Transcript Lecture One

Role of Metabolism in Nutrition
Definition: the sum of all biochemical changes that take
place in a living organism.
Group these reactions into two types:
anabolic
catabolic
Reactions: require energy
release energy
Produce:
more simple compounds
more complex
compounds
Modus
Operandi: Occurs in small steps, each of which is controlled
by specific enzymes.
Examples of each type of metabolism:
Anabolic Pathways
Protein Biosynthesis
Glycogenesis
Gluconeogenesis
Fatty Acid Synthesis
Catabolic Pathways
ATP
Generated
FOR
Provides
Energy
Glycolysis
TCA (Krebs cycle)
ß-oxidation
Respiratory Chain
Other useful generalizations:
Some of the steps in the anabolic path (going “uphill”) may not be
identical to the catabolic path--but some are shared.
Metabolism: Who Needs It?
Average American consumes ~ 1450 lbs of food
each year.
Assuming that 98.2% of this energy is
metabolizable, 1424 lbs is used to supply our
needs.
Supplies roughly 1 x 106 kcals/ year
How do we employ energy?
•
•
•
MECHANICAL- muscle contraction
ELECTRICAL- maintaining ionic gradients
(e.g., Na-K ATPase; 70% of
ATP used by kidney & brain
used to maintain gradient)
CHEMICAL- biotransformation of
molecules (e.g., synthesis
degradation, metabolism)
International Unit of Energy: Joule
: energy used when 1 Kg is moved
1 meter by a force of 1 Newton
: kJ = 103 J; MJ = 106 J
: 1 kcal = 4.184 kJ
:
Protein:
CHO:
Fat:
17 kJ or 4 kcal/g
17 kJ or 4 kcal/g
37 kJ or 9 kcal/g
Average Energy Needs:
European text:
American Biochem text:
100 kJ/ day x BW in kg
or
24 kcal/day x BW in kg
129-184 kJ/ kg
or
31-44 kcal/kg
Conversion Efficiency: Food to Usable Energy
40% used to make
high energy phosphate
bonds
60% “lost” (?) as
heat
What are the components of energy expenditure?
Basal metabolic rate
Definition:
Determinants:
Calculation:
Energy Expenditure Component 2:
THERMIC EFFECT OF FOOD
Definition:
Determinants:
Contribution to Total Energy Expenditure:
Components of Energy Expenditure- 3
Physical Activity
Contribution to Total Expenditure:
What about accounting for changes in energy expenditure due
to injury or trauma?
Maintaining Body Composition:
Fuel Utilization in Maintenance and Injury
Average Adult Composition
Water
%
(w/w)
55
Protein
19
Adipose Tissue
19
CHO
<1
Inorganic matter
7
Recommended Fuel Sources
(% of kcal)
Source
Fat
% of kcals
DRVs
Atwater*
30
33
Protein
10
15
CHO
60
52
*W.O. Atwater (1894), USDA Scientist credited with deriving
physiologic energy values of pro, CHO, fat.
PROGRESS!!!
Fuel Sources During Exercise
Normal ADL
LIGHT
MODERATE
HEAVY
OVERVIEW OF METABOLISM:
Too Much, Too Little, Too Stressed
Energy Economy in Feasting
Metabolic Adaptation to Starvation
•
WHO Guidelines for
Treatment of Severe Malnutrition
Fuel Utilization in Hypermetabolic States
Reclaiming Energy From Stored Fuel Sources:
By Choice = Fasting
By Necessity= Starving
Exhaustion of “labile” CHO:
Exhaustion of stored CHO:
Problem: certain tissues require glucose for energy
Tapping into stored protein:
Short-term effect and contribution:
If this contribution continues:
Adaptation to Starvation/ Fasting
Building glucose in the absence of labile or stored CHO:
After deamination, the carbon skeletons of some amino acids
can be used to make glucose or ketone bodies (ketoacids).
Gluconeogenesis: the formation of glucose from lactate,
some amino acids, and glycerol
Long-term dependence on GNG to fuel brain is not feasible.
Switch to ketone production within 10 d of fast -- provides
majority of energy for brain. Protein sacrificed for glucose
production for parts of brain requiring it.
Benefits of Ketosis:
•
•
provides needed source of energy;
suppresses appetite.
Concomitant Changes in Energy Expenditure
Wasting results in decreased energy expenditure
Heart mass
Lung mass
Skeletal muscle
Hormonal response to fasting leads to energy conservation
Metabolic Adaptations to Fasting/Starvation:
ADVANTAGES & DISADVANTAGES
Advantages
Energy Expenditure
Body Temperature
Enhanced Survival
Disadvantages
Wasting of
muscle mass
Decreased
immune
competence
See “guidelines for the inpatient treatment
of severely malnourished children”
London School of Hygiene and Tropical Medicine.
Full thickness
Burns
GI
Cardiac
trauma
sepsis
Renal
Cancer
Injury, Trauma, Surgery
Neurohormonal Activation of the Stress
Response
Glucocorticoid & Catecholamine
Activation, Hi Glucagon:Insulin
Ratio, Growth Hormone Release
Tachycardia, Tachypnea, Hyperglycemia,
Mobilization of Body Fat, Massive Catabolism
of Skeletal Muscle
In Critical Illness, Timing of Assessment is
Extremely Important!
Why?????
Metabolism in critical injuries
goes through at least three
distinct phases:
Ebb (1st 24 hrs post-injury)
Flow (Days 2-5)
Anabolic (7-10 days)
Immediate Needs to Sustain Life:
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•
•
Restore blood flow;
Maintain oxygen transport;
Prevent/treat infections.
If malnourished, introduce nourishment
cautiously, if not-Refeeding syndrome: malabsorption, cardiac
insufficiency, respiratory
distress, CHF, etc.
Fluid and Electrolytes
Many types of stress can cause massive
fluid losses.
Examples:
Severe burns= lose 12-15%
of BW is FIRST 24 hours!
Vomiting, diarrhea, wounds,
bleeding, and FEVER
Energy Metabolism in Critical Injuries
Response to Injury Separable into Two Phases
“Ebb” Phase : 1st 24 hours post-injury
Characterized by low cardiac flow, tissue perfusion
Priority of Metabolism in “Ebb”= resuscitation
maintain tissue perfusion
Hormonal response:
catecholamines increase availability
of energy-yielding substrates
(glucose, aa, ffa)
But…….substrate utilization in depressed.
No additional nutrition support needed
Second Phase: “Flow” or Hypermetabolic Phase
1. Massive increase in catabolic hormone release
(e.g., glucagon, catecholamines, etc.)
2. High cardiac output
3. Increased insulin secretion 2˚to #1;
insulin resistance may exacerbate
hyperglycemia.
N Balance Possible?
4. Energy sources? Glycogen gone.
No way, baby.
80% fat stores
20% endogenous
protein
Water/Na retention; urinary N losses
Late Flow Phase:
Anabolism now possible
Catecholamines decrease, energy needs
Decrease, N balance begins to approach “zero”.
Assessment of Energy Needs:
BEE X Activity Factor X Injury Factor
Warning! May overestimate needs!
Overfeeding may precipitate
Respiratory Failure.