Lesson 2 :
Role of Metabolism in Nutrition
Metabolism
• Metabolism – process by which living
systems acquire and use free energy to
carry out vital processes
• Catabolism (degradation)
– Nutrients and cell constituents are broken
down for salvage and/or generation of energy
– Exergonic oxidation
• Anabolism (biosynthesis)
– Endergonic synthesis of biological molecules
from simpler precursors
– Coupled to exergonic processes through
“high-energy” compounds
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.
Relationship Between Catabolic and
Anabolic Pathways
• Catabolic pathways
– Complex metabolites are transformed into
simpler products
– Energy released is conserved by the synthesis
of ATP or NADPH
• Anabolic pathways
– Complex metabolites are made from simple
precursors
– Energy-rich molecules are used to promote
these reactions
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 ( 600kg) of food
each year.
Assuming that 98.2% of this energy is
metabolizable, 1424 lbs ( 590kg) is used to supply our
needs.
Supplies roughly 1 x 106 kcals/ year
Energy needs
Measurement of Energy Intake
Diet Surveys
• Dietary and nutritional survey of British
Adults (Gregory 1990)
• Average UK dietary energy intake
• Men 10.2 MJ (2400 kcal/d)
• Women 7.02MJ (1650 kcal/d)
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
Energy needs
Measurement of Energy Intake
Metabolic Energy Yields
Fuel
KJ/g
Kcal/g
Fat
38
9
Alcohol
29
7
Carbohydrates 17
4
Protein
4
16
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
How to measure energy in food
• Direct Calorimetry
– Direct measurement of heat produced
– Bomb calorimeter
• Calculate
– Calories/g: 4 (cho), 9 (fat) ,4 (prt) and 7 (alcohol)
– based: lab analysis of food composition
• Calorie chart or nutrient database
Energy needs
Measurement of Energy Intake
Bomb Calorimeter
• Food is ignited electrically in the presence
of oxygen
• Heat of combustion is measured from a rise
in water temperature
Bomb Calorimeter measures heat
produced when food is burned
Text view of bomb calorimeter
Energy needs
Measurement of Energy Intake
Bomb Calorimeter
• Heat of combustion represents the gross
energy of the food
• Energy lost during digestion and absorption
• Affected by illness
Energy needs
Measurement of Energy Intake
% Energy from carbohydrates, protein and fat
Food
Total energy
% Energy
content of
from
serving (kJ) carbohydrate
% Energy
from
protein
%
Energy
from fat
Chocolate
1195
43
6
51
Peas
230
47
35
18
Chicken Breast
1138
0
75
24
Potato boiled
535
89
10
1
Energy Balance: Input vs Output
Energy Balance
• Energy In = Energy Out
– Weight Maintenance
• Energy In > Energy Out
– Weight Gain
• Energy In < Energy Out
– Weight Loss
Energy Balance
• Sources of fuel for energy
– Input from diet: carbs, fat, prot, alcohol
– Stored energy: glycogen, fat, muscle
• Energy outgo from:
– Basal metabolism
– Physical activity
– “Dietary thermogenesis”
Energy In
• Food and Beverages
– Food composition tables
– Bomb Calorimetry
• Complex social, environmental,
physiological control
Energy Out
• Energy of food = Body Energy = ATP
– Overall efficiency 25%, 75% released heat
• Energy out:
• 3 main components:
– Basal Metabolic Rate
– Thermic Effect Food
– Physical activity
BMR > Activity > Dietary Thermogenesis
Energy needs
Measurement of Energy Output
Energy Output
Energy of food
• 50% efficiency
ATP + Heat Loss
ATP
• 50% efficiency
“Work “+ Heat Loss
“Work “
Heat
Energy needs
Measurement of Energy Output
Energy Output
Measurement
of heat
• Two methods
• direct calorimetry
• indirect calorimetry
Measurement of
energy used
Energy needs
Measurement of Energy Output
direct calorimetry
• Measurement of heat loss
Energy needs
Measurement of Energy Output
Indirect calorimetry
• Utilisation of oxygen
• Oxygen consumption is proportional to ATP
synthesis
• Use oxygen consumption to determine heat
production
Energy needs
Measurement of Energy Output
Indirect calorimetry
• Glucose oxidation
C6H12O6 + 6O2
6H2O + 6CO 2 + 15.5 kJ/g of energy
• Starch oxidation
(C6H12O5)n + 6nO2
5nH2O + 6nCO 2 + 17 kJ/g of energy
Energy needs
Measurement of Energy Output
Indirect calorimetry
• Fat oxidation
(e.g. glyceryl butro-oleostearate (main fat in butter)
C3H5O3.C4H7O.C18H33O. C18H35O
43CO2 + 40H2O
+
+ 60O2
39 kJ/g of energy
Energy needs
Measurement of Energy Output
Indirect calorimetry
• Respiratory quotient (RQ)
• CO 2 Produced / O 2 Consumed
• RQ for Carbohydrates = 1.0
C6H12O6 + 6O2
6H2O + 6CO 2
• RQ for fats = 0.71 (average)
Energy needs
Measurement of Energy Output
Indirect calorimetry
• RQ value can be used to find the amount
energy produced per litre of oxygen
consumed
• Metabolic mix
Energy needs
Measurement of Energy Output
Indirect calorimetry
• Respiratory Gas Analysis
• Respirmeters
• Direct of measurement of O2 and CO2
• Heart Rate Monitoring
• Heart rate calibrated against oxygen utilisation
• Isotope Method
• Labelled water (2H and 18O)
• Difference of rates of loss of isotopes loss of CO2
What are the components of energy expenditure?
