Nutrition and Digestion

Where does food enter your body? Many would answer, "As soon as you put it into your mouth." Yet this is NOT where food actually enters your body. When food is in your mouth, or in your stomach or even in your intestines, it is still really outside of your body. The digestive tract or gut is really just a tube-like continuation of your body's external surface run through your body.

To enter the body, food must be absorbed across the epithelium that lines the digestive tract. Digestion prepares food for this journey. Digestion is a process that breaks or dissembles the things that you eat into molecules small enough to be absorbed by cells that line the digestive tract.

Ultimately these small molecules enter the cytoplasm of every cell in your body, where their nutritive value is utilized.

The human digestive tract is about 9 meters (30 feet) in length. It consists of:

mouth

esophagus

stomach

small intestine

large intestine

anus

The human digestive tract is a model food processing plant. As food passes through, it is mixed with various fluids, churned and moved by the musculature of the tract, broken down by various enzymes and absorbed by cells that line the tract.

Digestion begins in the mouth where food is cut and ground by teeth. This makes food easier to dissolve and affords a greater surface area for various fluids to act upon.

While in the mouth, saliva is added through secretions of the salivary glands which initiates chemical digestion. Saliva contains enzymes that initiate the digestion of starch. Saliva also contains mucin, a major protein of mucus, which acts as a lubicant. Ever tried to swallow a dry pill without salivating a bit?

Mucin also bunches the food together into a mass called a bolus. During swallowing, the bolus is pushed into the esophagus, the tubular channel that leads to the stomach. During the swallowing process, openings to the respiratory and nasal passages are automatically closed to ensure that food is kept out of these places. In spite of your best efforts, you simply cannot breathe while swallowing.

The walls of the esophagus contain muscles that contract in successive waves, a process called peristalsis. Peristalsis constricts the esophagus and pushes the bolus of food through a sphincter into the stomach.

Once in the stomach, food gets churned into a paste-like material and is mixed with gastric juices. Eventually this is called chyme. Gastric juice is produced by secretory cells that are located in pits in the wall of the stomach. Hydrochloric acid, a strong acid, is one of the compounds that makes up the gastric juice. Hydrochloric acid lowers the pH of the stomach contents to perhaps 2.0, providing an extremely acidic environment that kills most microbes in food, including many that could cause human illness.

If the acidic contents leaks back into the esophagus, the irritation of the lining there we perceive as "heartburn."

While most enzymatic digestion occurs in the small intestine, protein digestion begins in the stomach through the action of an enzyme called pepsin. Pepsin is secreted as an inactive material called pepsinogen. The acid environment of the stomach activates it, converting it to pepsin.

With all this digestion going on and the severe pH environment found within the stomach, why is the stomach itself not injured or digested? Because it protected by a fairly thick coating of alkaline mucus. When this protective lining is breached, one can develop gastric ulcers. For a long time, gastric ulcers have been treated with antacids. For many people, however, antacids were simply ineffective. Within the past several years, studies have shown that while stomach acidity may be a factor in stomach ulcers, the primary cause seems to be infection with a bacterium, Helicobacter pylori. Ulcer patients treated with antibiotics that kill these bacteria seem to enjoy a greater cure rate. What do you think of the possibility of a vaccine becoming available to prevent ulcers?

How is gastric secretion controlled? The first phase seems to involve stimulation by nerve impulses that reach the stomach from the brain as a result of the smell, taste or even the thought of food.

When food actually enters the stomach, other signals are generated. One signal is carried by sensory neurons from the stomach to the brainstem which responds by sending impulses down autonomic nerve fibers which stimulates the digestive glands of the stomach to release their products.

Another signal is a chemical message sent by the hormone gastrin which is released by endocrine cells in the stomach lining. The message is carried in blood vessels of the stomach wall to the stomach's glandular cells, triggering these cells to secrete gastric juices.

Only a few small molecules such as aspirin and alcohol are able to enter the bloodstream through the stomach wall. This explains the rapid onset of their effects. Other materials must wait for passage into the small intestine for absorption.

