Download Digestion and absorption of nutrients

GASTROINTESTINAL TRACT
PHYSIOLOGY
(PHG 222)
by
ADEJARE, A. A.
Department of Physiology
Faculty of Basic Medical Sciences
College of Medicine
University of Lagos
OUTLINE
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General organization/functional anatomy of the GIT
Review of smooth muscle function
GIT motility
GIT secretions and hormones
Digestion and absorption of food substances
Nutrition and metabolism
GIT disorders
Digestion
 Process whereby the body breaks down food into
absorbable nutrients.
 To digest food, five different body organs secrete digestive
juices: the salivary glands, the stomach, the small intestine,
the liver (via the gallbladder), and the pancreas.
 These secretions enter the GI tract at various points along
the way, bringing an abundance of water and a variety of
enzymes.
Mouth
 Teeth grind food to reduce the size.
 Saliva released to help moisten food.
 Digestion of carbohydrate begins in the mouth, where the
salivary glands secrete saliva, which contains water,
salts, and enzymes
 Tongue pushes food to the back of the mouth to start
swallowing reflex.
 Food passes through the esophagus and enters stomach.
 The salivary enzyme amylase begins digestion.
Mouth
• Protein
• Chewing and crushing moisten protein-rich foods and mix
them with saliva to be swallowed
• Fat
The sublingual salivary gland in the base of the tongue
secretes as salivary lipase. Some hard fats majorly neutral
fats and cholesterol begin to melt as they reach body
temperature.
 The enzymes in the mouth do not affect the fats, proteins,
vitamins, minerals, and fiber that are present in the foods
people eat.
Stomach
 Distended pouch.
 SME
 Secretes hydrochloric acid:
 Begins protein digestion.
 Kills microorganisms in food.
 Block salivary amylase activity
 mucus that coats and protects the stomach’s lining.
 Initiates breakdown of proteins. Both pepsin and the stomach acid
involved.
 Protein is converted to proteoses, peptones and polypeptides
 the attachment of a protein carrier to vitamin B12.
 Nutrients not absorbed except water and alcohol.
Stomach
• HCl uncoils protein strands and activates stomach enzymes
Fat
The acid-stable salivary/lingual lipase splits one bond of
triglycerides to produce diglycerides and fatty acids. The
stomach’s churning action mixes fat with water and acid. A
gastric lipase accesses and hydrolyzes a very small amount of
fat.
Small Intestine
 Most digestion and absorption occurs in the small intestine.
 Bile released to emulsify fat.
 Pancreatic enzymes released to digest carbohydrates,
proteins and fats.
 Final digestive enzymes in intestinal lining break down
carbohydrates, proteins and fats into absorbable units.
α dextrinase
Small intestine
Protein
Then enzymes on the surface of the small intestinal
cells hydrolyze these peptides and the cells absorb
them.
• Only a small percentage of the proteins are digested all the
way to their constituent amino acids by the pancreatic
juices.
• The last digestive stage of the proteins is achieved by the
enterocytes that line the villi of the small intestine
• Aminopolypeptidase and dipeptidases present in the
microvilli of the enterocytes split the remaining
polypeptides into tripeptides and dipeptides and a few into
amino acids.
• Both the amino acids plus the dipeptides and tripeptides
are easily transported through the microvillar membrane to
the interior of the enterocyte.
• Finally, the enterocytes contain peptidases that are specific
for the remaining types of linkages between amino acids.
• the last dipeptides and tripeptides are digested to form
single amino acids;
• these then pass on through to the other side of the enterocyte
and thence into the blood.
• More than 99 per cent of the final protein digestive products
that are absorbed are individual amino acids,
Fat
• The first step in fat digestion is physically to break the fat
globules into very small sizes so that the water-soluble
digestive enzymes can act on the globule surfaces. This
process is called emulsification of the fat.
• The emulsification is caused by bile salts and lecithin: make
the fat globules fragmentable
fat
 Bile is secreted continuously by the liver and is concentrated and
stored in the gallbladder. Bile is not an enzyme but an emulsifier that
brings fats into suspension in water .After the fats are emulsified,
enzymes can work on them.
Pancreatic lipase flows in from the pancreas (via the
pancreatic duct):
• Apart from causing emulsification, bile salts also cause
formation of micelles that help to accelerate fat digestion
• The bile salt micelles act as a transport medium to carry
the monoglycerides and free fatty acids, both of which
would otherwise be relatively insoluble, to the brush borders
of the intestinal epithelial cells.
