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"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. |