Peristaltic waves moving along the wall of the stomach propel small quantities of chyme into the small intestine. The small intestine is about 7 meters (21 feet) in length and is a highly coiled muscular tube about 2.5 cm (1 inch) in diameter.

It is here in the small intestine that the macromolecules we ingest as food are broken down into small, organic molecules such as simple sugars, amino acids and nucleotides. These are the things that are absorbed into the bloodstream.

The digestion of materials in the small intestine requires the help/participation of several major organs and their secretions. These include intestinal juice, pancreatic secretions, and emulsifying lipids from the liver and gall bladder.

When the inflow of chyme stretches the intestinal wall, the action triggers a nerve response that causes the cells of the intestinal lining to secrete intestinal juices and mucus.

Under normal conditions, the intestinal lining secretes 2-3 liters of fluid each day. This fluid is needed to dissolve the molecules for digestion and to facilitate absorption across the intestinal epithelium.

Cholera is a nasty human disease and results from a bacterial toxin that greatly increases the fluid released from the intestinal lining. When stimulated by cholera toxin, these cells can churn out over a liter of fluid each HOUR, most of which is simply lost through diarrhea.

Death by cholera is death due to severe, acute dehydration. Babies, and the very young are truly challenged by cholera. Treatment is largely one of replacing the lost fluids. Fortunately, cholera is rare in the US.

The pancreas is a gland that secretes digestive enzymes and releases them into the small intestine. The pancreas also released sodium bicarbonate, an alkaline material that neutralizes somewhat, the extremely acidic chyme that enters the small intestine from the stomach.

The secretion of pancreatic enzymes and bicarbonate is stimulated by two hormones-- cholecystokinin (CCK) and secretin. These are secreted into the bloodstream by endocrine cells found in the wall of the small intestine. All of this is in response to the inflow of chyme from the stomach.

You can refer to a diagram and re-trace the above steps through the digestive tract.

How does your body deal with fats. They aren't so soluble in water. Consider how washing dishes or greasy pans works. You add some soap and this promotes the solution of fats into the water.

Now suppose you eat a nice meal of raclette cheese (this stuff is perhaps 120% fat and is wonderful). How does your body handle it. In order for the large fat molecules to be efficiently broken down by the lipases (enzymes) the large clusters of this stuff must be broken down into much smaller clusters. This is done by the addition of bile salts produced by the liver and stored in the gall bladder. The gall bladder empties into the small intestine.

Bile salts are similar to detergents. In the presence of bile salts, fat globules are reduced to stable, microscopic droplets that can be attacked efficiently by the fat digesting enzymes, the lipases.

The first step in the absorption of food molecules is their movement from the lumen of the small intestine into the epithelial cells that make up its lining.

The inner surface of the small intestine is a highly textured surface because of the presence of microscopic finger-like projections called villi. These villi greatly increase the surface area available for absorption. Each villus is, in turn, covered with small projections called microvilli which further increase surface area. Together, the villi and microvilli yield a surface area more than 150 times that of your skin.

Each villus is laced with a rich capillary network surrounding a centrally located lymphatic vessel called a lacteal. The lacteal absorbs the products of lipid digestion such as fatty acids. From the lacteal, microscopic fat droplets (of fatty acid) are transported though a series of lymphatic vessels that eventually drain into a large vein in the neck.

Most non-fatty nutrients diffuse directly into the blood capillaries of the intestinal villi where they are carried to the liver and removed from the bloodstream. The liver is the major metabolic regulatory center, controlling blood-glucose levels and releasing glucose into the blood as needed.

By the time digested food gets to the end of the small intestine, virtually all of its nutrients have been removed along with most of its water. The nutrient depleted chyme is pushed into the large intestine or colon.

What happens here? Water is returned and the remaining contents are converted into a material known as feces. You may know this material by other names.

Pressure sensitive neurons detect when solids have accumulated at the terminal end of the large intestine which is called the rectum and respond by initiating a defecation reflex. Because one of the two anal sphincters is under voluntary control, we humans can consciously delay expulsion of feces until an appropriate time.

