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Cheryl Matrasko
James Loeser
Matthew O'Connor
Editor

 

THE BASICS OF ABSORPTION AND METABOLISM OF A MEAL

by James Loeser

"Are you hungry?" Your friend, Jennifer, asks.

You look at your watch then decide, "Yes, let’s get something to eat."

Americans typically eat 2-3 meals per day. The food we take in during each meal is digested by the body, by a process called metabolism, a term most people are familiar with; it can refer the to overall functioning of the body or it can refer to specific bodily function, such as food metabolism. Food taken in during a typical meal in absorbed by the body in approximately 2-4 hours, which is referred to as the absorptive state. During this time there are transient changes in the levels of glucose (a form of sugar that serves as the body’s most abundant energy source), amino acids (which are the building blocks of proteins), and triacylglycerols (the body’s storage form of fat). Also during this time, insulin (an anabolic hormone secreted by the pancreas) is released into the blood stream in response to eating. Insulin promotes the synthesis of triacylglycerols (fat stores), glycogen (sugar stores), and proteins (structural and functional molecules of the body). During the absorptive period virtually all tissues primarily use glucose as a fuel. Pretty dry stuff so far, but, it is my goal to introduce some commonly used dietary terms and to lay down a basic foundation of food metabolism so that the reader may be able to make more informed decisions about nutrition.

The first and most important organ in food metabolism is the liver. One of the largest organs in the body, it is located just below the diaphragm, on the upper right side of the abdomen. Food ingested during a typical meal is chewed up in the mouth, further broken down in the stomach, and delivered to the small intestine where most of the nutrient absorption takes place. Absorbed nutrients are sent directly to the liver via a special vein, called the hepatic portal vein. Because nutrients go directly to the liver via the hepatic portal vein and not into the general blood circulation, the liver gets first crack at processing absorbed nutrients. The liver takes up carbohydrates, fats, and amino acids. These nutrients are then metabolized or stored by the liver, or they are put into the general blood circulation for use by other tissues of the body. The liver serves a vital function by smoothing out a rapid influx of nutrients during the absorptive state, that is, 2-4 hours after a meal. Because your body is working hard to absorb nutrients from a meal, blood is diverted to the digestive tract and away from other organs, such as muscles and the brain, which is why you may feel sleepy after a big meal.

The liver is normally a glucose producing organ. The liver stores glucose (sugar) in long, branched chemical chains, called glycogen. When the body needs energy and is in a state of starvation, the liver breaks down glycogen and delivers glucose to the tissues of the body via the blood stream, thus defining its glucose producing function. However, during the absorptive state, the liver does the reverse: it builds up and stores glycogen from sugar taken in during a meal for use in a fasting state. The liver also builds fat molecules (triacylglycerols) from absorbed fat and delivers these molecules to other tissues of the body, in particular, to fat cells and muscle cells where they are stored and metabolized, respectively. Although not a major source of body fat, the liver builds fat from the building blocks of carbohydrates and proteins (simple sugar and amino acids) absorbed from a meal. The liver builds fat for a simple reason: dietary intake exceeds the body’s energy expenditure, in other words, the body takes in more fuel than it can use, so the body stores the energy as fat. What does this mean? If you eat meals containing large amounts of carbohydrates, then some of this fuel will be converted into fat. Many consumers do not know that the liver does this. However, many dieters who have tried one of the fad diets that promote very high carbohydrate, very low protein, and virtually no fat intake have discovered, among other things like reduced energy levels, weight gain, especially around the area where body fat tends to accumulate: on the hips and legs of women and around the waists of men. Also important in food metabolism is amino acid metabolism. During the absorptive period, more amino acids are delivered to the liver by the hepatic portal vein than the liver can use. The liver uses as many amino acids as it is capable of to build proteins for the body. The body does not store protein, per se, but merely replaces those proteins lost during normal body function. So the proteins delivered to the liver in excess of its capacity are either delivered in turn to the general blood circulation where they are picked up and used by other tissues of the body, or they are degraded by the liver into urea and excreted in the urine. Therefore, a high protein diet will show up as a high urea concentration in the urine. Overall, the liver is the main conductor and processor of nutrients absorbed after a meal, but also important in food metabolism is adipose (fat) tissue, skeletal muscle, and the brain.