• There the monoglycerides and free fatty acids are absorbed
into the blood
• the bile salts themselves are released back into the chyme
to be used again and again for this “ferrying” process.
DIGESTION IN THE LARGE INTESTINE
 Undigested residues, such as some fibers, are not absorbed but
continue through the digestive tract as a semisolid mass.
 Fiber also retains water, keeping the stools soft, and carries some
bile acids, sterols, and fat out of the body.
Overview of Digestion & Absorption (Cont.)
Vitamin
Water and Minerals
Mouth
and
salivary
glands
No action.
The salivary glands add water to
disperse and carry food.
Stomach
Intrinsic factor attaches to
vitamin B12.
Stomach acid (HCl) acts on iron to
reduce it, making it more absorbable.
The stomach secretes enough watery
fluid to turn a moist, chewed mass of
solid food into liquid chyme.
Small
intestine
Bile
emulsifies
fatsoluble vitamins and aids
in their absorption with
other fats. Water-soluble
vitamins are absorbed.
The small intestine, pancreas, and
liver add enough fluid so that
approximately 2 gallons are secreted
into the intestine in a day. Many
minerals are absorbed. Vitamin D aids
in the absorption of calcium.
Large
intestine
Bacteria produce vitamin
K, which is absorbed.
More minerals and most of the water
are absorbed.
Fibers
Most fiber passes intact through the digestive tract to
the large intestine. Here, bacterial enzymes digest some
fiber:
Fiber holds water; regulates bowel activity; and binds
cholesterol and some minerals, carrying them out of the
body as it is excreted with feces
Final Digestion Products
 Final digestion products absorbed by cells lining small
intestine.
 Carbohydrates:
 Monosaccharides
 Proteins:
 Amino acids
 Chains of 2 or 3 amino acids
 Fats:
 Fatty acids
 Glycerol
 Monoglycerides
 Vitamins, minerals, water and some larger fat-like
compounds such as cholesterol are not broken down
before they are absorbed.
The Absorptive System
 Most absorption takes place in the small intestine.
 Small intestine’s inner surface looks smooth, but viewed
through a microscope, it turns out to be wrinkled into
hundreds of folds (folds of Kerckring).
 Each fold is covered with thousands of fingerlike
projections called villi.
 A single villus, magnified still more, turns out to be
composed of several hundred cells, each covered with
microscopic hairs called microvilli.
 Thus, the combination of the folds of Kerckring, the
villi, and the microvilli increases the total absorptive
area of the mucosa perhaps 1000-fold.
 A villus has
 A vascular system: for nutrient and water absorption
 Central lacteal: for absorption into the lymph
 Pinocytic vesicles
 Microvilli or brush border
The Villus
Absorption in the small intestine
 Water: by osmosis from chyme to plasma
 Sodium ions: active transport of sodium from inside the epithelial
cells through the basal and side walls of these cells into paracellular
spaces and by facilitated diffusion from lumen to the inside of the
epithelial cells. Sodium is lost in diarrhea. Enhanced by aldosterone
 Chloride ions: passively “dragged” by the positive electrical charges
of the sodium ions.
 Bicarbonate ions absorption in the duodenum and jejunum:
hydrogen ions are secreted in exchange for the reabsorbed
sodium. The hydrogen ions combine with bicarbonate ions
to form carbonic acid (H2CO3), which then dissociates to
form water and carbon dioxide.
 The water remains as part of the chyme in the intestines,
but the carbon dioxide is readily absorbed into the blood
and subsequently expired through the lungs.
 Bicarbonate ions are however secreted in the ileum and
large intestine by HCO3-Cl antiport system: this
bicarbonate is to help neutralise the acid products
formed by bacteria in the large intestine
 Calcium ions: by active absorption. Regulated by PTH and
Vitamin D.
 Iron ions, potassium, magnesium, phosphate and others:
also by active absorption
Absorption of nutrients
 Carbohydrates: mainly glucose, galactose and fructose
 Co-transport with sodium through intestinal membrane.
 it is the initial active transport of sodium through the
basolateral membranes that provides the eventual motive
force for moving glucose also through the membranes.
 Fructose is transported by facilitated diffusion all the way
through the intestinal epithelium but not coupled with
sodium transport.
 Much of the fructose becomes phosphorylated, then
converted to glucose, and finally transported in the form of
glucose the rest of the way into the blood.
