Digestion

The Chemistry Of Food and Nutrition
by A. W. Duncan, F.C.S.

(Page 8 of 11)

The digestive juices contain certain unorganized ferments, which produce chemical changes in the food. If the food is solid, it has to be liquefied. Even if already liquid it has generally to undergo a chemical change before being fitted for absorption into the body. The alimentary canal is a tubular passage which is first expanded into the mouth, and later into the stomach. As the food passes down, it is acted upon by several digestive juices, and in the small intestine the nutritive matter is absorbed, whilst the residue passes away.

The saliva is the first digestive juice. It is alkaline and contains a ferment called ptyalin. This acts energetically on the cooked and gelatinous starch, and slowly on the raw starch. Starch is quite insoluble in water, but the first product of salivary digestion is a less complex substance called soluble-starch. When time is allowed for the action to be completed, the starch is converted into one of the sugars called maltose. In infants this property of acting on starch does not appear in effective degree until the sixth or seventh month, and starch should not be given before that time. Only a small quantity should be provided before the twelfth month, when it may be gradually increased.

Dr. Sims Wallace has suggested that the eruption of the lower incisors from the seventh to the eighth month, was for the purpose of enabling the infant - in the pre-cooking stage of man's existence - to pierce the outer covering of fruits so as to permit his extracting the soluble contents by suction; and accordingly when these teeth are cut we may allow the child to bite at such vegetable substances as apples, oranges, and sugar cane. Dr. Harry Campbell says that starch should be given to the young, "not as is the custom, as liquid or pap, but in a form compelling vigorous mastication, for it is certain that early man, from the time he emerged from the ape till he discovered how to cook his vegetable food, obtained practically all his starch in such a form.

If it is given as liquid or pap it will pass down as starch into the stomach, to setup disturbance in that organ; while if it is administered in a form which obliges the child to chew it properly, not only will the jaws, the teeth, and the gums obtain the exercise which they crave, and without which they cannot develop normally, but the starch will be thoroughly insalivated that much of it will be converted within the mouth into maltose. Hard well baked crusts constitute a convenient form in which to administer starch to children. A piece of crust may be put in the oven and baked again, and spread with butter. Later, we may give hard plain biscuits." Dr. Campbell continues, that he does not say that starch in the pappy form, or as porridge, should find no place whatever in man's dietary at the present day, but we should arrange that a large proportion of our food is in a form inviting mastication.

The teeth perform the very important function of breaking down our food and enabling it to be intimately incorporated with the saliva and afterwards with the digestive juices. The Anglo-Saxon race shows a greater tendency to degeneracy in the teeth than do other races; the teeth of the present generation are less perfect than those of previous generations. A dentist writes "I have had the opportunity of examining the teeth of many natives in their more or less uncivilized state, from the Red Indians of North America, the negroes of Africa, to the more civilized Chinese, Japanese, and Indians of the East, and I have usually found them possessed of sound teeth, but so soon as they come under the influence of civilized life in Washington, Montreal, London, Paris and other cities, their teeth begin to degenerate, though their general health may remain good."

In a long article on mastication in the Lancet from which we have already quoted, Dr. Harry Campbell gives as the effect of thorough and efficient mastication, that it increases the amount of alkaline saliva passing into the stomach, and prolongs the period of starch digestion within that organ. That it influences the stomach reflex by promoting the flow of gastric juice. That the frequent use of the jaws and the tongue, during the period of growth, cause the jaws to expand. If the jaws are not adequately exercised during this period, owing to the use of soft food, they do not reach their normal size, the teeth are overcrowded, do not develop fully, and are prone to decay. The effect of vigorous mastication is to stimulate the circulation in the tooth pulp, which promotes nutrition and maintains a firm dental setting. Dr. Campbell writes: "I am perfectly at one with Dr. Wallace, in believing that the removal of the fibrous portion of food is the main cause of the prevalence of caries among moderns."

When the food reaches the stomach, gastric juice is secreted. This juice contains a ferment called pepsin and hydrochloric acid. Pepsin is only active in an acid media. Starch digestion proceeds in the stomach to such a time - stated as from 15 to 30 minutes - when the acid gastric juice has been poured out in sufficient quantity to neutralize the alkalinity of the saliva. The gastric juice acts upon the Proteins only. After a time the liquefied contents of the stomach are passed into the first portion of the small intestine, called the duodenum. Here it meets with the pancreatic juice, which like the gastric juice attacks Proteins, but even more energetically, and only in an alkaline media.

The pancreatic, the most important of the digestive fluids, contains other ferments; one called amylopsin, takes up the digestion of any remaining or imperfectly converted starch left from the salivary digestion. Amylopsin is much more powerful and rapid than the ptyalin of the saliva, especially on uncooked starch. Its absence from the pancreatic juice of infants is an indication that starch should not be given them. The bile is alkaline and assists the pancreatic juice in neutralizing the acid mixture that leaves the stomach; it also assists the absorption of fats. The digestion of Proteins is not completed in the stomach. There are some who look upon the stomach as chiefly of use as a receptacle for the large mass of food, which is too quickly eaten to be passed at once into the intestines; the food being gradually expelled from the stomach, in such quantities as the duodenal digestion can adequately treat.

A frequently used table, showing the time required for the digestion of various foods in the stomach, is of little practical value. There is ample provision for the digestion of food, there is a duplication of ferments for the Proteins and starch. In health, the ferments are not only very active, but are secreted in ample quantities. The digestive or unorganized ferments must not be confused with the organized ferments such as yeast. The latter are living vegetable cells, capable of indefinite multiplication. The former are soluble bodies, and though capable of transforming or digesting some thousands of times their mass of food, their power in this direction is restricted within definite limits. Another and preferable name for them is enzymes.

The action of saliva on starch is powerfully retarded by tea, this is due to the tannin. Coffee and cocoa are without effect. Tea infused for two minutes only, was not found to have sensibly less restraining effect than when infused for thirty minutes. On peptic digestion both tea and coffee had a powerful retarding effect. When of equal strength cocoa was nearly as bad, but as it is usually taken much weaker, its inhibitory effect is of little consequence.

Bacteria are minute vegetable organisms, which exist in the dust of the air, in water and almost everywhere on or near the surface of the earth. They are consequently taken in with our food. They exist in the mouth; those in carious teeth are often sufficient to injuriously affect digestion and health. The healthy gastric juice is to a great degree antiseptic, but few bacteria being able to endure its acidity. When the residue of the food reaches the large intestine, bacteria are found in very great numbers. The warmth of the body is highly favorable to their growth. They cause the food and intestinal debris to assume its focal character. Should the mass be retained, the bacterial poisons accumulate and being absorbed into the body produce headaches, exhaustion, neurasthenia and other complaints. Protein matter, the products of its decomposition and nitrogenous matter generally, are especially the food of bacteria; this is shown in the offensiveness of the faces of the carnivore, notwithstanding their short intestines, compared with that of the herbivore.

Manchester the Vegetarian Society, 1905.