Transcript Physiology of metabolism and energy

Physiology of metabolism
and energy
The absorbed materials
Digested molecules of food, water
and minerals from the diet, are
absorbed from the cavity of the
upper small intestine.
The absorbed materials cross the
mucosa into the blood, and are
carried off in the bloodstream to
other parts of the body for storage or
further chemical change.
Protein
Foods such as meat, eggs, and beans consist of
large molecules of protein that must be digested
by enzymes before they can be used to build and
repair body tissues.
Digestion of proteins
An enzyme in the juice of the stomach starts the
digestion of swallowed protein. Further digestion
of the protein is completed in the small intestine.
Here, several enzymes from the pancreatic juice
and the lining of the intestine carry out the
breakdown of huge protein molecules into small
molecules called amino acids.
These small molecules can be absorbed from the
hollow of the small intestine into the blood and
then be carried to all parts of the body to build
the walls and other parts of cells.
Protein Metabolism
The first step in protein metabolism is to break it
into its constituent amino acids. These are absorbed
into the blood stream.
The second step is to break down the amino acids
into their constituent parts - catabolism. This
removes the nitrogen or amino group from the
amino acids. The process is called deamination.
Deamination breaks the amino group down into
ammonia and what is termed the carbon skeleton.
Ammonia is converted to urea, filtered through the
kidneys, and excreted in urine. The carbon skeleton-which is composed of carbon, hydrogen, and
oxygen--can then by used either for protein
synthesis, energy production (ATP), or converted to
glucose by gluconeogenesis.
Amino acids stimulate the release of
both glucagon and insulin
In a healthy person, a rise in blood amino
acid concentration stimulates the secretion
of both glucagon and insulin, so their
blood sugar remains stable.
The insulin is secreted to stimulate protein
synthesis - the uptake of amino acids into
muscle cells - making them less available
for gluconeogenesis. The glucagon is
secreted to stimulate the uptake of amino
acids into the cells of the liver for
gluconeogenesis.
Why are these two hormones battling for
opposing uses of the same amino acids?
Isn't that non-productive?
Actually, the phenomenon serves an important
purpose. The release of these two opposing
hormones ensures that the amino acids are used
for protein synthesis (because of the extra
insulin) but the blood sugar doesn't drop to
dangerously low levels, even if the meal was low
in carbohydrate.
As a result, blood glucose concentration remains
reasonably stable during protein metabolism.
The insulin and glucagon essentially cancel each
other out in terms of their effect on blood
glucose, while the insulin is still able to promote
protein synthesis.
Protein digestibility
An important aspect of protein metabolism is how
well or how poorly a given protein is digested by the
human body. Claims are sometimes made that
protein powders (especially predigested or
hydrolyzed proteins) are digested more efficiently
than whole food proteins.
Protein digestibility is measured by seeing how much
nitrogen is excreted in the feces compared to the
amount of nitrogen which is ingested. A correction is
made for the amount of nitrogen which is normally
lost in the feces. Therefore, digestibility research
examines how much more nitrogen is lost over
normal levels when a given protein is fed.
If an individual were fed 6,25 grams of protein, 1
gram of nitrogen was excreted in the feces.
Digestibility of common
proteins foods
Food source
Egg
Protein digestibility (%)
97
Milk and cheese
Mixed US diet
Peanut butter
Meat and fish
97
96
95
94
Whole wheat
Oatmeal
Soybeans
86
86
78
Rice
76
Protein quality
Protein quality is a topic of major debate, both in
the research world, as well as in the realm of
protein supplements. Arguments have been made
that one protein is of higher quality than another,
or that protein powders are superior to whole
food protein.
Protein quality refers, in a general sense, to how
well or poorly a given protein will be used by the
body.
More specifically, it refers to how well the
indispensable amino acid (AA) profile of a protein
matches the requirements of the body. However,
this should not suggest that the content of
dispensable AAs in a protein is irrelevant to
protein quality as the body.