 Proteins: mainly as amino acids, mono and dipeptides
 Also co-transport with sodium or through membrane
transport proteins
 Fats: mainly as free fatty acids and monoglycerrides
dissolved in bile micelles in chyme
 The micelles help to ferry them into the epithelial cells
 After entering the epithelial cell, the fatty acids and
monoglycerides are taken up by the cell’s smooth ER
where they are mainly used to form new triglycerides
that are subsequently released in the form of
chylomicrons through the base of the epithelial cell, to
flow upward through the thoracic lymph duct and empty
into the circulating blood.
TRANSPORT OF LIPIDS: LIPOPROTEINS
 Within the circulatory system, lipids always travel from
place to place bundled with protein, as lipoproteins.
 VLDL, LDL, HDL, and chylomicrons transport newly
absorbed lipids from the intestinal cells to the rest of the
body.
 The liver can assemble different lipoproteins, which are
known as VLDL. As the body’s cells remove triglycerides
from the VLDL, the proportions of their lipid and protein
contents shift. As this occurs, VLDL become cholesterolrich LDL.
 Cholesterol returning to the liver for metabolism or
excretion from other parts of the body is packaged in
lipoproteins known as HDL.
Indigestible Matter
 After digestion and absorption of nutrients, indigestible
matter, such as fiber moves into the large intestine.
 Indigestible matter is compacted by removing water.
 Little nutrient absorption occurs in large intestine.
Absorption in the large intestine
• Sodium ions: by active transport
• Chloride ions: by co transport with sodium
• Bicarbonate ions: are secreted by counter transport
with chloride ions
• Water: osmosis
Metabolism
 Chemical reactions that occur in the body:
 Building and maintaining body tissues
 Regulating body functions
 Supplying energy
 For metabolism to occur the body needs:
 Water
 Energy
 Oxygen
 Nutrients
NUTRITION
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A proper diet requires a balance
of carbohydrates, fats, and
proteins. In addition the body
requires many phytochemicals,
vitamins, minerals, enzymes,
and water.
Food Intake
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Food energy measured in Calories
Carbohydrates  obtained primarily through plants
 Monosaccharides used for cellular fuel
 Minimum carbohydrates = 100 g/day
Lipids < 30% of calories
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Mostly triglycerides
Saturated fats usually from animals
Cholesterol only from animals
Neutral fats provide insulation and energy
reserves
Phospholipids for membranes and myelin
Cholesterol for membranes, vitamin D,
steroid hormones, and bile salts
Proteins = 0.8 g/kg of body wt
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8 Essential amino acids
Plants usually lack 1 or more essential amino
acids / Animal protein usually contains all
Amino acids used to build structural proteins
and enzymes
VITAMINS:
"vita" = Latin word for life.
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Vitamins are organic substances
that act as coenzymes, chemicals
that assist the enzymes in the
bodies reactions. They do not
provide energy or calories.
Vitamins may be either Fat Soluble
or Water Soluble.
Fat soluble vitamins
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are stored in the body's fatty
tissues. Fat soluble vitamins
include the vitamins
A
 D
 E
 K.
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Vitamin A
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Found in fish, liver, eggs, butter,
yellow & green vegetables, fruits
Needed for healthy skin, eyes,
bones, teeth.
Deficiency causes night blindness,
skin disorders, kidney stones
Vitamin D
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Found in liver, fish, eggs,
milk, sunlight
Needed for growth,
healthy bones,
metabolism of calcium &
phosphorus
Deficiency causes rickets,
poor teeth and bones.
Vitamin E
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Found in whole grains, leafy
vegetables, milk, butter, vegetable
oils
Needed for healthy cell
membranes, red blood cells
Deficiency causes red cell rupture,
muscle disorders
Vitamin K
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Found in leafy vegetables,
soybeans, made by intestinal
bacteria
Needed for normal blood clotting
Deficiency causes slow clotting,
hemorrhaging.
Water soluble vitamins
can be dissolved in water but
cannot be stored in the tissues.
 They must be obtained each
day from food.
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Water soluble vitamins include
B1 (Thiamine)
 B2 (Riboflavin)
 Niacin
 B6 (Pyridoxine)
 Pantothenic
Acid
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Biotin
 B12
 Folic Acid
 C (Ascorbic acid)
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Vitamin B1 (Thiamine)
Found in organ meats, whole
grains, vegetables
 Needed for proper functioning
of heart, nervous system,
digestion
 Deficiency causes beriberi,
cardiovascular disorders.
