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.