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II:
PRINCIPLE of NUTRIENT UTILIZATION
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Adequate
nutritional support, that is, adequate provisions of energy and protein
substrate, is an essential aspect of the management of the surgical
patient.
The use of nutrients by the cell for energy and tissue synthesis
requires an adequate delivery of oxygen to the cells.
Maintenance of tissue, oxygen, delivery, and hemo-dynamic
stability are essential for nutritional management.
Energy requirements are
defined in terms of the calorie. One
calorie is the energy required to increase the temperature of 1 gram
of water by 1 degree centigrade. The
calorie used to define human metabolic needs is actually a kilocalorie
(1000 calories). Protein
requirements are defined in terms of nitrogen needs, nitrogen making up
about 15 percent of a protein.
The
respiratory quotient (RQ) is the ratio of carbon dioxide production
during metabolism to the amount of oxygen consumed.
RQ equals CO2 produced (Moles) /O2
consumed (Moles). The RQ value for carbohydrate is 1.0, for fat 0.7 and for protein
0.8: The grams of nitrogen in protein = grams of protein/6.25.
The calorie: nitrogen ratio is the number of calories per gram of
nitrogen. |
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Nutrient |
Calories/gm |
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Carbohydrates |
4
(3.4 hydrated) |
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Fats |
10 |
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Proteins |
4 |
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A: CARBOHYDRATE METABOLISM |
| Carbohydrate
is the primary fuel in man accounting for 60-70% of calories.
In its anhydrous form 1 gram of carbohydrate generates 4
calories. However,
carbohydrate is stored and used, for example, in solution in its
hydrated form from which 3.4 calories is generated per gram metabolized.
Insulin is required for glucose to enter the cell to be used for
energy. |
| Only
small amounts of glucose are stored.
About 150 grams can be stored in the adult liver and 300 grams in
muscle as glycogen, which can be rapidly mobilized to glucose for when
needed. Glucose requires
phosphorylation to initially enter the cell, and it cannot subsequently
leave that cell. The total
number of stored glucose calories is less than 1500 in the normal adult.
Excess glucose cannot be stored beyond that which is needed for
energy and a small amount of glucogen which tissues can use. Excess
glucose is rapidly conversed to fat.
This process requires energy, and the respiratory quotient
exceeds 1.0, resulting in a marked excess I carbon dioxide production. |
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Glucose
+ O2 = CO2 + Energy (RQ=1.0)
Glucose
(excess) + Energy = Fat + CO2
(RQ=8) |
| Carbohydrate
cycling occurs with the “stress response”, via activation of the
Cori cycle. Carbohydrate to
lactate and back to carbohydrate. This
process yields only a fraction of the ATP produced with the complete
burning of carbohydrates to CO2 and water.
In addition, the hormonal stimulus for gluconeogenesis exceeds
demands and a relative insulin resistant state is present due to
elevated anti-insulin hormone activity.
Providing
sufficient carbohydrate (60% of total calories) is still necessary to
attenuate the drive to breakdown protein with amino acids used to make
glucose. |
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B: FAT METABOLISM |
| Fat
is used fuel in unstressed man, accounting for about 30-40 percent of
the calories, depending on the diet.
Fat is used for 85-90% of fuel in starvation.
Fat is composed of triglycerides, which in turn comprise glycerol
and fatty acids. The
glycerol is burned like carbohydrate, while the fatty acids enter the
Krebs cycle via a process called beta-oxidation whereby two carbon
segments are cleaved off. Ketones
are then produced. The
ketones enter the Krebs cycle as acetylcoenzyme A, generating energy at
a respiratory quotient of 0.7 with 9 to 10 calories per gram of fat.
Increased ketosis signals the predominant use of fat through
ketones for energy. This
situation is seen in the starved surgical patients with a lack of
available carbohydrate where ketones become the predominant fuel.
With a large carbohydrate load, fat utilization is depressed.
In addition to the availability of carbohydrate, fat breakdown is
dependent on the endocrine environment.
Insulin decreases the utilization for energy in preference to
carbohydrate while catecholamines, and growth hormone increase fat
breakdown. The number of
available calories in stored fat in the normal adult with 10 kg of fat
is about 100,000 – about 100 times more than that in stored
carbohydrate. With the “stress response” fat is not used to decrease
amino acid use for energy. Instead there is an increase in the cycling of fatty acid to
fat and back. However, fat
may provide up to 50% of fuel in the absence of adequate carbohydrates.
Ketosis is not usually seen in the stressed surgical patient
reflecting the decreased fat utilization compared to that seen in
starvation. |
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