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Vitamin B2 (Riboflavin)
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Found in liver, poultry, milk, eggs,
cheese, fish, green vegetables, whole
grain
Needed for metabolism of protein,
carbohydrates, and fats, healthy skin
Used to make FAD for metabolism
Deficiency causes dim vision,
premature aging, sore mouth
Vitamin B6 (Pyridoxine)
Found in meats, liver, whole
grains, vegetables
 Needed for sodium and
phosphorus balance
 Deficiency causes anemia,
nausea, loss of appetite,
nervousness
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Vitamin B12
Found in Liver, meats, eggs,
cheese, dairy products
 Needed for red cell production,
healthy nervous system.
 Deficiency causes pernicious
anemia.
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Vitamin C
Found in citrus and other fruits, leafy
vegetables, tomatoes, potatoes
 Needed for healthy blood vessels,
resistance to infection, healing
 Deficiency causes scurvy, bruising,
bleeding gums
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Niacin
Found in red meats, organ meats,
fish, green vegetables
 Needed for metabolism, digestion,
nerves, skin
 Used to make NAD for metabolism
 Deficiency causes pellagra, sore
mouth, diarrhea, depression
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Folic Acid
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Found in green vegetables, liver,
whole grains, legumes
Needed for manufacture of proteins
and red blood cells, needed for cell
division, helps prevent spina bifida
Deficiency causes inflamed tongue,
diarrhea, B12 deficiency.
MINERALS:
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Inorganic substances that are used in
the chemical reactions of the body.
Major minerals needed include:
Calcium, Iodine, Iron, Magnesium,
Phosphorus, Potassium, and Sodium.
Calcium
Found in milk, cheese,
vegetables
 Needed for strong bones and
teeth, blood clotting
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Iodine
Found in
seafoods,
iodized salt
 Needed for
normal thyroid
metabolism,
prevents goiter
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Iron
Found in liver, meat, eggs
 Needed for red cell production,
prevents anemia
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Magnesium
Found in milk, meat, whole
grains, legumes
 Needed for proper nerve and
muscle functioning
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Phosphorus
Found in milk, whole grains,
meats, nuts, legumes
 Needed for tooth and bone
development, ATP, nucleic
acids
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Potassium
Found in whole grains, fruits,
legumes, meat
 Needed for proper nerve and
muscle function
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Sodium
Found in seafood, table salt
 Needed for water balance,
proper nerve and muscle
function
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Free Radicals
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charged molecules that become oxidized
by combining with oxygen or the removal
of hydrogen, causing electron deficiency.
seek to regain the electron by removing it
from other molecules, thus oxidizing them.
set up a chain reaction that may damage
cell structures such as DNA, cell
membranes, or needed enzymes.
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Free radicals may be produced by
normal metabolic processes, the
immune system in response to
disease, exposure to chemicals,
toxins, or radiation. Free radical
generation may be increased by
exercise and stress.
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Damage caused by free radical
generation is a major cause of the
degenerative effects of aging, may
cause cancers, damage to arterial walls
leading to heart disease and/or stroke,
and lead to other degenerative diseases
such as Alzheimer’s.
Antioxidants
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have a protective effect by neutralizing
free radicals.
best known antioxidants are Vitamin C,
Vitamin E, and beta carotene.
many others and possibly many yet to be
discovered.
proper number, types, and balance of is
an important part of nutrition.