1:
Basal metabolic rate
Definition:
Determinants:
Calculation:
Energy Out: Basal Metabolism
• Largest daily energy output
• Definition: “the sum total of minimal activity of
all tissue cells of the body under steady sate
conditions”
• Men estimate: lbs body weight X 11
• Women estimate: lbs body weight X 10
• affected by
– Muscle > Fat
Male > Female
– Young > Old
– Temperature: body and environment
Basal Metabolic Rate
• BMR = number of calories would need daily
simply to stay alive if were totally inactive, in bed,
awake for 16 hours & slept for 8 hours
• Harris-Benedict Equation:
• Women: 661+(4.38 x weight in pounds)+(4.38 x
height in inches)-(4.7 x age)=BMR
• Men: 67+(6.24 x weight in pounds)+(12.7 x height
in inches)- (6.9 x age)=BMR
James & Schofield
1) Basal Metabolic Rate
• 50-70% Energy Expenditure
• Maintain basic metabolic processes
Cells
Growth
•
•
•
•
•
•
•
Muscles
Temperature regulation
Osmotic pumps
Protein synthesis
Heart
Respiratory system
Digestive tract
Individual variation
Within individual variation
10%
Factors affecting BMR
• 1) Body Size & Composition
– Lean tissue BMR
– Body weight wt lean tissue (but also fat)
• 2) Age:
– age Lean tissue
• 3) Sex: Men lean
• 4) Activity: Exercise lean tissue
Factors affecting BMR
• 5) Growth BMR
– Children, pregnancy
• 6) Fasting/starvation: BMR
• 7) Fever/stress BMR
• 8) Smoking/caffeine: BMR
2:
Energy Expenditure Component :
THERMIC EFFECT OF FOOD
Definition:
Determinants:
Contribution to Total Energy
Expenditure:
2) Energy Out: Dietary
Thermogenesis
• Dietary thermogenesis
– Energy to digest, absorb, metabolize food
– About 10% of calories eaten
2) Thermic Effect of Food
• 3-6 hours following ingestion
• ~10% energy intake
– 2000 kcal diet = 200 kcal TEF
• Affected by:
– Meal size/frequency
– Composition: Protein > Carbs/fat
– Genetics
3:
Components of Energy Expenditure
Physical Activity
Contribution to Total Expenditure:
4:
What about accounting for changes in energy expenditure
due to injury or trauma?
3) Energy Out: Physical Activity
• Physical Activity affected by:
– Intensity -- how vigorous
– Time spent
– Body weight
3) Physical Activity
• Variable: 20-40%
• Working muscles require energy
– Heart/lung extra energy
• Amt energy used depends on:
– Muscle mass
– Body weight
– Activity nature & duration
Activity Level and Metabolism
• Activity can account for 20-30% of
metabolism
1. Sedentary = Multiplier 1.15 x BMR
2. Light activity (Normal Every day activities) =
Multiplier 1.3 x BMR
3. Moderately Active(exercise 3-4 x’s week) =
Multiplier 1.4 x BMR
4. Very Active (exercise more than 4 x’s week) =
Multiplier 1.5 x BMR
5. Extremely Active (exercise 6-7 x’s week) =
Multiplier 1.6 x BMR
Activity Level and Metabolism
• If you change Light activity (Normal Every
day activities) to Moderately Active(exercise 34 x’s week) daily caloric burning goes up 7.7%
• If you change Light activity (Normal Every
day activities) to Very Active (exercise more
than 4 x’s week) daily caloric burning goes up
23%
• If you change Light activity (Normal Every
day activities) to Extremely Active (exercise 67 x’s week) daily caloric burning goes up
38.5%
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:
•
•
•
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.
