Chapter 25
Metabolism and
Nutrition
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Nutrition
Major nutrients
 Carbohydrates, lipids, and proteins
Other nutrients
 Vitamins and minerals( & water)
A diet consisting of the five food groups: grains,
fruits, vegetables, meat & fish, milk products
provides adequate amount of all nutrients
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Metabolism and Nutrition
A nutrient is a “food or liquid that supplies the body’s
metabolic needs. Nutrients include:
 A necessary chemical that help regulate body
processes.(such as Na+ and other minerals)
 A substance that provides energy (such as lipids or
carbohydrates like glucose)
 Something that helps in growth and normal
metabolism (such as vitamins)
 A substance that repairs or maintains body functions
(such as proteins, or amino acids to make proteins)
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Metabolism and Nutrition
Metabolic reactions contribute to homeostasis by
harvesting chemical energy from consumed nutrients to
contribute to the body’s growth, repair, and normal
functioning
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Metabolism
Metabolism denotes the sum of all body chemical
reactions
 Catabolism is breaking down larger molecules into smaller
molecules. Catabolic reactions provide more energy than
they consume; they are exergonic – they liberate energy
 Anabolism is building larger molecules from smaller
molecules. Anabolic reactions consume more energy than
they produce; they are endergonic – they consume energy
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Metabolism
Metabolism is an energy-balancing act between
catabolic reactions and anabolic reactions
 The molecule that participates most often in energy
exchanges in living cells is ATP (adenosine triphosphate),
which couples energy-releasing catabolic reactions to
energy-requiring anabolic reactions
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ATP
ATP temporarily stores and transfers energy given off in catabolic
reactions and transfers it to anabolic reactions that require energy.
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Oxidation- Reduction
(Redox)Reactions
Chemical reactions in which electrons are exchanged as
a means of transferring energy are called REDOX
reactions
 Oxidation is the loss of electrons,
Remember:
OIL, RIG
or hydrogen atoms
 Reduction is the gain of electrons,
or hydrogen atoms
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REDOX Reactions
Since oxidation-reduction reactions always occur
together, the oxidation of glucose results in reduction of
the coenzymes NAD + and FAD+ as the electrons are
transferred to them
 Reduction, then, results in an
increase in potential energy;
energy taken from the
oxidized substrate (glucose in
our example)
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Carbohydrate Metabolism
Dietary sources
 Starch (polysaccharide) in grains and vegetables
 Sugars in fruits, sugarcane, sugar beets, honey and milk
Uses
 Glucose is the fuel used by cells to make ATP
(neurons and RBCs rely almost entirely on glucose)
 Excess glucose is stored as glycogen or fat
Dietary requirements
 130 g/day- 45–60% of total calorie intake; mostly
complex carbohydrates (whole grain & vegetables
recommended)
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Carbohydrate Metabolism
Glucose is the body’s preferred source of fuel
 During digestion, polysaccharides and disaccharides
are hydrolyzed into the monosaccharides glucose
(80%), fructose, and galactose
 These three monosaccharides are absorbed into the
villi of the small intestine and carried to the liver
◦ hepatocytes convert galactose and fructose to glucose
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Carbohydrate Metabolism
The oxidation of glucose to form ATP...
