The first step in the digestion of milk is that of coagulating or curding. Milk may be made to curdle by adding an acid to it, such as lemon juice or some other acid fruit juice or by the hydrochloric acid of the stomach. Normally, the coagulation of milk in the stomach of a young mammal is done by an enzyme secreted by glands in the stomach and known as rennin. This enzyme is especially abundant in the mucous lining of the stomach of young mammals and is extracted to be used in the manufacture of cheese.

The fifth edition of Harper’s Review of Physiological Chemistry (p. 177, 1955) says of rennin: “This enzyme causes coagulation of milk, and is important in the digestive processes of infants because it prevents the rapid passage of milk from the stomach. In the presence of calcium rennin changes irreversibly the casein of milk to a paracasein which is then acted upon by pepsin. This enzyme is said to be absent from the stomach of adults.”

Although, heretofore, it has been thought that the sole function of rennin is to coaugulate milk, it seems from Harper’s statement that it may be a true digestive enzyme. It changes casein to paracasein so that pepsin can act upon it. Enzymes are specific in their action. Each enzyme acts upon a particular type of food and certain of these can act upon a starch or a protein only after other enzymes have first acted upon them and changed them from their original composition. Harper’s statement seems to imply that pepsin acts upon paracasein rather than upon casein.

If this is the true relation of pepsin to the digestion of casein it means that rennin is essential to the efficient digestion of this protein. Rennin becomes of far greater importance in the digestion of milk than has heretofore been thought.

Rennin has been the subject of much controversy among physiologists. There was, first, the question: does rennin exist or does pepsin do the work ascribed to this enzyme? French and German investigators finally succeeded in establishing, to the satisfaction of everyone concerned, the existence of rennin as separate from pepsin. This did not end the controversy. While investigators now admit that rennin does, indeed, exist, many of them assert that it never exists in the human stomach, contending that it is found only in the fourth stomach of the calf.

In his Advances in Enzymology (London, 1954) Berridge defends the view that rennin is never found in the human stomach. He says that “Experiment tends to confirm the absence of rennin from human gastric juice.” On the other hand, Eusterman and Balfour, in The Stomach and the Duodenum (1936) state that, “according to a number of investigators, rennin tends to disappear from the adult stomach.” This statement implies that somebody, somewhere, found rennin in the human stomach, while its disappearance from the adult stomach has led to the suggestion that Berridge, who makes no distinction between infants and adults, made all of his experiments on adults.

In the second edition of his Textbook of Medical Physiology “1961” Arthur C. Guyton, M.D., says “rennin is found in the gastric juice of babies in large quantities, but it is present only to a very slight extent if at all in the gastric juice of adults. Also casein seems to be digested by babies much more easily than it is by adults, presumably because of rennin activity in the baby’s stomach.”

In the 1950 edition of The Physiological Basis of Medical Practice, Best and Taylor say that the rennin content of adults is “low” and provide us with the following data: “Rennin is especially abundant in the gastric mucosa of young animals, while pepsin is present in minimal amounts…The optimum pH for the action of rennin is between 5 and 6.5, and it is quite inactive at the pH of the gastric contents of the normal adult. In the infant, however, the pH of the gastric contents (5-6.5) is around the optimum for the action of this enzyme.

This indicates that the digestive processes required for the digestion of milk are somewhat different from those required for other foods and especially for other proteins. It is essential that the milk be coagulated and that the casein be converted into paracasein. I have seen two patients on a milk diet in which the milk did not coagulate, but was rushed along the digestive track into the colon and expelled in an unchanged fluid state. A glass of milk would be taken and in less than five minutes it would pass from the colon. Perhaps, in the absence of coagulation, milk would never be digested, but would pass through the digestive track too rapidly for the digestive enzymes to do their work.

In large numbers of other cases I have seen very large stools pass that were composed of large, hard milk curds that were white, apparently having undergone no digestion. Coagulation alone is not sufficient to assure the digestion of milk. In a few of these cases, the curds have been so large and there have been so many of them that bowel impaction resulted. We frequently see white curds in the stools of infants, indicating that, although the milk coagulated, apparently in a normal manner, the curds were not digested. We assume in these cases that milk has been taken in excess of enzymic capacity.

As rennin is active in low acid medium and is inactivated by the normal gastric juice of the adult, and as it is concerned solely with the digestion of milk, it should come as no surprise to us to learn that it is not secreted by the adult stomach. In this connection it should be stressed that the acidity of the juice poured into the stomach is determined by the food eaten.

Milk taken alone will occasion the flow of gastric juice that is low in acidity. Even in early childhood, when there is still a supply of rennin in the stomach, taking flesh, eggs or other protein at the same meal with milk will tend to result in the secretion of a highly acid gastric juice that will destroy or inactivate the rennin and interfere with or retard milk digestion, hence the wisdom of our rule: take milk alone or let it alone.

Rennin is apparently involved exclusively in the digestion of milk and tends to disappear from the gastric juice (is no longer secreted by the stomach) when the normal time to wean the child approaches. Some physiologists say that the concentration of rennin in the adult gastric juice is low; others say it isn’t detectable. A two year old baby normally has a mouth full of teeth and can begin eating solid foods.

