What is Digestion

Digestion: The Breaking Down of Big Food Particles

Hop on the intestinal train for a food-informative ride. From mouth to anus, your intestinal tract is a 25-foot long disassembly line. Whole food is taken in at one end and waste expelled from the other. Enroute nutrients are taken out of the food and absorbed into the bloodstream according to the body’s needs. The whole process is called digestion. (When things go wrong, it’s called indigestion.)

Digestion is basically a process of breaking down big food particles into individual molecules, tiny enough to squeeze through the intestinal lining into the bloodstream. Your body uses mechanical and chemical means to do this. By understanding the way the digestive process works – and how you can make it work better, you can improve your own “gut feeling.” Let’s follow a representative food – say a peanut butter and jelly sandwich – from top to bottom and study the stops along the way.

Digestion begins in the brain. Before your meal, you imagine how good the food is going to taste. You get an eyeful when you walk past the buffet table at the beginning of the party. Or, the aroma of freshly-baked bread draws you into the bakery. Your eyes and your nose get your body and mind in the mood for food, and just the thought of food gets the digestive juices flowing. Your mouth waters and your stomach churns at the very thought of what is soon to grace your pallet. Even before the first bite you think, sniff, and drool your digestion machine into action. Just anticipating eating gets the intestinal tract ready for the job coming its way.

Chew-chew! Mom’s admonition to “chew your food” is good advice. When the sandwich enters the mouth, the first step in food breakdown begins. Your teeth break the bread down into smaller particles, increasing the surface area through which the chemical food processors – enzymes – can penetrate the food. Chewing breaks up the fiber that holds the food together and unwraps the food package so that the digestive enzymes have easier access to the contents inside. The saliva already flowing in anticipation of eating bathes the broken-down bread and peanut butter with the first digestive enzyme, salivary amylase. Amylase breaks the chemical bonds between the carbohydrate molecules, changing them into smaller sugar molecules. Uncoupled from their friends, these are now free to be broken down even farther, if necessary. Even the fat in the food gets a head start on digestion while it’s in the mouth by receiving a tiny squirt of a fat-digesting enzyme called lingual lipase. (As we travel down the tube, you will notice that names of enzymes are a combination of the suffix –ase and the nutrient they work on, such as lipase digesting lipids, proteinase digesting proteins, and lactase dissolving lactose.) Saliva also lubricates the food, making that gooey peanut butter slip-and-slide down the ten-inch long esophagus, the tube that connects the mouth and the stomach. Saliva is the body’s own health juice. Besides helping with digestion, saliva contains a recently-discovered substance called epidermal growth factor (EGF), which facilitates the growth and repair of injured or inflamed intestinal tissue. Perhaps this is why animals lick their wounds.

Taking smaller bites, chewing the food well, and swallowing slowly are things you can do to better prepare the food package for the next part of its journey. Also, chewing foods slowly slows down your eating. You’ll swallow less air (so less burping), and you can be more aware of signals that tell your stomach it’s getting full. When each step along the disassembly line is done well, the next job is easier. By taking small bites, chewing your food well, and swallowing it slowly, you help the food particles go down the tube without discomfort. Mom not only chewed you out for “swallowing your food whole”, but you also probably got many eating sermons on how to “eat slowly.” Again, mother-nutritionist was right.

Churn-churn! When it enters the stomach, the mush from the mouth gets mixed. The stomach is your body’s mechanical and chemical food processor. It’s a pouch composed of sheets of muscle that encircle the stomach in different directions. When they contract, the stomach can mix the mush this way and that. The stomach muscles twist and churn the food like you would knead bread dough. The lining of the stomach secretes gastric juices, including hydrochloric acid, which dissolves the food, a protein-splitting enzyme called pepsin, and a fat-digesting enzyme called lipase. Like fruits or vegetables pureed in a blender, the food is churned and mixed with the digestive juices until it resembles thick soup. This glob is called chyme.

The circular muscles of the stomach also have the job of keeping the food in the stomach long enough to be broken down from a solid to liquid. Sometimes these muscles can malfunction. The muscle at the lower end of the stomach (called the pylorus) can become too thick and tight in the first few weeks or months of infancy, keeping the stomach contents from emptying. Instead, the food comes back up forcefully. This condition is known as pyloric stenosis and is manifested by vomiting of increasing frequency and severity. The muscle at the top end of the stomach, called the gastroesophageal sphincter, can become too loose, so the stomach contents don’t stay in during churning. This leads to a condition called gastroesophageal reflux.