Glucose (C6H12O6) + 6O2
6CO2 + 6H2O + ATP
... is known as “Cellular Respiration” and occurs in 4 steps:
 Glycolysis
 Formation of Acetyl CoA
 Krebs cycle
 Electron transport chain and oxidative phosphorylation
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Cellular Respiration- overview
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Cellular respiration (glucose
oxidation- overview)
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Cellular Respiration
Glycolysis
Anaerobic (does not require oxygen)
Occurs in the cytoplasm
Oxidation of one 6-carbon molecule of glucose into
two 3-carbon molecules of pyruvic acid
Glucose  2 pyruvic acid molecules + 2 ATP
2NADH formed
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Glycolysis
Fate of pyruvic acid depends on availability of oxygen
◦ If O2 is not available pyruvic acid - converted to
lactic acid
◦ If O2 is available- pyruvic acid enters
mitochondria
Acetyl-CoA will be formed from pyruvic acid and
aerobic cellular respiration continues
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Formation of Acetyl CoA
This stage is the “link” between glycolysis and Krebs citric acid
cycle- occurs in mitochondrial matrix
3carbon pyruvate converted to 2 carbon acetyle group by
removal of one CO2
 acetyle group and coenzyme A (CoA) combined to form acetyl
CoA
 For every glucose molecule (from two pyruvate molecules) the
final products are:
 2 acetyl CoA, 2 NADH, 2 CO2
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Cellular Respiration
Krebs cycle:
Completes breakdown of glucose in a mitochondrion
acetyl-CoA enters the the Krebs cycle
A series of oxidation reduction reactions
Products of 2 turns of Krebs cycle:
4 CO2
2ATP
6NADH
2FADH2
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Cellular Respiration
The electron transport system and oxidative
phosphorylation
Involves the transfer of electrons from NADH
and FADH2 to the electron transport system
energy released used to make ATP
oxygen is the the final electron
acceptor
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Cellular Respiration
In the total oxidation of 1 molecule of glucose, 36-38
molecules of ATPs are generated, depending on the tissue
 Only 4 ATP are generated by substrate level
phosphorylation (directly transferring a phosphate
from one molecule to another) in glycolysis and the
Krebs cycle
 Most of the ATP (either 32 or 34) is made by oxidative
phosphorylation using the electron transport chain
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Glucose Storage - Glycogenesis
If glucose is not needed immediately for ATP
production, glucose is stored as glycogen, a
polysaccharide that is the only stored form of
carbohydrate in our bodies
Insulin hormone stimulates glycogenesis
 75% glycogen stored in skeletal muscle,
rest in liver cells
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Glucose Release- Glycogenolysis
Glycogenolysis When cells require ATP, stored
glycogen is broken down into glucose and released into
the blood to be transported to cells
Only liver can release glucose from breakdown of
stored glycogen into blood ( skeletal muscles lack the
enzyme phosphatase)
Glycogenolysis stimulated by glucagon, epinephrine
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Glycogenesis & Glycogenolysis
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Making Glucose- Gluconeogenesis
Gluconeogenesis is the process of orming “new” glucose
from non-carbohydrate sources in the liver
Gluconeogenesis occurs on a large scale during fasting,
or eating a low carbohydrate diet
Lactic acid, amino acids, and the glycerol (from
triglycerides) are used to form glucose molecules or
pyruvic acid
Gluconeogenesis is stumulated by cortisol and glucagon
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Gluconeogenesis
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Lipids
Dietary sources
 Triglycerides- most abundant dietary lipids
Saturated fats- FA’s have single carbon-carbon bonds-- in
meat, dairy foods,
Unsaturated fats- FA’s have some double -- in seeds, nuts,
olive oil and vegetable oils
 Cholesterol in egg yolk, meats, organ meats, shellfish, milk
products- liver produces major amount
 Fatty acids linoleic acid & linolenic acid in vegetable oils
(essential fatty acids) must be ingested
Dietary requirements:
 Should be 30% or less of total caloric intake
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Lipoproteins & lipid transport
Most lipids are transported in the blood in combination
with proteins as lipoproteins
In general lipoproteins have:
 An outer shell that is made hydrophilic due to polar
proteins (plus amphipathic
phospholipids)
 An inner core that is
hydrophobic - a place
where mostly triglycerides
are transported
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Classes of Lipoproteins
Chylomicrons (2 % protein, 85% TGs, also cholesterol,
phospholipids)
 form in intestinal epithelial cells
 Transport dietary fat
 Enter lacteals, carried by lymphatic system to blood
◦ Lipases of blood vessel endothelial cells release fatty
acids from the chylomicron
◦ Taken up by adipose & muscle cells
◦ liver processes the remnants of chylomicrons
VLDLs (10% protein, 50% TGs, phospholipids &
cholesterol )
 VLDLs form in the liver
 Transport triglycerides from liver to fat cells
 converted to LDLs
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Classes of Lipoproteins
LDLs (25% protein, 50% cholesterol) --- “bad cholesterol”
 carry 75% of total cholesterol in blood – transport it to
to body cells
 LDL enters cells via LDL receptors
HDLs (40%, protein, 20% cholesterol) --- “good
cholesterol”
 carry cholesterol from cells to liver for elimination in
bile
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Cholesterol
There are two sources of cholesterol in the body: food we
eat and synthesis in the liver
For adults, normal blood cholesterol are:
 Total cholesterol- under 200 mg/dl
 LDL under 130 mg/dl
 HDL over 40 mg/dl.