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
Energy Requirements
• Difficult to estimate
• Direct measurement
– Research
• Estimates from averages
– Based on age/sex
– Assume light/moderate activity
– Estimate TEF
Energy
• We Need Energy for 3 Reasons:
– 1) Basal metabolism
– 2) Physical activity
– 3) Dietary thermogenesis
• How many calories do you need?
– Simple calculation
1) Basal Metabolism
• Definition: Energy required to maintain
normal body functions while at rest
• To estimate the calories you need for basal
metabolism
– For men: Multiply body weight (lbs) by 11
– For women: Multiply body weight (lbs) by 10
2) Energy for Physical Activity
ACTIVITY LEVEL
PERCENTAGE OF BASAL
METABOLISM CALORIES
Inactive: sitting most of the day;
<2 hours moving about slowly or
standing
30%
Moderate: sitting most of the
day; walking or standing 2-4
hours, no strenuous activity
50%
Active: physically active for >4
hours a day; little sitting or
standing; some strenuous activity
75%
3) Dietary Thermogenesis
• Definition: the energy expended during
digestion of food
• It accounts for approximately 10% of the
body's total energy need (basal needs and
energy needs)
Doing the Calculation
• Jane weighs 140 and is moderately active student
(she goes to classes and goes to the gym 1hr/day)
• Basal needs: 140 * 10 = 1400
• Physical activity needs: 1400 * .50= 700
• So far she needs 2100 calories, but wait, she has
to digest!
• 2100 * .10 = 210 calories
• Now, we add it up for her: 1400 + 700 + 210=
2310 calories
• How many calories do you need???
Energy Balance
• Balanced energy intake: not losing or
gaining weight
• Negative energy balance (weight loss):
energy intake < energy expended
• Positive energy balance (weight gain):
energy intake > energy expended
Hunger vs. Appetite
• Hunger: physical need for energy,
accompanied with unpleasant symptoms
such as weakness, stomach pains, irritability
• Appetite: desire to eat is driven by mental
stimuli
Obesity
• How do we define obesity?
–By culture
–By science
Is Obesity an Epidemic?
• Prevalence in US: 33% of adults and 25%
of children are obese (But according to
whom????)
• Risks associated with obesity: diabetes,
hypertension, stroke, heart disease, elevated
total cholesterol, low HDL-cholesterol,
certain types of cancer, gallbladder disease
What Causes Obesity
• 3 major factors contribute to the development of
obesity
– 1) Genetic background
• Heredity may account for approx. 25-40% of obesity
but this is very poorly understood
• Effects on metabolism (rare); traits that predispose
(common)
– 2) Dietary intake
– 3) Physical activity
Measuring Body Fatness
•
•
Weight-for-Height tables:
– Dietary Guidelines for Americans
– Metropolitan Life Insurance Company (allows for
increased weight with age)
– Limitations: not based on percentage body fat
Body Mass Index (BMI)- widely used
– Calculated by dividing body weight (in kg) by height
(in meters) squared
• 19-25 is considered acceptable
• overweight is btw. 25-30
• > 30 obesity
Some Methods for Assessing Body Fat:
• Scale weighing doesn't distinguish between
lean body mass and body fat
–
–
–
–
–
–
–
–
Skinfold thickness measurements
Bioelectrical impedance
Underwater weighing
Magnetic resonance imaging (MRI)
CAT scans
Ultrasound
Total body electrical conductivity
Magnetic resonance spectroscopy
We Do Need Body Fat
• For survival we need:
– 3-5% for men
– 10-12% for women
• Low body fat associated with
–
–
–
–
Delayed physical maturation during adolescence
Infertility
Accelerated bone loss
Symptoms of starvation
Role of Body Fat:
– Makes hormones
– Component of every type of body cell
– Cushions internal organs
– This fat is not available for energy
Location of Body Fat is Important to Health
• Central obesity is associated with more
health risks than lower obesity
• Assessing Body Fat Distribution:
– Waist to hip ratio
• More than .80 in women and .95 in men indicate
central body fat distribution
– Waist circumference
• Over 40" (102 cm) in men (increased risk for
health problems)
• Over 35" (88 cm) in women
Realities of Obesity
• Myth: any individual can get to any body weight
if they diet and exercise enough
• People do come in different shapes and sizes and
people should come in different shapes and sizes
• From a health perspective, the goal of obesity
prevention and treatment should be for people to
eat a health promoting diet, get regular exercise,
and pay attention to hunger and satiety cues
– What are the barriers with this goal?????