Methods of measuring
protein quality
There most spread methods available to measure
protein quality are: chemical score, biological
value, protein efficiency ratio, and protein
digestibility corrected amino acid score.
The quality of a protein is directly related to the
physiological needs of the subject being studied.
Diet and activity can affect how AAs are used in
the body. For example, long-duration endurance
activity tends to oxidize high quantities of the
branch-chain amino acids (BCAAs). In all
likelihood, there is no single protein that can be
rated as the highest quality for all situations.
Chemical score
Chemical score is method of rating proteins based on it's
chemical composition (more specifically it's indispensable AA
levels). To determine chemical score, a protein is picked as a
reference and other proteins are rated relative to that
reference protein.
Typically, egg protein has been used as the reference protein,
but this assumes that the amino acid profile of egg is the ideal
for humans.
Since chemical score is a relative, and not an absolute scale, it
is possible to have values greater than 100. If 5 grams of the
reference protein contains 800 mg of a certain amino acid, and
5 grams of the test protein contains 1000 mg of that same
amino acid, the second protein would be rated as 125% for
that amino acid.
The chemical score has little to do with how a food protein will
be used in the body and is rarely the only measure of protein
quality used to rate a protein.
Biological value (BV)
Biological value (BV) is probably one of the most commonly
used measures of a protein's quality. The BV of a protein is
given as the amount of nitrogen retained in the body
divided by the amount of nitrogen absorbed from that
protein. Therefore, digestibility of that protein is taken into
account. Thus:
BV = (nitrogen retained / nitrogen absorbed) * 100 %
A BV of 100 would indicate complete utilization of a given
dietary protein, in that 100% of the protein ingested was
stored in the body with none lost.
To measure BV, subjects are typically fed a zero protein diet
so that baseline losses of nitrogen can be measured (i.e.
the amount of nitrogen that is lost normally). Then the test
protein is fed at varying levels (generally 0.6, 0.5, 0.4 and
0.3 g/kg are fed) and a nitrogen balance study is done.
Some studies use longer periods of starvation and this is an
important consideration in evaluating the data.
BV of some
common
proteins
Table 2 presents the BV of
some common proteins.
Considering the high
protein intakes of most
strength athletes (2.0 g/kg
or higher) it is hard to see
how BV will play a
meaningful role in rating
proteins in this population.
In all likelihood, any decent
quality protein will be as
good as any other at these
types of protein intakes.
Protein
BV
whey
100
egg
100
milk
93
rice
86
casein, fish
and
75
beef
corn
peanut
flour
wheat
gluten
72
56
44
Protein efficiency ratio (PER)
PER is sometimes used to rate proteins and
represents the amount of weight gained (in grams)
relative the amount of protein consumed (in grams).
For example, a PER of 2.5 would mean that 2.5
grams of weight was gained for every gram of protein
ingested.
A recent animal study found that combinations of
animal (30% of total) and plant based proteins (70%
of total) had a higher PER value than the animal or
vegetable proteins eaten alone. This may have to do
with the proteins 'combining' to decrease the impact
of the limiting AA.
Individuals who wish to decrease their intake of
animal-based proteins may be able to achieve higher
PER values with a combination of animal and plant
based proteins than someone eating only animal
based proteins.
Protein digestibility corrected
amino acid score (PDCAAS)
PDCAAS is the newest method of protein quality to
be developed. It has also been suggested as the
ideal scale to rate proteins for their ability to meet
human requirements. Similar to chemical score, it
rates protein foods relative to a given reference
protein. In this case, the AA profile used is that one
determined to be ideal for children two to five years
old as its reference protein for adults.
Using the PDCAAS method, along with the proposed
AA reference patter, proteins which were previously
rated at poor quality, such as soy, have obtained
higher quality ratings. This is more in line with
studies showing that certain purified soy proteins,
such as Supro (tm) which is found in Twinlab Vegefuel, can maintain adults in nitrogen balance.
