Essential Nutrients and the Digestive System

In my previous post, I discussed the dietary choices of humans and what described a healthy diet.  However, what exactly makes a healthy diet healthy?  The answer lies in what it contains and how the body is able to extract and use the components in an optimal fashion.  The human body is remarkable in its form and function.  Fundamental to its operation is the need for energy.  Ultimately, this energy originates from the sun and is captured by plants.  Through the process of photosynthesis, plants convert and stores the sun's energy in their tissues, which is later consumed by humans either directly by eating the plants or by way of eating animals that eat plants.  Plants and animals possess many of the fundamental elements, known as nutrients, the body requires to function.  Thus, humans must consume sufficient quantities of nutrients to survive.  The human digestive system is a complex set of components that work together to extract the energy and nutrients contained in plants and animals and use them to sustain life (Sizer & Whitney, 2013).

The Essential Nutrients

As mentioned before, the fundamental components required by the body come in the form of nutrients, which are present in all foods that we consume.  Nutrients are families of molecules that perform a wide variety of functions within the body and are vital to the life of a human being.  Indeed, humans are actually comprised of the same materials as the foods they eat, in substantially different quantities, of course.  There are six kinds of nutrients the body requires, four of which are organic in nature, because they contain carbon and are derived from living things.  The organic nutrients include carbohydrates, fats, protein, and vitamins.  The two non-organic nutrients are water and minerals.  Although the body can make certain nutrients by itself, many are known as essential nutrients, in that the body cannot make them.  Instead, they must be taken in through the process of eating.  Each of the six families of nutrients has at least one essential nutrient.  In some cases, such as water, minerals, and vitamins, the entire family is comprised of essential nutrients (Sizer & Whitney, 2013).

Of the six nutrients, water is the most abundant, comprising almost 60 percent of a person's weight.  Water is crucial to the body's operation as it is the primary transport mechanism for all nutrients and a wastes throughout the body.  As it is lost constantly in the body, it must be replenished in a timely manner or death will occur. (Sizer & Whitney, 2013).

The vitamins and minerals are families of nutrients that provide no energy to the body.  Rather, they serve as regulators that are vital to all processes within the body, including digestion, muscle movement, converting nutrients to energy, and growing tissue.  Some minerals are used in the physical structures within the body.  For instance, calcium and phosphorous are primary components of bone in the skeletal system (Sizer & Whitney, 2013). 

The remaining three of the four organic nutrients are known as energy-yielding, in that they can be converted to energy that the body can use.  The energy-yielding nutrients are carbohydrates, proteins, and fats, also known as lipids.  Because they provided the bulk of energy to the body and must be consumed in relatively large quantities, the three energy-yielding nutrients are also called macronutrients.  While carbohydrates and fats primarily provide energy, proteins serve a dual-purpose, in that they provide energy and provide building structures to form the muscles and other tissues within the body.  It is important to note that alcohol can also provide energy, but because it is non-essential to life and can be harmful, it is not considered a nutrient (Sizer & Whitney, 2013).

The amount of energy contained in food is measured in kilocalories, commonly referred to as simply calories.  A gram of each of the three energy-yielding provides a fixed amount of calories, with carbohydrates and proteins each providing 4 calories per gram and fats providing 9 calories per gram.  Because each nutrient serves a purpose within the body, one must follow a diet that provides sufficient quantities of each nutrient and the average amount of energy required by the body.  Too little of some essential nutrients will lead to malnutrition, while too much of some nutrients can be toxic.  Moreover, to little energy can lead to being underweight, while too much energy can lead to being overweight, both of which can be problematic (Sizer & Whitney, 2013).

As mentioned in the previous blog post, the USDA's Dietary Guidelines provide guidance on what foods to eat from the five different food groups, as well as the recommended quantities from each.  Following a well-planned diet that adheres to these recommendations will supply most people with sufficient quantities of all essential nutrients, while keeping calorie intakes under control.  For instance, the fruits and vegetables food groups provide dietary fiber, potassium, magnesium, vitamin A, vitamin C, folate, and other important nutrients.  Foods from the grains food group provide iron, thiamin, folate, niacin, selenium, and substantial fiber to the diet, which helps in digestion and slows sugar absorption.  It is recommended that at least half of all grains consumed should be made up of whole grains, due to the additional fiber and health benefits of the whole-grain wheat kernels.  The protein food group provides protein, essential fatty acids, the B vitamins, iron, and other important vitamins and minerals.  Finally, the dairy food group provides a source of protein, calcium, potassium, and vitamins A and D via fortification.  All five food groups provide energy via one or more of the energy-yielding nutrients of carbohydrates, fats, and proteins (Sizer & Whitney, 2013).

The Digestive System

The consumption of nutrients via food does no good unless the body can extract the nutrients from the food and transform them to a form it can use.  The human digestive system does just that with incredible efficiency.  The digestive system is comprised of both mechanical and chemical components and it's primary function is to break down the food to its fundamental components, known as digestion, and then absorb the nutrients and other products.  Remainder material, such as fiber, is then excreted (Sizer & Whitney, 2013).