Hydrochloric acid is strong enough to eat through meat and potent enough to kill most of the harmful bacteria that may be in food. So the stomach not only digests, it disinfects. It is the body’s food processor and the body’s food purifier. Yet it does not destroy all the bacteria we ingest. Some of the bacteria that are able to survive the harsh conditions in the stomach eventually take up residence in the intestine where, in return for all they can eat, fulfill an important role in the health and digestive process.

For comfortable digestion to occur, the stomach lining should secrete just the right amount of acid at the right time – no more, no less. If the lining pours out acid when the stomach is empty (which can happen when you are under stress), the acid irritates the stomach lining, causing uncomfortable sensations, or indigestion. Also, if there is no food to absorb the acid, the excess acid enters the first part of the intestine, the duodenum, which is more sensitive. Since there is no food to digest, the acid digests the lining of the duodenum, causing what are called duodenal ulcers. Substances such as alcohol or coffee on an empty stomach (accompanied by stress) can literally eat you up inside. Or, the excess acid can squirt up into the esophagus, which also has a more sensitive lining, causing “heartburn” or reflux.

Belches, rumblings, and vomiting. When your stomach is empty and you are hungry or anticipating eating, the stomach contractions just squeeze air, making the noise experienced as “rumblings.” Burps, belches, rumblings, vomiting, and retches are occupational hazards for the hard-working gut. Most belches and burps follow the swallowing of air, which occurs not only when you swallow food but also when you swallow saliva, or when you get hungry or tense. Since you swallow several thousand times a day, it’s no wonder that the upper end of the intestines needs to kick back some air. Add to that the internal gases produced from fermentation in the lower intestines, and it’s no wonder that the average human is a bit of a “windbag.”

The vomiting reflex is designed to protect the intestines from unwanted material. It simply sends it back out the way it came. The brain triggers the muscles of the diaphragm, abdomen, and stomach to contract suddenly, forcing the stomach contents up and out. While vomiting is not always desirable, often (e.g., after experiencing food poisoning) vomiting brings gut feelings of relief.

Grazing for good digestion. The size of a meal shouldn’t be more than the size of your stomach. To help your eyes get a general picture of the size of your stomach, open your fist so that the tips of the thumb and forefinger touch. Your stomach is about the width of this open fist and twice as long. Next time you are about to binge, place your fist next to your heaping plate and notice the mismatch. (Compare the tiny fist of a tiny toddler next to a full plate. You can now see why tiny tummies can get upset easily.)

All parts of the intestinal tract – from top to bottom – were designed for grazing, that is small frequent mini meals rather than three big squares a day. The term “square meal” doesn’t imply a complete balanced meal must be eaten at every sitting. In some ways, children are smarter eaters than adults. Left to their own foraging, they tend to nibble and graze throughout the day. Most adults would experience less indigestion if they ate five mini meals a day rather than three maxi meals.

Seep-seep! By the time the peanut butter sandwich is chewed and churned, it’s ready to be pushed from the stomach into the small intestines where the real digestive action is. Most of the sandwich’s proteins and carbs empty from the stomach into the small intestine within a couple of hours, yet the fats, since oil and water (digestive juices) don’t easily mix, remain in the stomach a few hours longer. (This is why you feel fuller longer after a high-fat meal than you do after a high carbohydrate or high protein meal).

As the chyme, the food mush enters the small intestine the most important part of digestion – absorption – begins. The first part of the small intestine is called the duodenum (Latin for “twelve” – the adult duodenum is around 12 finger breadths long). More digestive processes occur in the duodenum than in any other part of the intestinal tract.

In order for food to become part of you, it has to get through the intestinal lining, so here’s where exciting changes occur. First, because the more delicate lining of the small intestine doesn’t like the irritating stomach acids, it secretes the body’s own antacids – bicarbonates – to neutralize the food. As the food moves down the intestinal disassembly line, it passes by stations where it gets squirts of digestive juices that further break down the proteins, carbohydrates, and fats into molecules small enough to seep into the bloodstream.