Projecting from the large intestine is a short, blind tube, the appendix. Inflammation of the appendix, literally appendicitis, if unattended, can lead to a rupturing of the structure which releases intestinal contents into the abdominal cavity. There are numerous bacteria present in feces and their presence in the abdominal cavity can lead to a fatal infection.

The huge numbers of bacteria that live in the large intestine are not freeloaders. Bacteria constitute almost half the dry weight of large intestine contents. These bacteria metabolically attack organic substances remaining in chyme and often produce unpleasant smelling by-products. If gaseous, we refer to these by-products as flatus. You may know this gaseous material by other names.

These same bacteria also produce vitamin K, biotin, folic acid and other nutrients that we humans absorb. Bacteria also contribute by competing for space and nutrients with less welcome bacteria. Consider how upset things become after taking substantial amounts of antibiotics that diminish intestinal bacteria. The result is often a bout of diarrhea and/or an opportunistic yeast infection.

There is still a lot controversy over such topics as the impact of dietary sugar, cholesterol and saturated fats on our health.

Nutritionists, however, generally agree that our diet should balance carbohydrates, triglyceride lipids (fats and oils) and proteins. Foods that balance these groups of foodstuff should also easily provide us with enough energy, organic building blocks, vitamins and minerals for sustenance.

An average person engaged in a relatively sedentary life style requires about 2,500 Calories per day to maintain their bodies in a stable state. A person who engages in strenous activity, maybe a professional athlete, may require over 4,000 Calories.

Metabolic rates also change with age.

Carbohydrates provide the most readily available form of glucose and therefore, the most readily available form of usable energy. Glucose is an all purpose energy source. It is also the one sugar usable by all brain and nerve cells.

Not all carbohydrates are easily digestible. Cellulose is a carbohydrate but we humans cannot digest it. Yet cellulose is useful to humans. Consider that when we eat celery, we consume considerable cellulose which ends up contributing to the bulk in our digestive tract and provides something for peristalsis to act upon. Low-cellulose diets can lead to constipation and have been linked to cancer. The only reason one can have for suggesting foods rich in both polysaccharides and undigestible fibers over that of polysacchride rich candy is the presence of the fibers is useful for healthy digestion.

Fats are chemical, highly reduced and are rich sources of energy. Two fatty acids, linolenic and linoleic, are essential fatty acids necessary for biological membrane construction and cannot be manufactured by the human body. Diets rich in saturated fats and cholesterol MAY predispose susceptible individuals to cardiovascular disease. Nutritionists tend to agree that a healthy diet should be low in fat, providing less than 30% of our caloric intake.

This is not so easy to do because fats tend to provide attractive taste and texture to foods. We humans LIKE fats. One of the large food processing companies has a product called Olestra, which is a synthetic material that provides the taste and texture of fat but is not digestible. It simply passes through like cellulose. It has just won FDA approval for human consumption. Seems it gives some individuals diarrhea though.

Dietary proteins are needed to provide amino acids from which we assemble our enzymes, antibodies and other kinds of proteins. We can obtain protein from virtually any food. We can manufacture all but eight amino acids. These eight essential amino acids must be ingested. The absence of even one of these essential amino acids can halt protein synthesis.

These required amino acids are synthesized by plants and bacteria.

We also need vitamins. A vitamin is an organic compound needed in trace amounts for normal health but is one that we cannot synthesize. There are 13 that we humans must acquire through our diet or risk suffering vitamin deficiency diseases, some of which can be fatal.

Do you need to take vitamin pills? If you eat a well-balanced diet, probably not.

A dietary supply of certain inorganic minerals is also necessary for proper nutrition. Without calcium or magnesium, a large number of enzyme mediated reactions in your body would simply not be able to occur. These elements act as factors necessary for the enzymes to operate.

Calcium and phosphorous are needed for bone growth. Calcium is also needed for muscle function. Iron forms the core of electron transport pigment proteins and hemoglobin. Phosphorous is also needed for ATP synthesis.

A number of other elements such as iodine, copper, molybdenum, manganese and chromium are needed in tiny amounts for certain enzyme reactions or as co-factors and are referred to as trace elements required in our diets.

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