Summary of protein
quality
Although a variety of methods of measuring protein
quality have been proposed, none are perfect in rating
proteins for human use. While some methods of rating
protein are based on how well (or poorly) an animal
grows (or the nitrogen balance which is attained),
these methods provide no information on specific
amino acid requirements or protein synthesis at a
given tissue. Rather, only data regarding growth in the
whole body are obtained.
Another strategy to rate proteins is to compare the AA
profile in food protein to some reference protein.
Previously, food proteins such as egg or milk were
used as a reference but there has been a recent move
toward the use of an idealized reference pattern of
AAs to rate proteins. This assumes that the true
requirements for a given AA are known.
Carbohydrates
An average American adult eats about half a
pound of carbohydrate each day. Some of our
most common foods contain mostly
carbohydrates. Examples are bread, potatoes,
pastries, candy, rice, spaghetti, fruits, and
vegetables. Many of these foods contain both
starch, which can be digested, and fiber, which
the body cannot digest.
The digestible carbohydrates are broken into
simpler molecules by enzymes in the saliva, in
juice produced by the pancreas, and in the lining
of the small intestine. Glucose and other
monocaccharides is carried through the
bloodstream to the liver, where it is stored or
used to provide energy for the work of the body.
Fates of dietary glucose
The major source of dietary carbohydrate for humans
is starch from consumed plant material. This is
supplemented with a small amount of glycogen from
animal tissue, disaccharides such as sucrose from
products containing refined sugar and lactose in milk.
Digestion in the gut converts all carbohydrate to
monosaccharides which are transported to the liver
and converted to glucose. The liver has a central role
in the storage and distribution within the body of all
fuels, including glucose.
Glucose in the body undergoes one of three metabolic
fates: it is catabolised to produce ATP; it is stored as
glycogen in liver and muscle; it is converted to fatty
acids. Once converted to fatty acids, these are stored
in adipose tissue as triglycerides.
Extracting Energy from
Glucose
Two different pathways are involved in the
metabolism of glucose: one anaerobic and one
aerobic.
The anaerobic process occurs in the cytoplasm
and is only moderately efficient.
The aerobic cycle takes place in the mitochondria
and is results in the greatest release of energy.
As the name implies, though, it requires oxygen.
Anaerobic Metabolism
Glucose in the bloodstream diffuses into the
cytoplasm and is locked there by
phosphorylation. A glucose molecule is then
rearranged slightly to fructose and
phosphorylated again to fructose diphosphate.
These steps actually require energy, in the form
of two ATPs per glucose. The fructose is then
cleaved to yield two glyceraldehyde phosphates
(GPs).
Finally, two more ATPs are produced as the
phosphoglycerates are oxidized to pyruvate.
Aerobic
Metabolism
Pyruvate is the
starting
molecule for
oxidative
phosphorylation
via the Krebb's
or citric acid
cycle.
In this process,
all of the C-C
and C-H bonds
of the pyruvate
will be
transferred to
oxygen.
Summary of
metabolism of glucose
Basically, the pyruvate is
oxidized to acetyl coenzyme
A, which can then bind with
the four carbon oxaloacetate
to generate a six carbon
citrate.
Carbons and hydrogens are
gradually cleaved from this
citrate until all that remains
is the four carbon
oxaloacetate we started
with. In the process, four
NADHs, one FADH and one
GTP are generated for each
starting pyruvate.
Anaerobic
Consumed
:
2 ATP
Produced:
8 ATP
Net:
6 ATP
Aerobic
Consume
d:
0 ATP
Produced: 2x 15 ATP
Net:
30 ATP
Gluconeogenesis
The process of conversion of lactate to
glucose is called gluconeogenesis, uses
some of the reactions of glycolysis (but in
the reverse direction) and some reactions
unique to this pathway to re-synthesise
glucose.
This pathway requires an energy input (as
ATP) but has, due to kidneys, the role of
maintaining a circulating glucose
concentration in the bloodstream (even in
the absence of dietary supply) and also
maintaining a glucose supply to fast twitch
muscle fibres.
