Download Mechanical digestion

Chapter 4
Proteins
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1
Key Concepts
• Food proteins provide the amino acids
necessary for building and maintaining
body tissue.
• Protein balance, both within the body and in
the diet, is essential to life and health.
• The quality of a protein food and its ability
to meet the body’s needs are determined by
the composition of amino acids.
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2
Amino Acids: Basic Building
Material
• Each protein is composed of hundreds of amino
acids
• Amino acids form unique chain sequences to form
specific proteins; the sequencing determines the
function of the completed protein
• When protein foods are eaten, proteins are broken
down into amino acids
• Amino acids are reassembled in the body to form
a variety of proteins
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3
The Nature of Proteins


Proteins are relatively large, complex molecules
May be subject to mutations or malformations
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Dietary Importance
• Amino acids
– Named for chemical structure
– Basic structure of carbon, hydrogen, and
oxygen
– Primary source of nitrogen in diet
– Some contain small amounts of valuable
minerals
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5
Classes of Amino Acids
• Indispensable amino acids
– Body cannot manufacture in sufficient quantity
so it is essential to eat them
• Dispensable amino acids
– Amino acids that the body can synthesize from
indispensable amino acids
• Conditionally indispensable amino acids
– Normally synthesized but some health
conditions may require dietary intake
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6
A Healthy Balance
• Protein balance
– Catabolism: breakdown
– Anabolism: synthesis
• Nitrogen balance (intake = excretion)
– Positive nitrogen balance: body stores more than it
excretes, storing and using protein instead of excreting
it (periods of rapid growth or recovering from illness
– Negative nitrogen balance: body takes in less than it
excretes, losing more that it can build up (malnutrition
or illness)
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Functions of Protein
•
•
•
•
•
•
Tissue building & repair
Energy
Water and PH balance
Metabolism and transportation
Body defense system
Hormones, immunoglobulins, muscles, albumin,
& antibodies are all proteins
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8
Tissue Building
• Fundamental structural material of every cell
• Comprises bulk of:
– Muscles
– Internal organs
– Brain and Nerves
– Blood plasma
• Protein repairs worn-out, wasted, or damaged
tissue
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Energy
• May provide body fuel if the supply of
carbohydrate and fat is insufficient for needs
• Body’s third choice for energy production
• Less efficient
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10
Water Balance
• Plasma proteins (especially albumin) attract
water, resulting in maintenance of normal
circulation volume
• Proteins have a unique structure to act as
buffering agents in pH balance
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Metabolism & Transport
• Enzymes
– Digestive enzymes: amylases, lipases, proteases
• Transport agents
– Lipoproteins
– Hemoglobin
• Hormones
– Insulin and glucagon
– Thyroxine, calcitonin
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12
Body Defense
• Immune system defends against disease
and infection
– White blood cells
– Antibodies
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13
Key Concepts
• Food proteins provide the amino acids necessary
for building, maintaining and repairing body
tissue.
• Protein balance, both within the body and in the
diet, is essential to life and health.
• The quality of a protein food and its ability to
meet the body’s needs are determined by the
composition of amino acids.
