Role of Metabolism in Nutrition
Definition: the sum of all biochemical changes that takeplace in a living organism.
Group these reactions into two types:
anabolic catabolic
Reactions: require energy release energy
Produce: more complex more simple compoundscompounds
ModusOperandi: Occurs in small steps, each of which is controlled by specific enzymes.
Examples of each type of metabolism:
Anabolic Pathways Catabolic Pathways
Protein Biosynthesis GlycolysisGlycogenesis TCA (Krebs cycle)Gluconeogenesis ß-oxidationFatty Acid Synthesis Respiratory Chain
Other useful generalizations:
Some of the steps in the anabolic path (going “uphill”) may not beidentical to the catabolic path--but some are shared.
ATPGeneratedProvidesEnergy
FOR
Metabolism: Who Needs It?
Average American consumes ~ 1450 lbs of foodeach year.
Assuming that 98.2% of this energy is metabolizable, 1424 lbs is used to supply ourneeds.
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: 17 kJ or 4 kcal/g CHO: 17 kJ or 4 kcal/g
Fat: 37 kJ or 9 kcal/g
Average Energy Needs:
European text: 100 kJ/ day x BW in kg or24 kcal/day x BW in kg
American Biochem text: 129-184 kJ/ kg or 31-44 kcal/kg
Conversion Efficiency: Food to Usable Energy
40% used to makehigh energy phosphatebonds
60% “lost” (?) as60% “lost” (?) asheatheat
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 dueto injury or trauma?
Maintaining Body Composition: Fuel Utilization in Maintenance and Injury
Average Adult Composition %
(w/w)Water 55
Protein 19
Adipose Tissue 19
CHO <1
Inorganic matter 7
Recommended Fuel Sources (% of kcal)
Source % of kcals DRVs Atwater*
Fat 30 33
Protein 10 15
CHO 60 52
*W.O. Atwater (1894), USDA Scientist credited with derivingphysiologic 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 = FastingBy 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 acidscan 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 -- providesmajority of energy for brain. Protein sacrificed for glucoseproduction 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 Disadvantages
Energy Expenditure Wasting of muscle mass
Body Temperature Decreased immune
Enhanced Survival competence
See “guidelines for the inpatient treatment of severely malnourished children” London School of Hygiene and Tropical Medicine.
Burns trauma sepsis
GI Cardiac Renal Cancer
Full thickness
Injury, Trauma, Surgery
Neurohormonal Activation of the StressResponse
Glucocorticoid & CatecholamineActivation, Hi Glucagon:InsulinRatio, Growth Hormone Release
Tachycardia, Tachypnea, Hyperglycemia, Mobilization of Body Fat, Massive Catabolism of Skeletal Muscle
In Critical Illness, Timing of Assessment isExtremely Important!
Why?????
Metabolism in critical injuriesgoes through at least threedistinct phases:
Ebb (1st 24 hrs post-injury)Flow (Days 2-5)Anabolic (7-10 days)
Immediate Needs to Sustain Life:
• Restore blood flow;• Maintain oxygen transport;• Prevent/treat infections.
If malnourished, introduce nourishmentcautiously, if not--
Refeeding syndrome: malabsorption, cardiacinsufficiency, respiratorydistress, CHF, etc.
Fluid and Electrolytes
Many types of stress can cause massivefluid 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.
4. Energy sources? Glycogen gone.80% fat stores20% endogenous protein
Water/Na retention; urinary N losses
N Balance Possible?No way, baby.
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.