Fats
Fat molecules are a rich source of energy for
the body. The first step in digestion of a fat is
to dissolve it into the watery content of the
intestinal cavity.
Fat digestion
The bile acids produced by the liver act as natural
detergents to dissolve fat in water and allow the
enzymes to break the large fat molecules into smaller
molecules, some of which are fatty acids and
cholesterol. The bile acids combine with the fatty
acids and cholesterol and help these molecules to
move into the cells of the mucosa. In these cells the
small molecules are formed back into large molecules,
most of which pass into vessels (called lymphatics)
near the intestine.
These small vessels carry the reformed fat to the
veins of the chest, and the blood carries the fat to
liver and than to storage depots in different parts of
the body.
Fat metabolism and
gluconeogenesis
Fatty acids cannot be used directly to
produce glucose. However, gycerol, a
product of fat metabolism, can and does
go through the gluconeogenic pathway to
produce glucose. Glycerol is a minor
component in fats, and accounts for only 9
to 15% of the total mass.
Fats are much less important than
proteins in the gluconeogenic process
Vitamins, water and salt
Another important part of our food that is absorbed
from the small intestine is the class of chemicals we
call vitamins. There are two different types of
vitamins, classified by the fluid in which they can be
dissolved:water-soluble vitamins (all the B vitamins
and vitamin C) and fat-soluble vitamins (vitamins A,
D, and K).
Most of the material absorbed from the cavity of the
small intestine is water in which salt is dissolved. The
salt and water come from the food and liquid we
swallow and the juices secreted by the many digestive
glands. In a healthy adult, more than a gallon of
water containing over an ounce of salt is absorbed
from the intestine every 24 hours.
Oxydation and ATP
Food energy is released through a chemical reaction with
oxygen in a process called oxidation. When this occurs
outside the body - for example the burning of oil (a fat) in a
lamp or the use of a flaming sugar cube (a carbohydrate)
as a decoration in a dessert - this energy is released as
heat and light. In the body however, food energy needs to
be released more slowly and in a form that can be
harnessed for basic cell functions and transformed into
mechanical movement by the muscle cells.
This is accomplished by "refining" the three basic food
materials (carbohydrate, fat, and protein), converting them
into a single common chemical compound adenosine
triphosphate (ATP). It is this ATP, synthesized as the cell
metabolizes (or breaks down) these three basic foods that
transfers the energy content of all foods to muscle action.
Isodinamia of substances
The energy contained in equal weights of
carbohydrate, fat, and protein is not the same.
Energy content is measured in Calories.
Carbohydrates and protein both contain 4.1
Calories per gram (120 Calories per ounce) while
the energy "density" of fat is more than double at
9 Calories per gram.
The disadvantage of fat as a fuel for exercise is
that it is metabolized through pathways that
differ from carbohydrates and can only support
an exercise level equivalent to 50% VO2 max. It
is an ideal fuel for endurance events, but
unacceptable for high level aerobic (or sprint)
type activities.
Energy Requirements
for Daily Activities
An average man of 70 kilograms who lies in bed all day uses
about 1650 Calories of energy. The process of eating and
digesting food increases the amount of energy used each day by
an additional 200 or more Calories, so that the same man lying in
bed and eating a reasonable diet requires a dietary intake of
aboul 1850 Calories per day. If he sits in a chair all day without
exercising, his total energy requirement reaches 2000 to 2250
Calories. Therefore, the approximate daily en- ergy requirement
for a very sedentary man performing only essential functions is
2000 Calories.
The amount of energy used to perform daily physical activi- ties
is normally about 25 per cent of the total energy expendi- ture,
but it can vary markedly in different individuals, depend- ing on
the types and amounts of physical activities. For example,
walking up stairs requires about 17 times as much energy as
lying in bed asleep. In general, over a 24-hour period, a person
performing heavy labor can achieve a maxi- mal rate of energy
utilization as great as 6000 to 7000 Calo- ries, or as much as 3.5
times the energy used under conditions of no physical activity.