At this age, also, the salivary glands begin the secretion of the enzyme ptyalin, which is necessary to starch digestion! Intestinal enzymes essential to starch digestion begin to be secreted at this time, also.

Thus, both the presence of adequate chewing apparatus and the secretion of digestive enzymes indicate that now is the time to begin the feeding of solid foods. In an article by a dentist, which appeared a few years ago, the author makes the statement that the baby should be weaned when the first two teeth are cut, as this signifies that solid foods are now to be taken.

Of all persons a dentist should know that two teeth do not enable a baby to chew foods adequately. As these first two teeth are in the front (are biters and not chewers), the baby is certainly not physiologically or anatomically equipped for such chewing.

When the first teeth are through and the starch-splitting enzymes are being secreted, there starts a decline in the production of rennin; this is to say, its secretion begins to decline at the age of two. It continues to be secreted in decreasing amounts during the next three to four years, that is, during what I have called the transition period (see Hygienic Care of Children), in which the child is normally making the transition from the exclusive milk diet of infancy to the solid food diet of later life.

During this transition period milk is normally taken. Should it surprise us to learn that when the child reaches the age at which it should normally be fully weaned its digestive glands cease to secrete the enzymes that are specially related to milk digestion?

One question comes to mind that I can find no data for an answer. It is this: Does the individual who continues to take milk regularly, from infancy into adulthood, continue to secrete rennin for a longer period of time than does the individual who is weaned at the normal weaning time of three to five years? This is to ask: Does the stomach continue to secrete rennin for an abnormally long period of time if the infant diet is persisted into late childhood and thereafter?

Does the persistence of the need for rennin cause the body to continue to secrete it? If it does continue to secrete this digestive enzyme longer than normal, at what time of life does it disappear from the digestive juice of the stomach? If it continues to secrete rennin but in much decreased amounts, in those who continue to take milk, does this account for the fact that some physiologists find no trace of rennin in the subjects they use in their tests?

While the answer to this question (I have cut it up into several subordinate questions) may prove to be of no great practical value, it would prove interesting as well as instructive concerning the power of the body to adjust itself to varying circumstances of life, especially its power of the body to adapt its digestive juices and enzymes to the food eaten. Every such advance in knowledge of the chemistry of digestion provides us with added data to assist us in determining, not only the normal diet of man, but, also, and of equal importance, the normal mode of feeding.

Today’s nutritionists wholly neglect all natural indications of the normal feeding of man. One food is as good as another and any food is equally as good at any time of life as at another, providing the commercial manipulators of our foodstuffs can prepare it in a manner that it is acceptable to all ages.

For example, although we know that during the first period of life after birth, even the lion cub does not eat flesh, our nutritionists do not hesitate to advocate flesh food for the human infant at a period of its development when the young carnivores of all kinds are still drawing their nutriment from the maternal font. Arrogance and egotism cause them to assume that they can improve the normal order of feeding. The results of their efforts are not encouraging.

While considerable confusion exists about rennin, there is one thing about which there is general agreement: namely, the adult human stomach has no rennin. Berg and others have shown that the adult organism does not use milk as efficiently as the rapidly growing organism and that, milk is base-forming in the infant and acid-forming in the adult. Berg attributed this to the more rapidly growing organism.

May it not be, in light of the foregoing that the greater efficiency of the young animal in assimilating milk rests upon the fact that the infant and young child digest milk better? This suggestion derives support from the fact noted by Berg and others, that, adult organisms handle sour milk more efficiently, the characteristics of the milk having been greatly altered by the ferment action of bacteria.

Every indication of nature is contrary to the present dogma of the dairy industry and the medical profession that we must take our quart of milk every day so long as we live; that, even at the age of a hundred, we are not to be weaned. Nature indicates that we are to be weaned at an early age. In medical circles the tendency of the pendulum, just at present, is to swing violently and far in the opposite direction.

In increasing numbers, medical men are blaming milk for a growing number of illnesses in children and adults. From being the all-good milk is rapidly becoming the all-bad. Certainly the assumption that invalids and convalescents, who have weak digestive powers, should be fed milk like an infant, because in their enfeebled condition they can handle milk better than other foods, is no longer tenable.

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The Hygienist will see in all these facts a justification of Graham’s condemnation of the use of milk and milk products by the adult and his observation that the use of milk by the adult makes him logy and lazy. Trall also pointed out that milk is not a normal part of the adult diet.

Those of us who have had an extensive experience with the milk diet, formerly so strenuously advocated as a near-panacea, will discover in the foregoing facts at least partial explanation of the many troubles that the milk diet produced. Polyuria, constipation, diarrhea, bowel impaction, nausea, much gas and discomfort, increased blood pressure, a water-logged state of the tissues (edema), catarrh, indigestion and other troubles arising out of “the harmless practice of overfeeding on milk,” necessitated all manners of manipulations to make it acceptable to patients.

From Dr. Shelton ‘s Hygienic Review, pages 274-276, Aug. 1969.

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