Ever wonder how food is digested?
We eat it every day, but you may not have asked your why your body actually needs food. Yes, food can taste good, especially processed foods that have been designed to tweak your salt and sweet taste buds. (Do you think the flavor “salted caramel” is an accident?) But food has much more importance to us than the sensual pleasure of taste.
The food industry developed to feed people who were unable to grow their own food. As populations grew and more people moved to cities, the need to feed larger numbers of people increased. With increasing competition, food companies began to develop strategies to gain market share. Markets and supermarkets opened, providing all the food that had previously been purchased from individual vendors like the milkman, the farmer’s market, the butcher, etc. The results of this evolution are delicious, visually appealing food products that last a very long time on store shelves. The industrialization and “perfection” of whole foods like fruits and vegetables has also occurred over time. The oversized, perfectly shaped, shiny bright apple is by human design, not by natural selection.
But as biological organisms who developed in a natural world, our digestive systems are adapted to eat foods in their natural state or processed through heat or fermentation, not by industrial chemistry. Organs such as the pancreas have evolved to produce specific proteins called enzymes, which are secreted into the intestine to help us digest the food we eat. Chemically altered food may or may not be able to be digested by this set of enzymes and instead may be evacuated partially digested or undigested. Synthetic chemicals can create havoc on the beneficial bacteria lining the system. In addition, depending on the integrity of the cells lining the tract, some of these manmade molecules may be absorbed into the bloodstream.
How Food Is Digested: The Anatomy/Physiology
The system responsible for how food is digested is called the digestive system. The digestive system typically begins as food enters a long tube at an orifice in your face called a mouth. From there, food passes through many compartments, including the esophagus, stomach, small intestine, and large intestine. Waste material typically exits your body through the anus. Surgical techniques can alter this arrangement at any step along the way. The digestive tract is not pretty, but its basic function isn’t too complicated to understand.
In your mouth, the mechanical process of chewing grinds and macerates food into smaller bits. While chewing, food is moistened with saliva, which not only softens the food, but also begins the digestion of starches. Your tongue, given the presence of numerous taste buds all along its surface, is able to provide sensation to your brain about the quality of the food in your mouth before you swallow it. Is it sweet? Sour? Salty? Bitter? A combination of these?
Once swallowed, the food travels through the esophagus and into the stomach, where it usually sits for several hours while it contracts and churns, mixing with acid and enzymes to begin the digestion of proteins. A muscular ring, called a sphincter, which separates the top of the stomach from the esophagus, is normally tightly closed during this process to prevent refluxing of the acids and food material back into the esophagus.
Once the food is sufficiently mashed up, the chyme, as it is now referred to, passes out of the stomach, a bit at a time, and into the first portion of the small intestine, called the duodenum. As this happens, two different fluids, bile and pancreatic juice, empty into the duodenum through a duct. Both fluids mix with the chyme to help further digest it. Bile emulsifies fats, breaking them down into their components, fatty acids and glycerol. The pancreatic enzymes work on breaking down proteins, fats, and starches into their building-block components. Lactase, the enzyme able to break down the milk protein lactose, is one of the pancreatic enzymes released.
The resulting mixture passes through many feet of small intestine, which provides plenty of time and surface area for the food materials to break down into their molecular components. Once these molecules are small enough, the resulting peptides (or amino acids), sugars, and fatty acids are able to pass through the lining of the small intestine and into a bloodstream that leads directly to the liver.
The liver is a very complex organ with hundreds of functions, but you can think of it as a gatekeeper that regulates the amounts of sugar, fat, and protein that are allowed to pass into the bloodstream. From the components it receives from the small intestine, the liver assembles various proteins, fats, and cholesterol. Along with the pancreas, the liver is also involved with carbohydrate metabolism, or the storage and release of sugars into the bloodstream. The liver also very importantly acts as a toxin waste dump for those materials that pass through the lining of our intestines but aren’t supposed to gain access to the rest of our body. We can survive without portions of the liver, but our bodies cannot survive without some functioning liver tissue.
Materials that are not able to pass through the wall of the small intestine collect in the large intestine, also referred to as the colon. The large intestine has three main functions. One is to act like a sponge and absorb excess water so we don’t dehydrate. When the colon is irritated and not functioning properly, it doesn’t absorb as it should and we can end up with diarrhea. Billions of bacteria also live in the large intestine, making it a very important part of the microbiome (the individual collection of bacteria that lines our guts, airways, and skin). These bacteria consume the food residue that passes by, and in exchange provide us with important minerals and vitamins including vitamin K and biotin. The third important function of the colon is to absorb these vitamins, which are important for good health. The colon creates a solid material from the undigested remainder, referred to as stool, which passes into the rectum, where it is stored in preparation for evacuation through the anus.
In short, the answer to the question of how food is digested is this: Our digestive systems, by utilizing chemical and mechanical processes, enable our bodies to break down foods into their components, absorb the nutrients, and expel the waste. Nutrients from the food we eat are absorbed through the small intestine, and the large intestine absorbs water, along with the vitamins and minerals released by bacteria.
Our bodies are composed of trillions of cells which are continually performing their daily functions: making proteins, reproducing, and dying. Nutrients, including fatty acids, cholesterol, amino acids/peptides, sugars, vitamins, and minerals, are needed to create more DNA and the material to build more cells. Without the influx of nutrients, our cells cannot function or reproduce and will eventually die. With prolonged starvation, organs fail and eventually, the whole body will die. Dietary sugars, fats, and proteins are all important foodstuffs we need for normal metabolism.