Notion about basal
metabolism
Physiology of temperature
regulation and water-solt balance
Invertebrates generally
cannot adjust their body
temperatures and so are at
the mercy of the
environment. In
vertebrates, mechanisms
for maintaining body
temperature by adjusting
heat production and heat
loss have evolved. These
species are called "coldblooded"
(poikilothermic) because
their body temperature
fluctuates over a
considerable range.
In birds and mammals ,
the ' 'warm-blooded ' '
(homeothermic) animals,
a group of reflex responses
that are primarily
integrated in the
hypothalamus operate to
maintain body temperature
within a narrow range in
Poikilothermic
and
homeothermic
organisms
Temperature
balance
The balance between heat
production and heat loss is
continuously being
disturbed, either by changes
in metabolic rate (exercise
being the most powerful
influence) or by changes in
the external environment
that alter heat loss or gain.
The resulting changes in
body temperature are
detected by
thermoreceptors, which
initiate reflexes that change
the output of various
effectors so that heat
production and/or loss are
changed and body
temperature is restored
Normal Body Temperature
In homeothermic animals, the actual
temperature at which the body is
maintained varies from species to species
and, to a lesser degree, from individual to
individual. In humans, the traditional
normal value for the oral temperature is
37 °C (98.6 °F), but in one large series of
normal young adults, the morning oral
tem- perature averaged 36.7 °C, with a
standard deviation of 0.2 °C.
Temperature receptors
There are cold and warmth receptors.
Nerve fibers respond differently at
different levels of temperature. So a
person determines the different gradation
of thermal sensation by the relative
degrees of stimulation.
It is believed that the cold and warmth
receptors are stimulated by changes in
their metabolic rates or from chemical
stimulation of the endings as modified by
the temperature.
Sensory Nerve Endings in the
Skin
Central and peripheral
thermoreceptors
There are two categories of
thermoreceptors, one in the skin
(peripheral thermoreceptors) and
the other (central thermoreceptors)
in deep body structures, including the
hypothalamus, spinal cord, and
abdominal organs.
Since it is the core body temperature,
not the skin temperature, that is being
maintained relatively constant, the
central thermoreceptors provide the
essential negative-feedback component
of the reflexes.
Central control of touch and
temperature sensation
Almost all sensory information from the somatic
segments of the body enters the spinal cord through the
dorsal roots from the spinal nerves.
Sensory signals are carried through one or two
alternative sensory pathways: 1) the dorsal colomnmedial lemniscal system; 2) the anterolateral system.
All these fibers belong to spinothalamic tract.
Sensory information that must be transmitted rapidly or
with great spatial fidelity is transmitted mainly in the
dorsal colomn-medial lemniscal system.
Sensory impulses, which do not need to keep these
conditions, are transmitted mainly in the anterolateral
system. The anterolateral system can transmit pain,
warmth, cold and crude tactile sensation. Because of
the crossing of the medial lemnisci in the medulla, the
left side of the body is represented in the right side of
the thalamus, and the right side of the body is
represented in the left part of the thalamus.
Central processing of impulses
Cerebral cortex processes somatic sensory information
in somatosensory area I, and somatosensory area II.
Somatosensory area I has much more extensive spatial
orientation of the different parts of the body.
Somatosensory area II helps in association of somatic
sensory information with visceral sensation and body
activity. In general, thermal signals are transmitted in
pathways parallel to those for pain signals. On entering
the spinal cord the signals travel for a few segments
upward or downward and than terminate in dorsal
horns. Then nerve fibers cross to opposite anterolateral
sensory tract and terminate both the reticular areas of
the brain stem and the ventrobasal complex of
thalamus.
A few thermal signals are also relayed to the somatic
sensory cortex from the ventrobasal complex.
Furthermore, it is known that removal of the
postcentral gurus in the human brain being reduced but
does not abolish the ability to distinguish gradations of
temperature.