Chapter 24 – Nutrition, Metabolism and Thermoregulation

G.R. Pitts, J.R. Schiller, and James F. Thompson, Ph.D.

Use the video clip, CH 24 Nutrition for a review of general nutrition

Nutrition

We eat, we digest, we absorb, then what?

3 fates for food = nutrients

1) Most are used to supply energy for life2) Some are used to synthesize structural or

functional molecules3) The rest are stored for future use – love

handles!

Nutrition for College Students

Four Groups:

Grease

Salt

Sugar

Alcohol

Weight ManagementIf 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

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 nucleiwhen stimulated, they cause

cessation of eating, even if one has been starved for days

Controls for their set points are unknown!

Regulation of Food Intake (cont.)

Feeding center is always active; but the satiety center can inhibit it• May be driven by changes in blood

compositionglucostatic theory – blood glucose levels varylipostatic theory - blood lipid levels vary; fat

released from adipose tissue between meals

• Other influencesblood amino acid levelstemperature - high temp decreases appetiteGI tract distension (a slow and variable reflex)social and psychological factorshormones (CCK), neurotransmitters, ions (Zn+)

MetabolismAll biochemical reactions in the bodyBalance between synthesis (anabolic) and breakdown (catabolic) reactions

• Anabolism chemical reactions that combine simple, smaller molecules into more complex molecules uses energyprotein formation from amino acidscarbohydrate formation from simple sugarsetc.

• Catabolismchemical reactions that break down complex organic molecules into simpler onesreleases energyproteins are broken down by various proteasesetc.

Relationship of Catabolism to Anabolism

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

Adenosine Tri-Phosphate

3 Phosphates

ATP MetabolismAllows for transfer of small but useful amounts

of energy from one molecule to anotherCell's entire amount of ATP is recycled

approximately every minuteATP 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

ATP Metabolism (cont.) energy is released by

breaking the third phosphate group’s bond

• ATP ADP + Pi

a reversible reactionthe energy released is enough

to drive anabolic reactions

• ATP ADP + CrP creatine provides energy

storage in skeletal muscleallows for more ATP to be

formed when O2 is less readily available during skeletal muscle contraction

Energy ProductionEnergy is stored in chemical bonds Oxidation-Reduction (Redox) reactions:

• Oxidation component:also known as dehydrogenation reactionsremove electrons from molecules

o decreases the energy remaining in the oxidized moleculeo generally, 2e- (and 2H+) are removed simultaneously

Can also be the gain of oxygen

Energy Production (cont.)Oxidation-Reduction

reactions (cont.)• Reduction

component:addition of electrons to

a moleculeincreases the energy of

the reduced molecule

These 2 component reactions are always coupled: oxidation-reduction reactions

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

Energy Production (cont.) ATP Generation

• Adding a phosphate ion to a molecule is phosphorylation

• 3 mechanisms:(2) oxidative

phosphorylation o electrons (H+)

removed from molecules

o enzymes combine H+ with O2 releasing enough energy for ATP formation

(3) photo-phosphorylation - photosynthesis

Carbohydrate MetabolismGeneral

• 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)

Carbohydrate Metabolism (cont.)

Glucose anabolism• Glucose storage:

glycogenesisglycogen formation is

stimulated by insulinglucose 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

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 hormoneo cortisol (glucocorticoids) mobilizes proteins, making AA's

availableo 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.”

Glucose Metabolism

Glucose Catabolism• glucose oxidation is known as cellular

respirationcomplete catabolism of each molecule of

glucose to CO2, H2O maximum yield of 36 ATP molecules/glucose

o 38% of the energy present in a glucoseo excellent efficiency for a biological systemo the rest of the energy is “waste heat”

2 linked enzymatic pathways are involved in glucose catabolism

o glycolysiso Kreb’s cycle

Glucose Metabolism (cont.)Glycolysis - OverviewOccurs in cytosol1 glucose 2

pyruvates (pyruvic acid)

net gain 2 ATP’s• 2 ATP’s used• 4 ATP’s made

net gain 2 NADH + 2H+ (aerobic conditions)

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 cellpyruvate converted to acetyl coenzyme A (acetyl CoA)this reaction couples glycolysis to the Krebs cycle

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)

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 NADH2‘s and

FADH2‘s to make additional ATPs

Glucose Metabolism (cont.)

Glycolysis and Krebs Cycle combined total:

6 CO2 (waste) + 6 H2O10 NADH2 + 2 FADH2 +4 ATP (energy harvest)

Electron Transport

Animal Physiology, Hill et al., 2004

Electrons Source: NADH2/FADH2 from glycolysis and Krebs cycle

High‑energy electrons enter the system, and low‑energy electrons leave

Electron Transport System

• Oxidative phosphorylation O2 is the final electron

acceptor for low‑energy electrons from last of the carrier molecules

NADH2 3 ATPFADH2 2 ATP

• Enzyme cytochrome oxidase splits apart O2 moleculesCombines each O atom

with 2 H+ ions to makes H2O = water

Animal Physiology, Hill et al., 2004

Glucose MetabolismOverview

C6H12O6 + 6 O2

6 CO2 (waste) + 12 H2O + 36 ATP (useful energy)

LipidsBeta 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.

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.

Nutrient Catabolism Pathways Are All Interconnected

Ketone bodies result from excessive lipolysis and fat catabolism; a symptom of diabetes mellitus

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.

The Absorptive State

InsulinDominates

theAbsorptive

State

All Cells Rely on Glucose from the Meal for Energy

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.

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.

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.

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

Thermogenesis

Heat Loss to the Environmentradiation away of infrared radiationconvection and conduction of heat to air

or water surrounding the bodyevaporation from sweating and from

ventilating respiratory membranesvasodilation of cutaneous capillary bedsdecreased hormonal activity leading to

decreased basal metabolic rate (BMR)behavioral: stop exercising; move to the

shade, take off clothes, turn on a/c, etc.

Heat Production/Conservationincreased hormone activity (thyroxine,

epinephrine) leading to increased (BMR)increased sympathetic ANS activity

leading to increased (BMR)shivering of skeletal musclesvasoconstriction of dermal capillary

bedsbehavioral: start exercising, huddle

together, use clothing and shelter, use fire or other means of heating the surroundings

Thermogenesis (cont.) A complex regulation involving negative feedback

control through endocrine and autonomic pathways:

Pathology of Temperature Control

fever

heat cramps, heat exhaustion, heat stroke

heat-induced dehydration

burns

hypothermia

alcohol-induced hypothermia

End Chapter 24