Chapter 24

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Chapter 24 Nutrition, Metabolism and
Thermoregulation
Use the video clip, CH 24 Nutrition for a review
of general nutrition
G.R. Pitts, J.R. Schiller, and James F.
Thompson, Ph.D.
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Nutrition

• We eat, we digest, we absorb, then what?
• 3 fates for food nutrients
• Most are used to supply energy for life
• Some are used to synthesize structural or
  functional molecules
• The rest are stored for future use love handles!

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Nutrition for College Students
Four Groups Grease Salt Sugar Alcohol
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Weight Management

• If energy consumption (food intake) equals energy
  utilized (activity), then body weight will remain
  constant
• Activity and consumption levels vary day to day,
  but individuals keep relatively constant weight
  for long periods of time
• Many individuals in affluent nations have an
  imbalance between intake and use ? obesity

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Regulation of Food Intake

• Hypothalamus - complex integrating center
  receiving sensory information from all parts of
  the body
• Two hypothalamic centers control eating
• Feeding (hunger) center
• located in lateral hypothalamus
• when stimulated, it initiates feeding, even if
  one is full
• Satiety center
• ventromedial nuclei
• when stimulated, they cause cessation of eating,
  even if one has been starved for days
• Controls for their set points are unknown!

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Regulation of Food Intake (cont.)

• Feeding center is always active but the satiety
  center can inhibit it
• May be driven by changes in blood composition
• glucostatic theory blood glucose levels vary
• lipostatic theory - blood lipid levels vary fat
  released from adipose tissue between meals
• Other influences
• blood amino acid levels
• temperature - high temp decreases appetite
• GI tract distension (a slow and variable reflex)
• social and psychological factors
• hormones (CCK), neurotransmitters, ions (Zn)

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Metabolism

• All biochemical reactions in the body
• Balance between synthesis (anabolic) and
  breakdown (catabolic) reactions
• Anabolism
• chemical reactions that combine simple, smaller
  molecules into more complex molecules
• uses energy
• protein formation from amino acids
• carbohydrate formation from simple sugars
• etc.
• Catabolism
• chemical reactions that break down complex
  organic molecules into simpler ones
• releases energy
• proteins are broken down by various proteases
• etc.

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Relationship of Catabolism to Anabolism
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Adenosine Tri-Phosphate

• ATP is the link between anabolism and catabolism
• ATP energy is the currency used in most
  cellular energy exchanges
• catabolic reactions provide the ATP energy that
  most anabolic reactions require
• only about 10-30 of the energy released by
  catabolic reactions can be used
• most chemical energy is lost as waste heat
• waste heat is not wasted it is essential in
  maintaining a constant body temperature

3 Phosphates
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ATP Metabolism

• Allows for transfer of small but useful amounts
  of energy from one molecule to another
• Cell's entire amount of ATP is recycled
  approximately every minute
• ATP is NOT for long term energy storage
• too reactive in the cell
• other molecules available for energy storage
  (neutral fats, glycogen, creatine phosphate,
  etc.)
• About 8kg (17 lb) of ATP is produced every hour
  in an average male
• Total amount of ATP present in the body at any
  time is only about 50g

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ATP Metabolism (cont.)

• energy is released by breaking the third
  phosphate groups bond
• ATP ? ADP Pi
• a reversible reaction
• the energy released is enough to drive anabolic
  reactions
• ATP ? ADP CrP
• creatine provides energy storage in skeletal
  muscle
• allows for more ATP to be formed when O2 is less
  readily available during skeletal muscle
  contraction

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Energy Production

• Energy is stored in chemical bonds
• Oxidation-Reduction (Redox) reactions
• Oxidation component
• also known as dehydrogenation reactions
• remove electrons from molecules
• decreases the energy remaining in the oxidized
  molecule
• generally, 2e- (and 2H) are removed
  simultaneously
• Can also be the gain of oxygen

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Energy Production (cont.)

• Oxidation-Reduction reactions (cont.)
• Reduction component
• addition of electrons to a molecule
• increases the energy of the reduced molecule
• These 2 component reactions are always coupled
  oxidation-reduction reactions

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Energy Production (cont.)