 Normally, triglycerides are in the range of 10-190
mg/dl.
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Triglyceride Storage
Adipose tissue removes fatty acids from chylomicrons and
VLDLs and stores them as triglycerides in:

50% subcutaneous tissue, around kidneys, in omenta,
genital area, between muscles and other areas

They are catabolized and mobilized constantly
throughout the body.
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Lipid Metabolism
The conversion of glucose or amino acids into
triglycerides is called lipogenesis.
Occurs when we consume more calories than are needed
to satisfy ATP needs
Insulin stimulates lipogenesis
amino acids ---acetyl coA---fatty acids---TGs
glucose---acetyl coA---fatty acids---TGs
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Lipid Metabolism
Lipolysis refers to breakdown of TGs to glycerol and
fatty acids
 Glycerol can be converted into glucose, or pyruvic
acid
 In beta oxidation of fatty acids, carbon atoms are
removed in pairs from fatty acid chains-- acetyl
coenzyme A formed--enters the Krebs cycle.
Lipolysis can occur under the action of epinephrine,
norepinephrine, thyroid hormones and cortisol
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Lipid Metabolism
Ketogenesis : occurs as a part of normal fatty acid
catabolism
From acetyle coA the liver forms acetoacetic acid which
can then be converted to beta-hydroxybutyric acid and
acetone ( ketone bodies)
Ketogenesis is a normal part of fat breakdown, but an
excess will cause a metabolic acidosis
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Pathways of lipid metabolism
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Proteins
Dietary sources
 Animal proteins contain greatest amounts of
essential amino acids- eggs, milk, fish, meats
 Beans, nuts, and cereals lack some essential amino
acids
 Daily intake- 0.8 g per kg body weight
Uses
Structural proteins: keratin, collagen, elastin, muscle
proteins
 Functional proteins: enzymes, some hormones
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Proteins
During digestion, proteins are hydrolyzed into amino acids
Amino acids are absorbed by the blood capillaries of villi and
enter the liver via the hepatic portal vein
Amino acids, under the influence of growth hormone and
insulin, enter body cells by active transport.
 Inside cells, amino acids are synthesized into proteins
 Amino acids may also be stored as fat or glycogen( after
conversion to glucose) or used for energy
Essential amino acids are the 10 amino acids that can’t be
synthesized by the body ( have to be taken in diet)
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Protein Catabolism
Liver cells convert amino acids into substances that can
enter the Krebs cycle to produce ATP
 First deamination removes the amino group (NH2)
◦ converts amino groups to ammonia (NH3) & then
urea
◦ urea is excreted in the urine
In liver cells amino acids can also be converted into:
 Glucose ( gluconeogenesis)
 fatty acids ( lipogenesis)
 ketone bodies ( ketogenesis)
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Protein Anabolism
 Involves the formation of peptide bonds between amino
acids to produce new proteins.
◦ Structural proteins: keratin, collagen, elastin, muscle
proteins
◦ Functional proteins: enzymes, hemoglobin, antibodies,
some hormones
 Stimulated by human growth hormone, testosterone,
thyroxine, and insulin.
 Protein synthesis carried out on the ribosomes of almost
every cell in the body, directed by the cells DNA and RNA.
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Metabolic Adaptations
During the absorptive state ingested nutrients entering
the blood stream and glucose is readily available fot
ATP production
During the postabsorptive state absorption of nutrients
from GI tract has been completed and energy needs
must be met by fuels in the body
An average meal requires about 4 hours for complete
absorption, and given three meals a day, the body
spends about 12 hours of each day in the absorptive
state.