Body Composition
Body Weight
• Fat Mass + Fat Free Mass
• FFM: muscle, lean tissue, bone, water
• Diseases associated with:
– Excessive fat mass
– Depleted fat mass
– Depleted FFM
Body Weight
• Body weight = Fat + FFM
– Not % fat % FFM
Healthy Body Weight
• Weight associated lowest mortality
• Techniques for Assessing body weight
– Life Insurance Tables
– Relative weight
– Body Mass Index
• Life Insurance Tables
– Rarely used research/clinical practice
• Relative weight
– Actual weight/desirable weight
– 110-120%: overweight
– 120-130% obese
• Disadvantages relative weight
– Desirable difficult to define
– Not sex specific
– Not adjusted for age
Body Mass Index (BMI)
Weight (kg)
BMI =
Height (m2)
Classification
Weight (lb)
Height (in2)
BMI (kg/m2)
X 705
Risk co-morbidity
Normal values
18.524.9
Average
Overweight
Pre-obesity
Obesity class I
Obesity class II
Obesity class III
25
2529.9
30.034.9
35.039.9
40.0
Increased
Moderate
High
Very High
World Health Organization,
1998
Limitations of BMI
Both men have a BMI of 31
Find % body fat by: Underwater
Weighing
Skinfold
• Measures
subcutaneous fat
• Accuracy depends
on caliper skill
Other High Tech Methods
• Bioelectrical
Impedance BMI
• Magnetic
Resonance Imaging
MRI
• “Bod-Pod”
measures air
displacement
Bioelectrical Impedance
•
•
•
•
•
•
•
•
Estimation of body composition
Most used in clinical practice
Based on electrical conduction through organism
At higher frequencies (eg 50 kHz) resistance of
cell membranes reduced so that current penetrates
both extra- and intracellular fluids
Bio impedance is a reliable prediction of the
body’s main conduction material : water
73.2% FFM consists of water and total FM + FFM
= total body weight
Electrodes placed on dominant side of body; legs
apart and not in contact
More accurate in obese women than DEXA
Fat intake  but Overweight 
Total
calories
Physical
activity
1 lb (0.4kg) body fat =~ 3500 kcal
Energy Balance
• Positive - Gain 1 lb - eat 3,500 kcal more
than need
• Negative -Lose 1 lb - eat 3,500 kcal less
than need
• If energy bal = - 500 kcal / day
3500/500 = 7 days to lose 1 lb
• Best to combine with physical activity, e.g.,
mile walk = - 100 kcal
What happens in weight loss?
 Water
 Fat
 Muscle mass
May  Bone density
Gradual weight loss minimizes loss of
muscle & bone
• Drastic methods: fasting, surgery,
liposuction
• Other methods: diets, pills
•
•
•
•
•
Body fat location is important
• Apple = Abdomen
• Pear =
Hips & thighs
• Apple -> risk of heart
disease
• waist/hip ratio:
>0.8 F, > 0.95 M
indicates apple shape
Assessing obesity:
bmi, waist circumference and disease risk
Disease Risk Relative to Normal
Weight and Waist Circumference
Men <102 cm
Women <88 cm
Men >102 cm
Women >88 cm
Category
BMI
Underweight
—
—
Normal*
<18.5
18.5-24.9
—
—
Overweight
25.0-29.9
Increased
High
Obesity
30.0-34.9
35.0-39.9
High
Very high
Very high
Very high
>40
Extremely high
Extremely high
Extreme obesity
*An increased waist circumference can denote increased disease risk even in persons of normal weight.