Adipose tissue is next in line after the liver in its ability to distribute fuel molecules, namely fat. Adipose tissue is made up of adipose cells, in which triacylglycerol (fat) droplets fill nearly its entire volume. In a 70 kg (155 lbs) man, adipose tissue weighs approximately 14 kg (31 lbs) or about half as much as the total muscle mass. In morbidly obese individuals adipose tissue can constitute up to 70% of body weight and around 50% of body weight for severely obese individuals. Therefore, in a severely obese man weighing 113 kg (250 lbs.), adipose tissue constitutes approximately 57 kg (125 lbs.), that’s about half, of his total body weight. After consumption of a fat containing meal, triacylglycerols not processed in the liver are sent into the general circulation to be utilized by other tissues of the body, namely, fat tissue. In the absorptive state, elevated levels of glucose and insulin promote storage of triacylglycerols into fat tissue for use in a state of starvation. Therefore, if the absorptive state is frequently maintained and if total energy consumption is more than energy expenditure, then elevated levels of glucose and insulin would continually favor storage of triacylglycerol, resulting in a increase of adipose tissue and an overall gain in weight. While diet determines energy consumption, skeletal muscle determines the greatest portion of energy expenditure.

Skeletal muscle during a resting state accounts for approximately 30% of the total oxygen consumption of the body; however, during vigorous exercise skeletal muscle consumes up to 90% of the total oxygen consumption. Since the energy requirements vary so greatly between different levels of activity, muscle is unique in its ability to respond to substantial changes in energy requirements. Oxygen consumption of a specific tissue, such as muscle, is directly related to its energy requirement, and ultimately related to the number of calories it burns. In order to meet increased energy demands, muscle tissue maintains its own surplus of glycogen, similar to liver glycogen but exclusive for use in the muscle. After a meal, carbohydrates passed on by the liver are readily taken up by muscle tissue and subsequently stored in the form of muscle glycogen. In the well-fed state, glucose broken down from muscle glycogen is primarily used to fuel muscle tissue during exercise. Fat is secondary to glycogen usage for increased energy requirements in muscle in the well-fed state. After a meal, depleted muscle glycogen stores are readily replenished, and as long as glucose is available to the muscle it will use it and not fat. Thus, in order for muscle tissue to burn fat as an energy source, glucose circulating in the blood and muscle glycogen stores must first be exhausted. Also important after a protein containing meal is the muscle’s uptake of amino acids from the general circulation that are used to replace degraded or broken down muscle protein –the structural and functional molecules of muscle. While muscle tissue constitutes the majority of oxygen consumption and thus energy requirements, the brain is the single most important consumer of energy.

Although the brain contributes only 2% of the total body weight of an adult, it consumes 20% of the total oxygen consumption of the body in a resting state. Because the brain never rests, it uses energy at a constant rate. In fact, brain cells will begin to die after just a few minutes of oxygen or energy deprivation. Therefore, special metabolic pathways exist to provide the brain with a steady source of oxygen and energy to ensure its continual function. The brain is restricted to glucose as its principal energy source; it cannot use fat or amino acids as an energy source. In addition, no appreciable stores of glycogen or fat are found in the brain. Therefore, the brain must have a constant supply of glucose from the general circulation, and the body will break down stores of energy in other tissues, such as liver glycogen stores and muscle protein, in order to supply the brain with energy. The basics of energy metabolism during a state of starvation will be discussed in an upcoming article.

I would like the reader to recall the beginning quote. While our schedules usually determine when we eat, we should not let the timing of our meal affect the volume consumed. We live in a fast-food, "super-size for 39c" and "clean your plate" society. We eat quickly and we eat high volumes. These two factors, among others, contribute the ever increasing obesity problem in this country. This is not to say that you shouldn’t follow the dietary guidelines set out by organizations such as the American Heart Association, American Medical Association, or American Dietary Association. On the contrary, this is to say that even by strictly following dietary recommendations, you can still gain weight because the volume of food intake is greater than the energy burned in bodily function, in other words, eating too much and exercising too little. So follow the dietary recommendations given by the aforementioned organizations and exercise vigorously at least three times a week.

Please Note: I am pursuing a dental education, but I am neither a registered dietician nor a licensed physician, and anyone considering a lifestyle or dietary change or new exercise regimen should seek professional advice before starting such a program.

© 2000, James Loeser. All Rights Reserved

james@aikido-world.com

James Loeser has given AWJ permission to place these articles here in our web pages. 


James Loeser has his M.S.from Northwestern University, in Biotechnology - Specializing in Medicinal Chemistry / Bioinformatics. 
He is a dental student at the University of Illinois at Chicago, and practices Aikido.


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6/11/2002