10.3: The Chemical Principle of Digestion

There’s no telling how long this mechanical view of digestion might have persisted if it had not been for a pet bird and a wounded trapper. Between the two, they opened the way for our modern understanding of digestion.

The Pet Bird

The bird (a kite, relative of the owl), came along toward the end of the 1700s, the beloved pet of French scientist Rene de Reaumur. It had eating habits which, while repugnant to some, were fascinating to Reaumur.

The bird would gobble down its food in gluttonous chunks—then later spit up parts of the food, such as bones. To Reaumur, who was familiar with the mechanical ideas of digestion, it was another of nature’s ways of separating the nutritive from the non-nutritive. But as he looked closely at what was regurgitated, he had another impression. Some of it seemed to be decomposed, rather than just broken up or torn apart. Could something more than mechanical be happening in the bird’s stomach?

Actually, this observation wasn’t unique. Too many people had seen food returned after being partially digested. After all, the Roman wealthy had vomitoria adjacent to their dining rooms, in order to enable banquet guests to keep eating long after normal capacity had been reached. It was perfectly clear that such returned food had changed. But how—and why?

Reaumur had an idea. He put meat into a bit of metal tubing fastened to a string, lowered it into the bird’s stomach, and after an interval, pulled the tube up again. The meat had deteriorated.

Again and again, he experimented and was able to extract some kind of stomach juice from the meat. The juice was acid. When this juice was mixed with meat in a test tube, the meat began to disintegrate.

Word of Reaumur’s work intrigued others, who—stringing tubes and sponges—trapped the stomach juice of many kinds of animals and verified Reaumur’s discovery. Scientists even went so far as to fashion a perforated little metal ball which they used to get substances in—and out of—peoples’ stomachs. It became clear that the body had a chemical means of digestion, which caused food to be broken down and absorbed. But beyond this, the process remained a mystery.

The Wounded Trapper

The work of Reaumur and his colleagues seemed to have reached a dead end until 1822, when a severely wounded Canadian trapper named Alexis St. Martin was brought into the primitive surgery of young Dr. William Beaumont. Beaumont was the military surgeon of a frontier U.S. Army post on MacKinac Island, Michigan. St. Martin had been shot accidentally in the stomach.

With great care and skill, Beaumont managed to save the trapper, who was overwhelmingly grateful. But during convalescence, it appeared that St. Martin would retain a curious kind of souvenir. In healing, the wound formed a tube of flesh and muscle, following the path of the shot, from the skin of the belly into the stomach.

The surrounding abdominal muscles closed this odd hole, but Beaumont could open it and actually peek in to see the stomach at work. He was fascinated and asked permission to perform some experiments. The grateful St. Martin agreed—at first. Beaumont took the trapper into his home and provided financial support, but the experiments— eventually, over a hundred of them—dragged on for years.

As one can imagine, it became a constant struggle to get St. Martin to cooperate. (The repeated peeping into his stomach did no apparent harm. The trapper went on to father 17 children, and lived to the ripe old age of 86.)

Beaumont’s meticulously recorded observations opened up many new avenues of research. For example, copious amounts of acid juices seemed to come out of the stomach wall whenever food came through the trapper’s mouth. Yet, these digestive juices didn’t appear when Beaumont put food directly into the stomach through St. Martin’s special opening.

Beaumont also was fascinated to find that the appearance of the stomach was changed by thoughts and emotions—whether a fantasy of dinner or anger or fear. Thoughts and emotions also seemed to change the blood supply, the secretion of juices, and the activity of the stomach.

Nutrient:	Digested to:
Carbohydrate	Glucose, other single sugars
Protein	Amino acids
Fat	Fatty acids, monoglycerides

Table 10-1: Nutrients and Digestive Products.

The extent of the stomach’s activity surprised Beaumont. Far from being a passive sack, the stomach somehow twisted and kneaded, slowly churning food with the digestive juice. Liquids went through the stomach fairly quickly. Solid foods took much longer. The food seemed only to break down in the stomach, and not to be absorbed there into the body. With each stomach contraction, Beaumont noted that a small amount of partially broken-down food was pushed down into the intestine.

Beaumont’s observations led to further study of what happened in the intestine. By the middle of the 19th century, scientists perceived some dim outlines of the digestive truth.

The most active chemistry of digestion appeared to go on in the intestine. But little was then known of human nutrient needs—beyond some general ideas about carbohydrate, fat, and protein. It was evident that the digestive process had to proceed until it had refined the nutrients in some way. Indeed, scientists had a glimmer that even a refined nutrient had to be broken down further before it could be absorbed into the body. They even theorized about catalysts that would enable this ultimate digestion to take place.

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Digestion breaks down food into a form that can be absorbed. Absorption is the process of taking substances from the digestive tract and into the body.

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We now know that the processes of digestion and absorption are far more subtle and complex—and involve much more of the body—than Beaumont ever imagined.