Adapted from Clinical guidelines. National Heart, Lung, and Blood Institute Web site. Available at:
http://www.nhlbi.nih.gov/nhlbi/cardio/obes/prof/guidelns/ob_gdlns.htm. Accessed July 31, 1998.
• Correlates with body fat content
• European Origin
–
–
–
–
Asian: healthy weight <23.5
Polynesian: healthy weight 18.5-26
Elderly: healthy weight up to 27
Requires further research
• Not suitable for athletes
Body Fat Distribution
• disease risk/mortality with excessive body
fat
• ‘Healthy’ % body fat
– 12-20% Men
20-30% Women
• Fat distribution associated disease risk
– Abdominal fat risk
– Lower body fat: no increase risk
• Central Obesity: Abdominal fat
–  prevalence Men, postmenopausal women
• Measures of Central Adiposity
• 1) Waist Circumference
– > 88 cm women, 102cm men
• 2) Waist:Hip Ratio
– >0.8 women, 0.95 men
• 3) Fat fold measures
– Training required
Obesity
•
•
•
•
Prevalence increasing
Serious health concern
body weight 10% significant health risk
Prevalence obesity increases
– Age
– Lower socio-economic status
– Women (USA)
• 0.5kg fat stores ~ 4500 kcals
• Recommended weight loss: 0.5kg/wk max
– Not greater than 1kg/wk
•
•
•
•
 intake by ± 600 kcals
Slow reduction: decreased loss lean tissue
Rapid loss: Large loss fluids
Long term: 75% fat loss, 25% lean loss
Weight Control
• Case Study
Underweight
• <5% US population
• Causes:
– Hunger, illness, appetite, psychological traits,
hereditary…
• If malnourished weight gain nec
– 1lb/month
– 750-800kCals
– Establish new eating habits
Weight Gain Strategies
• Energy dense foods
– Whole milk not skim
•
•
•
•
•
Regular meals
Large portions
Extra snacks
High energy beverages
Exercise: add lean muscle
Anorexia Nervosa: definition
•
•
•
•
•
•
Starvation intentional
Fear of fatness
Significant weight loss
altered body image
Bizarre food behavior
Amenorrhea, delayed menses
Diagnosis
• Refusal maintain body weight
• Intense fear weight gain/fatness
• Undue influence body weight/shape self
esteem
• Amenorrhea or delayed menses
– 3 consecutive periods
• 2 types
• Restricting type
– No regular binge eating/purging
• Binge eating/purging
– Regular binge eating/purging
Demographics
•
•
•
•
•
•
Inc dramatically
2-5% adolescent & young women
5% mortality rate
Upper SES
Family history
Stress/perfectionist
Physical consequences
• PEM: similar to Marasmus
– LBM BMR
•
•
•
•
Decreased growth
Irregular heart beat, blood pressure
Electrolyte imbalance
brain tissue, impaired immune system,
anemia, digestive functioning
• Development body hair
Treatment
• Multidisciplinary approach
– Involve family
• Restore nutritional status
– Add 200kcal/week
– Supplements initially
• Psychological treatment
– Reward eating not weight
• Exercise education
Bulimia Nervosa: diagnosis
• Recurrent episodes binge eating
– Large amount food short time
– Sense of lack control
• Inappropriate compensation
• 2/week for 3 months
• Self esteem unduly influenced body shape
Demographics
• More prevalent AN
• More common in men than AN
• Single white female
– Well educated
– Close to ideal body weight
• Family history
– Obesity, depression, abuse, conflicts, high
expectations.
Physical consequences
immune system
Fluid & mineral imbalance
Abnormal heart rhythms, kidney damage
UTI
Damage pharynx, esophagus, salivary
glands, teeth
• Calloused hands
•
•
•
•
•
Treatment
•
•
•
•
•
•
Avoid finger foods
Prolong eating time, add bulk
Satisfy hunger
Plan meals, snacks
Depression, additive behavior therapy
Goal: Weight maintenance
– Avoid cyclic weight,
Binge Eating Disorder: BED
• Binging without purging
• Consume less during binge than BN, less
restraint following
• Associated with
– Self loathing, depression, anxiety
• Treatment: psychological counseling
Female Athlete Triad
Eating disorder
•Restrictive dieting,
•over exercising,
•lack body fat
Amenorrhea
•hormones
Osteoporosis
•Bone
mineralization
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