The digestive tract is flexible and muscular tube, around 26 feet long, that includes the mouth, throat, esophagus, stomach, small intestine, large intestine, the rectum, and the anus.  Interestingly, the digestive tract is entirely outside of the human body and is merely a cavity running from the mouth all the way to the anus.  Food passing through the digestive tract only enters the body when it is absorbed through the digestive tract walls (Sizer & Whitney, 2013).  An illustration of the major components of the digestive tract is shown below.

The Digestive System (Science In Everyday Terms, 2012)

The digestive system works in a progressive manner to break down food and absorb what the body needs from it, while excreting the unneeded components.  As mentioned earlier, digestion works at both a chemical level, using enzymes and acid, as well as a physical level.  As food enters the mouth, the physical act of chewing breaks the food into smaller pieces, increasing its surface area.  The salivary glands in the mouth produce saliva, which help to lubricate and break down the food using a variety of enzymes.  For instance, salivary enzymes already begin to break down break into glucose, before you even swallow it.  The chewed, partially broken down, mass of food is known as a bolus and is now ready to be swallowed (Mayo Clinic, 2014).

Upon swallowing a bolus, the food is sent to the esophagus, which contains a number of ringlike muscles.  Through a synchronized muscle process called peristalsis, the bolus is transferred from the throat to the stomach.  After entering the stomach, the food is broken down even further.  From a physical perspective, the stomach is very muscular and performs contractions to squeeze, churn, and mix its contents to liquefy the food.  In addition, the stomach uses chemical actions to assist in the process, releasing stomach acid and enzymes.  The end result of the processing in the stomach is that the food is turned into a liquid substance known as chyme (Mayo Clinic, 2014).  By the time the chyme is ready to leave the stomach, it looks nothing like the original food that entered the mouth.  Instead, it is in prime condition to be absorbed unraveled and prepped, the starches have been partly split, and the fats have been separated from the rest of the mass (Sizer & Whitney, 2013).

The chyme then proceeds through the pyloric valve, at the bottom of the stomach, and enters the small intestine through a series of squirts, so that small amounts enter at a time.  In fact, only about an eighth of an ounce enters the small intestine at a time.  The rest of the chyme continues to be mixed in the stomach until it, too, gets squirted into the small intestine.  Upon entering the small intestine, the chyme gets mixed with a number of digestive juices that are produced from the pancreas, liver, and gallbladder.  The pancreas produces digestive enzymes that work to break down carbohydrates, fats, and proteins.  The pancreatic juice also contains a bicarbonate to neutralize the acid that has entered the small intestine from the stomach.  Because fats need a little help in digestion, the liver produces bile, an emulsifier that helps to digest them.  Finally, the gallbladder stores the bile, so that it can be excreted in precise amounts (Mayo Clinic, 2014).

The digestive enzymes in the intestine and pancreatic juice work together to act on the chemical bonds that hold the larger nutrients together.  As this happens, the molecules continue to get broken down into smaller and smaller pieces, suspended in the intestinal fluids.  Additional enzymes are present on the intestinal walls themselves, to help break the components down even further, just before absorption.  Once this final breakdown occurs, the pieces that once were food are now small enough to be absorbed and pass through the intestinal walls.  Through this process, virtually all macronutrients are absorbed before leaving the small intestine (Sizer & Whitney, 2013).  The remaining material is a combination of water, electrolytes, and waste, such as undigestible plant fiber and dead cells from the digestive tract.  As this passes through the colon, nearly all of the water is reabsorbed, leaving a soft substance called the stool.  The muscles of the colon separate this into small segments and push it into the lower colon and rectum for later excretion through the anus (Mayo Clinic, 2013).

Although much cannot, some kinds of fiber can also be broken down further by the large quantity of bacteria living in the colon (Sizer & Whitney, 2013).  Indeed, up to 1,000 bacterial species live in the bowel and the mass of bacterial cells in the colon weighs over 3 pounds.  This bacteria, and its microflora, are essential in aiding digestion and increasing the body's immune function (Joshi & Lamb, 2014).

Conclusion

The body's ability to break down food into its most basic components and absorb them for its own use is remarkable.  Without this incredibly efficient and complex system, the essential nutrients from each of the six nutrient families could not be obtained and human life would not be possible.

References

Joshi, M. R., & Lamb, N. (2014). The use of probiotics to help manage changes in the gut as people age. Primary Health Care, 24(8), 35-41.

Mayo Clinic. (2014). Slide show: See how your digestive system works. Retrieved from Mayo Clinic: http://www.mayoclinic.org/digestive-system/sls-20076373?s=1

Science In Everyday Terms. (2012). The Digestive System: How it Works! Retrieved from Science In Everyday Terms: http://scienceineverydayterms.blogspot.com/2012/12/the-digestive-system-how-it-works.html

Sizer, F., & Whitney, E. (2013). Nutrition: Concepts and Controversies (13th ed.). Mason, OH: Cengage Learning.