• ATP Generation
• Addition of phosphate to a chemical compound is
  phosphorylation
• 3 mechanisms for this
• (1) substrate-level phosphorylation a
  high-energy phosphate group is transferred
  directly from a molecule to ADP to make ATP
• For example, when the energy stored on a
  high-energy phosphate group on creatine phosphate
  is transferred to ADP to make ATP in skeletal
  muscles

CK transfers its high energy phosphate to ADP
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Energy Production (cont.)

• ATP Generation
• Adding a phosphate ion to a molecule is
  phosphorylation
• 3 mechanisms
• (2) oxidative phosphorylation
• electrons (H) removed from molecules
• enzymes combine H with O2 releasing enough
  energy for ATP formation
• (3) photo-phosphorylation - photosynthesis

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Carbohydrate Metabolism

• General
• 80 of carbohydrates ingested contain glucose
  remainder fructose, galactose
• glucose is the body's preferred carbohydrate
  energy source
• Fate of carbohydrates -- depends on needs of body
  cells
• ATP production
• Amino acid synthesis
• Glycogenesis
• Lipogenesis
• Excretion in urine (minimal)

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Carbohydrate Metabolism (cont.)

• Glucose movement into cells
• facilitated diffusion - insulin controls (?) rate

• glucose is immediately phosphorylated upon entry
  (glucose-6-phosphate)
• maintains the diffusion (concentration) gradient
• traps glucose inside the cell

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Carbohydrate Metabolism (cont.)

• Glucose anabolism
• Glucose storage glycogenesis
• glycogen formation is stimulated by insulin
• glucose not needed immediately is stored in the
  liver (25) and in skeletal muscle (75)
• Glucose release glycogenolysis
• converts glycogen to glucose
• occurs between meals, stimulated by glucagon and
  epinephrine

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Carbohydrate Metabolism (cont.)

• Glucose anabolism (cont.)
• Formation of glucose from proteins, fats
  gluconeogenesis
• when blood glucose level is low, you eat if
  glucose remains low, body catabolizes some
  proteins and fats
• stimulated by cortisol and thyroid hormone
• cortisol (glucocorticoids) mobilizes proteins,
  making AA's available
• thyroid hormone mobilizes proteins (AA's) and may
  mobilize lipids
• epinephrine, glucagon, hGH also stimulate
• These five hormones are often referred to as the
  insulin antagonists.

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Glucose Metabolism

• Glucose Catabolism
• glucose oxidation is known as cellular
  respiration
• complete catabolism of each molecule of glucose
  to CO2, H2O
• maximum yield of 36 ATP molecules/glucose
• 38 of the energy present in a glucose
• excellent efficiency for a biological system
• the rest of the energy is waste heat
• 2 linked enzymatic pathways are involved in
  glucose catabolism
• glycolysis
• Krebs cycle

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Glucose Metabolism (cont.)

• Glycolysis - Overview
• Occurs in cytosol
• 1 glucose ?2 pyruvates (pyruvic acid)
• net gain 2 ATPs
• 2 ATPs used
• 4 ATPs made
• net gain 2 NADH 2H (aerobic conditions)

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Glucose Metabolism (cont.)

• Fate of pyruvate (pyruvic acid) - depends on
  availability of O2
• without O2 NADH H pyruvate ? lactic acid
• with O2 available to the cell
• pyruvate converted to acetyl coenzyme A (acetyl
  CoA)
• this reaction couples glycolysis to the Krebs
  cycle

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Glucose Metabolism (cont.)

• Pyruvic acid - formation of acetyl coenzyme A
  (Acetyl CoA) CO2
• lose one carbon from pyruvate to form CO2 (waste)
• the remaining two carbons, the acetyl group, join
  with CoA, to generate NADH H (1 from each
  pyruvate 2 NADH 2H total from one
  glucose)

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Glucose Metabolism (cont.)