Each nutrient requires specific intestinal juices that work on its molecules in particular ways. Trypsin and peptidases are enzymes that disassemble the protein necklaces into individual amino acids, which enter the bloodstream through “doors” in the intestinal lining specially marked for “amino acid entry only”. Carbohydrates are disassembled into individual sugar molecules by the enzymes lactase, sucrase, maltase, and pancreatic amylase, and the individual sugar molecules enter the bloodstream through doors marked “sugar only.” (If the intestinal lining is injured, these doors are not so selective, allowing allergens to pass through that are potentially harmful to the body – a condition known as the leaky gut syndrome.) Behind each door runs a blood vessel that quickly ferries the nutrients throughout the body where they can be burned for energy or reassembled into tissues.

Meanwhile back in the stomach, the fats are finally getting ready to leave. As they enter the small intestine, the fats get a squirt of bile from the gallbladder and some lipase from the pancreas. The bile emulsifies the fat, the way soap breaks up grease. Like soap, bile does not really dissolve the fat but rather breaks it down into tiny particles, which are then more easily broken down by the intestinal enzyme lipase for absorption into the bloodstream. The individual fatty acid molecules exit the intestine through doors marked “fats only.” However, unlike proteins and carbs which go through their own doors and into the bloodstream quickly, fats go into microscopic holding rooms within special cells in the intestinal lining. Here they are stuffed into microscopic bags, which are then taken out the back door and loaded onto ferry boats in the bloodstream called lipoproteins. These molecular ferries then circulate throughout the bloodstream loading and unloading fat molecules at cell loading docks. Picture each cell of the body as having millions of loading docks on the membrane. If the cell doesn’t need any more fat, it shuts down the docks, so that the lipoprotein ferry boats can’t dock and are forced to circulate around the bloodstream until they find some other place to deposit the fat. The two places always receiving more fats are the liver and the fat cells. So the ferry boats either deposit the excess fat around the waist, the hips, or wherever, or transport the extra fat into the liver where it is dissolved by bile and excreted out into the intestines as waste.

A supply-and-demand process. The enzymes involved in digestion work on a supply and demand basis. If the glands in the intestinal lining and the pancreas secrete enough enzymes to break down and absorb all the food that comes by, the intestines feel fine. But if there is more food than there are enzyme workers to process it, the doors close, and the excess travels down into the lower intestines where it is not very welcome. The result is indigestion. How do you keep this supply and demand digestive process in balance? You guessed it. Give the enzymes a fighting chance by grazing or eating smaller meals. Overwhelming the intestines with too much food sets you up for indigestion.

The luxurious intestinal lining. The lining of the intestines is more like a plush carpet than a smooth sheet. Here’s why. Trillions of microscopic projections, called villi, grow out from the cells of the intestinal lining. This increases the cell’s surface area, thus increasing the contact between the food and the cells. The more contact these cells (and their rich blood supply) have with the food, the more nutrients can be absorbed.

The intestinal lining is only one cell thick, which is a good news / bad news phenomenon. The good news is that because the lining is thin, nutrients are easily absorbed. They come in the front door of the cell and go out the back door into the bloodstream. The bad news is that this delicate lining is easily injured by irritants and infection. When this happens, food cannot be absorbed efficiently, leading to diarrhea, abdominal pain, gas, and bloating. When the cells are damaged, there may not be enough enzymes available to digest certain foods, for example, lactose. Temporary lactase deficiency is common following intestinal irritation or infection. This is the reason why doctors advise smaller, more frequent feedings of more easily-digestible foods during recovery from an intestinal infection. It may also be necessary to avoid milk and other lactose-containing foods. The cells of the intestinal lining do regenerate and heal, though complete healing may take several weeks. Diarrhea or looser stools may occur for 4 to 6 weeks following an intestinal infection.

Water and waste. By the time the peanut butter and jelly sandwich has completed its 20-foot-long, ten-hour journey through the small intestines, most of the nutrients have been absorbed. The leftovers enter the final five feet of the journey, called the large intestine, or colon. Little digestion occurs in the colon since it has few villi and low levels of intestinal enzymes. Food processing in the colon is often described as the calm after the storm. Yet the colon is not just a passive waste collector and eliminator. It plays an active and important role in the health and well-being of the whole body, accounting for the phrase, “Your body is only as healthy as your colon.”

The first vital function of the colon is to regulate the body’s water balance. As the waste from the food passes through the lower part of the intestines (called the jejunum) and enters the colon, the colon absorbs excess water from the food and furnishes it to the water-thirsty body. If, however, the waste matter lacks water, the colon fills the stool with water to prevent constipation. Healthy water balance in the colon leads to healthy stool patterns. In fact, one of the most important changes a person will notice after following the eating program in this book is having several smaller, soft (but not diarrhea) stools a day rather than the usual American pattern of one huge bowel movement once a day or every other day. Passage of 4 to 6 soft bowel movements a day suggests that your colon is in biochemical balance.

