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 complex more simple compounds compounds Modus Operandi: Occurs in small steps, each of which is controlled by specific enzymes.

Examples of each type of metabolism: Anabolic PathwaysCatabolic Pathways Protein BiosynthesisGlycolysis GlycogenesisTCA (Krebs cycle) Gluconeogenesisß-oxidation Fatty Acid SynthesisRespiratory 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. ATP Generated Provides Energy FOR

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 10 6 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 = 10 3 J; MJ = 10 6 J : 1 kcal = 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 or 24 kcal/day x BW in kg American Biochem text: kJ/ kg or 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 % (w/w) Water55 Protein19 Adipose Tissue19 CHO<1 Inorganic matter 7

Recommended Fuel Sources (% of kcal) Source % of kcals DRVs Atwater* Fat Protein CHO *W.O. Atwater (1894), USDA Scientist credited with deriving physiologic energy values of pro, CHO, fat. PROGRESS!!!

Fuel Sources During Exercise Normal ADLLIGHT 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 AdvantagesDisadvantages Energy ExpenditureWasting of muscle mass Body TemperatureDecreased immune Enhanced Survival competence See “ guidelines for the inpatient treatment of severely malnourished children” London School of Hygiene and Tropical Medicine.

Burns trauma sepsis GICardiacRenal Cancer Full thickness

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: 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. 4. Energy sources? Glycogen gone. 80% fat stores 20% 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.