• Krebs cycle (Citric Acid Cycle or Tricarboxylic
  Acid Cycle (TCA)
• oxidation of acetyl Coenzyme A
• reduction of coenzymes (NAD, FAD)
• Oxidative phosphorylation
• uses NADH2s and FADH2s to make additional ATPs

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Glucose Metabolism (cont.)
Glycolysis and Krebs Cycle combined total 6 CO2
(waste) 6 H2O 10 NADH2 2 FADH2 4 ATP
(energy harvest)
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Electron Transport

• Electrons Source NADH2/FADH2 from glycolysis and
  Krebs cycle
• High-energy electrons enter the system, and
  low-energy electrons leave

Animal Physiology, Hill et al., 2004
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• Electron Transport System
• Oxidative phosphorylation
• O2 is the final electron acceptor for low-energy
  electrons from last of the carrier molecules
• NADH2 ? 3 ATP
• FADH2 ? 2 ATP
• Enzyme cytochrome oxidase splits apart O2
  molecules
• Combines each O atom with 2 H ions to makes H2O
  water

Animal Physiology, Hill et al., 2004
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Glucose MetabolismOverview

• C6H12O6 6 O2 ?
• 6 CO2 (waste) 12 H2O 36 ATP
  (useful energy)

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Lipids
Beta oxidation breaks down fatty acids to form
acetyl Coenzyme A. Lipids are more reduced (have
fewer oxygens) therefore, they have more
potential chemical energy and can be more fully
oxidized as an energy fuel. Therefore, we gain
more energy, gram for gram, from fats than from
carbohydrates.
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Protein Metabolism
Amino acids may be deaminated and the resulting
carbon skeletons of whatever composition, can
be entered into the glycolytic or Krebs cycle
pathways to yield an energy harvest of ATPS. The
amino groups will be joined with CO2 molecules to
form the nitrogenous waste urea.
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Nutrient Catabolism Pathways Are All
Interconnected
Ketone bodies result from excessive lipolysis and
fat catabolism a symptom of diabetes mellitus
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The Daily Metabolic Cycle

• The body shifts back and forth physiologically
  between the absorptive state and the
  postabsorptive state.
• The absorptive state occurs for approximately 4
  hours after each regular meal.
• The postabsorptive state takes over until the
  next meal can be absorbed.

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The Absorptive State
Insulin Dominates the Absorptive State All Cells
Rely on Glucose from the Meal for Energy
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The Post-Absorptive State
Glucagon dominates the post-absorptive state and
is assisted by the insulin antagonists
(glucocorticoids, thyroid hormones, epinephrine,
and hGH).
Only Brain and Spinal Cord Cells Rely on Glucose
for Energy Most Other Body Cells Rely on Fatty
Acids for Energy.
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The Post-Absorptive State
Glycogenolysis provides glucose fuel for skeletal
muscle. Glycogenolysis and gluconeogenesis in
the liver provide plasma glucose for nervous
tissue. Lipolysis supplies lipids to fuel all
other cells.
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Lipid Transport by Lipoproteins
Lipoproteins transport hydrophobic lipids in a
droplet which is emulsified by an external layer
of phospholipids and proteins which make the
surface water soluble.
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Lipid Transport by Lipoproteins

• Chylomicrons carry absorbed fat from the meal to
  adipose tissue via lymph
• VLDL and LDL carry cholesterol synthesized in the
  liver and fat stored in the liver to body tissues
• HDL carries excess cholesterol back to the liver
  for catabolism and excretion
• Increased total cholesterol and increased LDL are
  linked to vessel and heart disease

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Thermogenesis
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Heat Loss to the Environment

• radiation away of infrared radiation
• convection and conduction of heat to air or water
  surrounding the body
• evaporation from sweating and from ventilating
  respiratory membranes
• vasodilation of cutaneous capillary beds
• decreased hormonal activity leading to decreased
  basal metabolic rate (BMR)
• behavioral stop exercising move to the shade,
  take off clothes, turn on a/c, etc.

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Heat Production/Conservation

• increased hormone activity (thyroxine,
  epinephrine) leading to increased (BMR)
• increased sympathetic ANS activity leading to
  increased (BMR)
• shivering of skeletal muscles
• vasoconstriction of dermal capillary beds
• behavioral start exercising, huddle together,
  use clothing and shelter, use fire or other means
  of heating the surroundings

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Thermogenesis (cont.)

• A complex regulation involving negative feedback
  control through endocrine and autonomic pathways

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Pathology of Temperature Control

• fever
• heat cramps, heat exhaustion, heat stroke
• heat-induced dehydration
• burns
• hypothermia
• alcohol-induced hypothermia

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End Chapter 24