Bugs in the bowels. Billions of bacteria reside in the intestines, primarily in the large intestine. Called intestinal flora, because they are like the plant life of the intestines, they contribute to the healthy life of the colon – and the whole body. These intestinal bacteria are also known as probiotics because they add health to life. (The term “probiotics” is also used for bacteria supplements in a bottle, either in powder or capsule form.)

The two main healthful bacteria in the colon are lactobacilli and Bifidus bacteria. They live symbiotically in the colon, meaning in a mutual give-and-take relationship. These bugs give good stuff to the body in return for a warm place to live. The healthy bacteria keep the harmful bacteria in check. They also ferment the soluble fiber in food, forming short-chain fatty acids (SCFA’s), which nourish the cells of the large intestine, stimulating healing and reducing the development of intestinal cancer. SCFA’s are also absorbed from the intestines and travel to the liver where they decrease the liver’s production of cholesterol. In addition, SCFA’s inhibit the growth of yeast and disease-causing bacteria. Probiotic supplements with lactobacilli and Bifidus bacteria are available in most drug and vitamin stores. A few brands we recommend are Hyperbiotics, Regular Girl, and Sunfiber.

More gut sounds. Air-swallowing produces belches and burps. Gas at the other end is another worry. When the normal bacteria that live in the large intestines “eat” certain foods (such as the polysaccharides in beans), the byproduct of the bacteria’s own digestive process is gas. While the colon absorbs some of this gas, some goes up and accounts for bad breath, and the rest of the gas is expelled as flatus. The odorous gas is a combination of methane and hydrogen sulfite.

Most of the gas you hear, feel, or smell is a result of the intestinal bacteria digesting the sugars and fiber that manage to reach the large intestine. Since these bacteria love starch, the prime gas producers are polysaccharides in starchy foods, such as beans, brussel sprouts, prune juice, and just about every type of grain, except rice, which seems to be the most socially-acceptable starch. Galactose from milk and the soluble fiber pectin are also favorite foods of the resident bacteria.

How much gas is produced by what foods is very individual? To put your large intestine on emission control, keep your own food-gas diary and adjust your diet accordingly. Most of the time the volume of food you eat is more at fault than the type of food. Overwhelming the intestines with more starch than they can absorb sets you up for unwanted blasts.

The end. The rhythmic contractions of the colon move waste material (called feces) into the last five inches of the large intestine, called the rectum, where the final waste products are eliminated. Normal passage of feces or stools (called defecation) occurs when the nerves lining the rectum sense the presence of feces that need to be evacuated. The presence of feces in the rectum stimulates a reflex (called the defecation reflex) which causes the muscles of the abdomen and upper rectum to contract and the muscle encircling the anus to relax, allowing easy passage of stools. This reflex is very efficient as long as people don’t do something to mess it up. Eating too little fiber, not drinking enough fluids, eating too much fat (fat slows intestinal transit) and simply not paying attention to the evacuation warnings of the rectum will cause the reflex to stop working. (Kids between five and ten, especially boys, are prone to ignore this reflex.) Feed your intestines properly and listen to their signals, and they will work well for you.

Dr. Bill Sears

Dr. Bill Sears

Dr. Sears, or Dr. Bill as his “little patients” call him, has been advising busy parents on how to raise healthier families for over 40 years. He received his medical training at Harvard Medical School’s Children’s Hospital in Boston and The Hospital for Sick Children in Toronto, the world’s largest children’s hospital, where he was associate ward chief of the newborn intensive care unit before serving as the chief of pediatrics at Toronto Western Hospital, a teaching hospital of the University of Toronto. He has served as a professor of pediatrics at the University of Toronto, University of South Carolina, University of Southern California School of Medicine, and University of California: Irvine. As a father of 8 children, he coached Little League sports for 20 years, and together with his wife Martha has written more than 40 best-selling books and countless articles on nutrition, parenting, and healthy aging. He serves as a health consultant for magazines, TV, radio and other media, and his AskDrSears.com website is one of the most popular health and parenting sites. Dr. Sears has appeared on over 100 television programs, including 20/20, Good Morning America, Oprah, Today, The View, and Dr. Phil, and was featured on the cover of TIME Magazine in May 2012. He is noted for his science-made-simple-and-fun approach to family health.