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The Absorptive State
Soon after a meal glucose, amino acids, and triglycerides
(as chylomicrons) enter the blood
There are 2 metabolic hallmarks of this state:
 Glucose is oxidized to produce ATP in all body cells
 Any excess fuel molecules are stored in hepatocytes,
adipocytes, and skeletal muscle cells
Pancreatic beta cells begin to release insulin
Storage: nutreints stored as glycogen or triglycerides
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Regulation of Metabolism During the Absorptive State
Rise in blood glucose concentration stimulate insulin
release from pancreatic beta cells.
Insulin’s functions
promotes entry of glucose into skeletal muscle & adipose
tissue
stimulates storage of glucose as glycogen in liver & muscle
enhances synthesis of triglycerides in adipose tissue &
liver
Promotes entry of amino acids into cells & protein
synthesis
 stimulates protein synthesis along with thyroid &
growth hormone
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The Postabsorptive State
About 4 hours after the last meal absorption in the small
intestine is nearly complete and blood glucose levels start to
fall. The main metabolic challenge at this point is to maintain
normal blood glucose levels (70 to 110 mg/100 ml)
 As blood glucose levels decline, glucagon secretion
increases
◦ Blood glucose levels are sustained by:
◦ the breakdown of liver glycogen & gluconeogenesis
using lactic acid and/or amino acids
◦ lipolysis
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The Postabsorptive State
 Cells use alternative fuel sources (conserves
glucose for brain & RBCs)
◦ fatty acids from fat tissue fed into Krebs as acetyl
CoA
◦ lactic acid produced anaerobically during exercise
◦ ketone bodies by heart & kidney
◦ Homeostasis of blood glucose concentration is
especially important for the nervous system and red
blood cells.
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Regulation of Metabolism During the
Postabsorptive State
The most important hormone is glucagon.
 stimulates gluconeogenesis & glycogenolysis within
the liver
Hypothalamus detects low blood sugar- increases
sympathetic output:
 release of norepinephrine & epinephrine
◦ stimulates glycogen breakdown & lipolysis
◦ raises glucose & free fatty acid blood levels
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Basal Metabolic Rate
The metabolic rate is the overall rate at which metabolic
reactions use energy.
Basal metabolic rate (BMR) is measured with the body in
a quiet, fasting condition
 The BMR is 1200–1800 Cal/day in adults, or about 24
Cal/kg of body mass in adult males and 22 Cal/kg in
adult females
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Body Temperature
Body temperature reflects the balance between heat
production and heat loss
homeostatic mechanisms maintain a normal range for
internal (core) body temperature at 37°C (98.6°F)
 Peripheral tissues can be much cooler (“shell temperature
1-6°C lower)
◦ Body temperature is maintained by hormonal regulation
of the BMR, and sympathetic nervous system
stimulation
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Metabolism & Heat Production
Factors that affect metabolic rate and thus the production
of body heat
 exercise increases metabolic rate as much as 15 times
 thyroid hormones increase BMR
 sympathetic nervous system’s release of epinephrine &
norepinephrine increases metabolic rate
 higher body temperature increases metabolic rate
 ingestion of food raises increases metabolic rate by 10-
20%
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The Hypothalamic Thermostat
The body’s thermostat is a group of neurons in the
hypothalamus that receives impulses from
thermoreceptors scattered throughout the body
Nerve impulses from the thermostat propagate to
the heat-losing center and the heat-promoting
centers of the hypothalamus
Several negative feedback loops work to raise body
temperature when it drops too low or to lower body
temperature
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Thermoregulation
If the core temperature declines,
skin blood vessels constrict and
thyroid hormones and
catecholamines (epinephrine and
norepinephrine) are released.
Cellular metabolism increases and
shivering my ensue
If core body temperature rises,
blood vessels of the skin dilate,
sweat glands are stimulated, and
the metabolic rate is lowered
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Regulation of Food Intake
Nuclei in the hypothalamus regulate hunger & satiety
The brain receives signals from:
 Neural signals from the digestive tract ( via vagus nerve)
 Rising blood levels of glucose, amino acids, fatty acids
indicate fullness
 Hormones
◦ Insulin & leptin (released from fat cells) suppress
hunger
◦ glucagon stimulates hunger
 To a lesser extent, body temperature (increase inhibits
eating) and psychological factors
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