. Such experiments, made with different substances, under precisely similar conditions, will enable us to determine the relative speed of digestion of the different substances.

In the mid 19th century, a unique opportunity of observing the rapidity of digestion in the human subject, under ordinary conditions, was afforded to Dr. William Beaumont, in the person of one Alexis St. Martin, a Canadian, who had a permanent opening into the stomach through the skin, owing to a gunshot wound. Dr. Beaumont was able to introduce substances into the stomach through the opening and observe the rate of digestion. Since then many experiments and observations have furnished data as to the rate of digestion and solution of food in the stomach and of how quickly it passes from the stomach into the small intestine.

Factors that affect digestion

Drinks. Dr. Beaumont found that cold drinks introduced into the stomach in any quantity during digestion had a markedly slowing influence upon the process. Drinking iced water, or cold fluids in any quantity, eating of ices after meals, etc., are therefore not to be encouraged.

Exercise. Beaumont found that the time required for digesting the same substances varied on different days with varying conditions of the person himself, as well as with varying external conditions. An average meal was digested more readily than a small and insufficient one, and an excess of food slowed the process. The rapidity also varied with the nature and amount of previous exercise, and with the length of time since the preceding meal. After prolonged and exhaustive exercise the digestive organs are generally depressed, and food ought not to be taken immediately after exercise; similarly if a meal is eaten too soon after the previous one, the next one finds the stomach unprepared to receive it. In the case of St. Martin the state of the weather was also found to affect the rapidity of digestion. Active exercise immediately after a full meal also tended to interfere seriously with the digestive process, even to the point of stopping it altogether. But if the meal was small, moderate exercise was found to facilitate its digestion, and moderate exercise was also of advantage an hour or so after a full meal. As a general rule, after a full meal, such as dinner, it is better to pass an hour or more in pleasant conversation or some similarly light occupation, before any active work is begun. After lighter meals, snacks, tea, etc, this is not necessary.

Sleep. Somewhat conflicting views are held as to the influence of sleep after a meal, many maintaining that the after-dinner nap, customary with many people, interferes with the rapidity of digestion. It is unwise to lay down hard and fast rules. It is an undoubted fact that a light sleep after dinner is useful to many, though it may be harmful to others. But it can be said with some confidence that a sound and prolonged sleep is hurtful, because in sleep the activity of the vital processes is diminished. This, lasting for any time, would undoubtedly retard digestion.

Chewing. One extremely important factor in the rapidity of digestion of solid food is the degree to which it has been broken down by chewing. If the food remains in large pieces, the gastric juices have difficulty penetrating to the interior of the masses; whereas if the food has been broken up into minute portions, or shredded, they are all attacked at once, and digestion is rapidly completed. This is probably the reason why milk is easier to digest if it has been boiled rather that drunk raw. In the process of digestion milk curdles, the curd forming masses of considerable size, which take some time before they are broken down by the gastric juice. If the milk has been boiled, the curd formed is in much smaller masses, and hence the greater ease in digestion.

The time taken by some common food-stuffs to be brought into a state of solution and pass into the intestines is given in the following lists.

1. Foods which leave the stomach 2 to 3 hours after being swallowed.

Boiled milk ( ¾ pint); eggs (2), raw, poached, or omelette; beef sausage (4 oz); oysters (10); white bread, slice and a half (3 ½ oz.); rusks (3½ oz.); biscuits (1¾ oz.); sweetbreads (7 oz.); fish (7 oz.); asparagus (5 oz.); potatoes (5 ½ oz.).

2. Foods which leave the stomach 3 to 4 hours after being swallowed.

Skim or sour milk (1 pint); eggs, hard-boiled (2); chicken ( ½ lb.); lean beef (9 oz.); boiled ham (6 oz.); beef steak ( ¼ lb.); roast veal (¾ lb.); coarse bread 2 ½ slices (5 ½ oz.); boiled rice (5 ½ oz); boiled cabbage (5 ½ oz).

3. Foods which leave the stomach 4 to 5 hours after being swallowed.

Lentil porridge (5 ½ oz); pease porridge (7 oz.); salt herring (7 oz.); salmon (7 oz.); smoked beef (3 ½ oz.); smoked tongue (9 oz.); roast goose (9 oz.)

Of course, in using these tables it must be recognized that a mixture of two or three of these foodstuffs will be undergoing solution at the same time, thus a meal of 7 oz. white fish, 5½ oz. boiled cabbage, 9 oz. roast goose, would leave the stomach in about four to five hours and not the sum of the time of digestion of its component parts. An ordinary dinner then leaves the stomach in three to four or five hours according to its composition.

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