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Spotlight on
Metabolism and
Energy Balance
Energy: Fuel for Work
• Energy source
– Chemical energy in carbohydrates, fat, and
protein
• Transferring food energy to cellular energy
– Stage 1: digestion, absorption, and
transport
– Stage 2: breakdown of molecules
– Stage 3: transfer of energy to a form cells
can use
What Is Metabolism?
• Catabolism
– Reactions that break down compounds into
small units
• Anabolism
– Reactions that build complex molecules
from smaller ones
What Is Metabolism?
• Cell is the metabolic processing center
– Nucleus
– Cytoplasm: cytosol + organelles
• Mitochondria are the source of many energy
pathways
What Is Metabolism?
• Who are the key energy
players?
– ATP is the body’s energy
currency
• ATP = adenosine
triphosphate
• Form of energy cell use
– NAD and FAD: transport
shuttles
• Accept high-energy
electrons for use in ATP
production
Breakdown and Release of Energy
• Pathways initiate breakdown of
macronutrients
– Anaerobic
• Breakdown glucose
• Do not require oxygen
– Aerobic
• Breakdown glucose, fat, and protein
• Require oxygen
Breakdown and Release of Energy
• Extracting energy from
carbohydrate
– Glycolysis
• Pathway splits
glucose into two
pyruvates
• Transfers electrons
to NAD
• Produces some ATP
Breakdown and Release of Energy
• Extracting energy from carbohydrate
– Pyruvate to acetyl CoA
• Releases CO2
• Transfers electrons to NAD
Breakdown and Release of Energy
• Extracting energy from carbohydrate
– Citric acid cycle
• Releases CO2
• Produces GTP (like ATP)
• Transfers electrons to NAD and FAD
Breakdown and Release of Energy
• Extracting energy from carbohydrate
– Electron transport chain
• Accepts electrons from NAD and FAD
• Produces large amounts of ATP
• Produces water
– End products of glucose catabolism
• ATP, H2O, and CO2
Breakdown and Release of Energy
• Extracting energy from fat
– Split triglycerides into glycerol and fatty acids
– Takes place in mitochondria
– Beta-oxidation
• Breaks apart fatty acids into acetyl CoA
• Transfers electrons to NAD and FAD
Breakdown and Release of Energy
• Extracting energy from fat
– Completing fatty acid breakdown
• Acetyl CoA from beta-oxidation enters cycle
• Citric acid cycle and electron transport chain
– Fat burns in a flame of carbohydrate
• End products of fat breakdown
• ATP, H2O, and CO2
Breakdown and Release of Energy
• Extracting energy from protein
– Split protein into amino acids
– Split off amino group
• Converted to urea for excretion
– Carbon skeleton enters breakdown pathways
– End products
• ATP, H2O, CO2, urea
Biosynthesis and Storage
• Making carbohydrate (glucose)
– Gluconeogenesis
• Uses pyruvate, lactate, glycerol, and certain
amino acids
• Storing carbohydrate (glucose  glycogen)
– Liver and muscle make glycogen from glucose
• Making fat (fatty acids)
– Lipogenesis
• Uses acetyl CoA from fat, amino acids, and
glucose
• Storing fat (triglyceride)
– Stored in adipose tissue
Biosynthesis and Storage
• Making ketone bodies (ketogenesis)
– Made from acetyl CoA
• When inadequate glucose in cells
• Making protein (amino acids)
– Amino acid pool supplied from
• Diet, protein breakdown, and cell
synthesis
– Biosynthesis
• Different pathways used to build amino
acids from carbon skeletons
Special States
• Feasting
– Excess energy intake from carbohydrate, fat,
protein
• Promotes storage
–Fat  adipose tissue
–Amino acids  protein synthesis
–Carbohydrate  adipose tissue
Special States
• Fasting
– Inadequate
energy intake
• Promotes
breakdown
– Prolonged fasting
• Protects body
protein as
long as
possible
Special States
• Fasting
– Survival priorities and potential energy sources
• Preserve glucose-dependent tissue
–RBC, brain cells, central nervous system
• Maintain muscle mass
Special States
• Fasting
– The prolonged fast: In the beginning
• Blood glucose drops, liver breaks down
glycogen to glucose
• Gluconeogenesis
• Fat and protein are primary fuel
Special States
• Fasting
– The early weeks
• Several energy-conservation strategies
– Several weeks of fasting
• Rely on stored body fat
– The end is near
• Muscle atrophy and emaciation
• Sacrificed muscle tissue in attempt to
preserve brain tissue
Energy Balance
• Energy intake vs. energy
output
• Energy equilibrium
– Intake = output
– Maintain weight
Energy Balance
• Positive energy balance
– Intake > output
– Gain weight
• Negative energy balance
– Intake < output
– Lose weight
Energy In
• Regulation of intake
– Internal cues
• Hunger
–Prompts eating
• Satiation
–Signals to stop eating
• Satiety
–Tells when you are ready to eat again
Energy In
• Regulation of intake
– External cues
• Appetite
–Psychological desire to eat
–Influenced by the eating environment
Energy In
• Control by committee
– What stimulates our cues?
• Internal, physiological response
• Eating environment
Energy In
• Internal factors
– Gastrointestinal sensations
• Sense of fullness
– Neurological and hormonal factors
• Neuropeptide Y
• Ghrelin
• Leptin
Energy In
• External factors
– Diet composition
• Energy density, balance of energy sources,
and form
• Macronutrients
– Sensory properties
• Taste
Energy In
• External factors
– Portion size
• Super-size culture
– Environment and social factors
• Hypothalamus
– Emotional factors
• Hypothalamus
Energy Out: Fuel Uses
• Total energy expenditure
• Major components of energy expenditure
– Energy expenditure at rest (basal energy
expenditure)
• Energy for basic body functions
• Affected by body size, composition, age,
and gender
Energy Out: Fuel Uses
• Major components of energy expenditure
– Physical activity
• Highly variable
• Affected by body size, fitness level, type
of activity
– Thermic effect of food (TEF)
• Energy to digest, absorb, metabolize
food
Energy Out: Fuel Uses
• Estimating total energy expenditure
– Resting energy expenditure (REE)
• 1.0 kcal/kg/hr for males
• 0.9 kcal/kg/hr for females
– Physical activity
• Add a % of REE (see Table 8.2)
– Thermic effect of food
• 6% to 10% of (REE + physical activity)
Estimating Energy Expenditure
• Estimated Energy Requirement (EER)
– Equations for males and females
• Factors for age, weight, height, and physical
activity
– Predicts total energy expenditure (TEE)
Body Composition
• Body composition
– Fat and lean muscle mass
• Assessing body weight
– Body mass index (BMI)
• Weight (kg) × height2 (m)
• BMI ≤ 18.5 kg/m2 = underweight
• BMI 18.5 to ≥ 25 kg/m2 = normal weight
• BMI 25 to ≤ 30kg/m2 = overweight
• BMI ≥ 30 kg/m2 = obese
Body Composition
• Assessing body fatness
– DXA
– Underwater weighing
– BodPod
– Skinfold
measurements
– Bioelectrical
impedance
Body Composition
• Body fat distribution
– Gynoid obesity (“pear”)
• Excess fat in hips and
thighs
– Android obesity
(“apple”)
• Excess fat around
abdomen
– Waist circumference