METABOLISM
Sum of all the chemical
reactions occurring within the
body
Types of Metabolic Reactions
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Anabolic reactions - energy requiring
synthesis reactions
Catabolic reactions - energy releasing
reactions that generate ATP
Enzymes - globular proteins
that act as catalysts
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Increase reaction rates
Holoenzyme - a two-part enzyme
consisting of a protein part and an
organic cofactor
Apoenzyme - the protein portion
 Coenzyme - the organic cofactor; usually
a vitamin
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Energy Production
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Oxidation reactions - loss of an
electron by an atom or molecule
Reduction reactions - involves the gain
of electrons by a molecule
Coupled redox reactions
Cellular Respiration
Oxidation of Glucose
Glucose Metabolism
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Glycolysis
Acetyl Coenzyme A
Krebs Cycle
Electron Transport Chain
Glycolysis
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Glucose molecules are broken down into
two molecules of pyruvic acid in the
cytoplasm of the cell
Net gain of 2 molecules of ATP
No oxygen required
Fate of pyruvic acid depends on the
oxygen availability
Glycolysis
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Glucose C6H12O6
Glucose-6-phosphate
ATP
Fructose-6-phosphate
ADP
Fructose 1,6, diphosphate
Glyceraldehyde-3-Phosphate or
Dihydroxyacetone Phosphate
2Pyruvate (pyruvic acid) + 2NAD
2C3H4O3
+ 2NADH+
ATP
ADP
+ 4ATP
+ 2ATP (net)
Acetyl CoA Formation
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Pyruvic acid is decarboxylated by the
removal of CO2 into a two carbon acetyl
group
Occurs in the mitochondria of the cell
Krebs Cycle - TCA Cycle
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Formation of citric acid when
oxaloacetic acid combines
with acetyl CoA
Organic molecules are
broken down, carbon dioxide
is released and hydrogen
atoms are removed &
transferred by coenzymes
NAD & FAD
Kreb’s Cycle
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Acetyl CoA + Oxalocetic Acid
Citric Acid
Isocitric Acid
CO2 NADH2
alpha-Ketoglutaric Acid
CO2 NADH2
Succinyl CoA
ATP
Succinnic Acid FADH2
Fumaric Acid
Malic Acid
NADH2
Electron Transport
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Involves electron carrier molecules that
will release energy in a controlled way
This energy is used to generate ATP
Occurs inner mitochondrial membrane
Chemiosmosis
Glucose Anabolism
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Glycogenesis - conversion of glucose to
glycogen; stimulated by insulin
Glycogenolysis - hydrolysis of glycogen
to form glucose; stimulated by
glucagon
Gluconeogenesis - synthesis of glucose
from non-carbohydrates such as fats
and amino acids
Lipid Metabolism
Lipid Catabolism - Lipolysis
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Hydrolysis of triglycerides into glycerol
and fatty acids
Glycerol converted to G 3-P and then
into pyruvic acid, then into the Kreb’s
cycle
Beta -oxidation of fatty acids occurs
forming two-carbon fragments which is
then attached to coenzyme A, forming
acetyl CoA
Protein Metabolism
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Proteins are converted into substances
than can enter the Kreb’s cycle by
 deamination - loss of (NH2) from amino
group
 decarboxylation - loss of CO2 molecule
 dehydrogenation - loss of hydrogen
atom
Protein synthesis involves transcription and
translation
The Central Pathway of Energy Metabolism
Basal Metabolic Rate
What Is Your BMR?
• Your BMR measures the minimum calorie
requirement your body needs to stay alive in
a resting state
• It is the amount of calories your body would
need if you were to stay in bed all day
How Many Calories Is This?
• About 70% of your calorie intake is
responsible for just supplying your BMR
• You need calories to:
– Pump your heart
– Breathe
– Control your body temperature
– Any many other things
Do We All Have The Same BMR?
• We all have different BMR and there are
many things that will affect what that rate is
• Your BMR is the largest factor in
determining your overall metabolic rate
(how your body burns calories)
Genetics
• Some people are born with slower
metabolisms than others
• Some people are born with faster
metabolisms than others
Gender
• Men have a greater muscle mass and a lower
body fat percentage (10-15% higher BMR
than womem)
• The higher your muscle mass, the higher
your metabolism
Age
• Your BMR will reduce as you age
• After 20 years of age, your BMR drops about
2% every year
Weight
• The more you weigh, the higher your BMR
• The BMR of an obese woman is 25% higher
than a woman of an appropriate weight
Body Surface Area
• The greater your body surface area, the
higher your BMR
• Tall, thin people have higher BMRs
Body Fat Percentage
• The lower your body fat percentage, the
higher your BMR
Diet
• If you reduce your calorie intake suddenly,
your BMR can drop by 30%
• Your body wants to ensure that it always has
the calories it needs to survive, so cutting
calories quickly will switch your body into a
“survival” mode
Body Temperature
• For every 1 degree increase in your body
temperature, your BMR increases by
approximately 14%
• Chemical reactions occur faster in your body
at higher temperatures
• You burn a lot more calories while you are
sick or have fever
External Temperature
• Exposure to cold temperatures will increase
your BMR
• Prolonged exposure to heat will also
increase your BMR
Glands
• Your thyroid gland (butterfly-shaped gland
in your neck) is responsible for making
thyroxin
• The more thyroxin produced, the higher
your BMR
Exercise
• Exercise helps to build lean muscle tissue
• The more lean muscle tissue, the higher your
BMR
• This means you will burn more calories –
even when you are sleeping!
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