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14
Food Sources of Protein
• Complete proteins
– Meat, fish, poultry, seafood
– Soy
• Incomplete proteins
– Plant-origin foods
• Grains and Legumes
• Nuts and Seeds
• Fruits and vegetables
• Complementary proteins -matching plant foods
so that the amino acids missing in one food are
supplied by another
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Vegetarian Diets
• Must combine foods to cover all amino acid needs
(makes a complete protein)
• Types of vegetarian diets
– Lacto-ovo vegetarian
– Lacto-vegetarian
– Ovo-vegetarian
– Vegan
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Lacto-Ovo Vegetarian Diet
Pyramid
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Vegetarian Diets
• Health benefits and risks
– Less saturated fat and cholesterol
– More fruits, vegetables, whole grains, other
healthful foods
– Lower BMI and risk of obesity
– Lower rate of cardiovascular disease
– Lower risk of renal disease
– Lower risk of type 2 diabetes
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Digestion of Proteins
• Mouth – mechanical, no digestion
• Stomach: enzymatic breakdown of protein by
proenzymes
– Pepsinogen & Hydrochloric acid = Pepsin
– Rennin
• Small intestine
– Pancreatic secretions
• Trypsin, chymotrypsin, carboxypeptidase
– Intestinal secretions
• Aminopeptidase, dipeptidase
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Summary of Protein Digestion
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Body Needs for Protein
• Tissue growth and repair
• Dietary protein quality is important
–
–
–
–
Chemical score (CS)
Biological value (BV)
Net protein utilization (NPU)
Protein efficiency ratio (PER)
• Additional needs are caused by illness,
disease and skin injury like decubiti or
burns
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Dietary Deficiency or Excess
• Protein energy malnutrition
– Kwashiorkor
– Marasmus
• Excess protein
– Usually also means excess fat intake
– Protein displaces other healthy foods in diet
– Extra burden on kidneys to rid body of nitrogen
waste
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Dietary Guides
• Recommended Dietary Allowances (RDAs)
– Relate to age, sex, weight
– Highest at birth and slowly decline into
adulthood
– Men and women: 0.8 g/kg of desirable weight
– 10% to 35% of total caloric intake from
protein (children and adults)
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Calculating Protein Needs,
cont’d
• DRI:
– Assumes individual consumes 2200 kcal/day;
based on recommendation of 10% to 35% of
total kilocalories from protein
– 2200 kcal x 0.1- 0.35 = 220-770 kcal/day from
protein
– 220-770 kcal
4 kcal/g = 55-192.5 g/day of
protein
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Summary
• Protein provides the body with its primary tissuebuilding units, amino acids
• *20 common amino acids
• Nine of the 20 amino acids are indispensable in
the diet
• Body can manufacture the remaining 11 amino
acids
• Complete proteins are foods that supply all the
indispensable amino acids
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Summary, cont’d
• Complete proteins are usually of animal origin
and soy
• Plant foods are considered incomplete proteins
because they lack one or more of the indispensable
amino acids (with the exception of soy)
• Vegetarian diets can be strict with only plant
proteins (vegan), whereas other variations include
dairy, egg, and sometimes fish
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Summary, cont’d
• Adequate dietary protein and a reserve “pool” of
amino acids help maintain overall protein balance
• A powerful digestive team of six protein-splitting
enzymes acts to break down the protein into amino
acids for vital tissue-building tasks
• Protein requirements are influenced by growth
needs and nature of diet in terms of protein quality
and energy intake
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Summary, cont’d
• Clinical influences on protein include fever, disease,
surgery, or other trauma to the body
• Protein needs are calculated based on RDA standards
related to age, sex, and weight, & is set at 0.8 g of
high quality protein per kilogram of body weight/ day
• Adjustments for protein intake are required for:
– infants and pregnant and breastfeeding women
– individuals following a vegan diet
– critical illness
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28
Williams' Basic Nutrition &
Diet Therapy
14th Edition
Chapter 5
Digestion, Absorption,
Transport and Metabolism
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Key Concepts of Digestion,
Absorption and Metabolism
• Through a balanced system of mechanical
and chemical digestion, food is broken
down into smaller substances and the
nutrients are released for biologic use.
• Special organ structures and functions
conduct these tasks through the successive
parts of the overall system.
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30
Digestion
• Basic principles
– Principle of change
• The body cannot use food as it is eaten.
• Food must be changed into simpler substances to be
absorbed and then used by cells to sustain life.
– Principle of wholeness
• The parts of the digestive process comprise a
continuous whole.
• Food components travel through the gastrointestinal
(GI) system until they are delivered to cells or
excreted.
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Mechanical Digestion
• GI motility: Beginning in the mouth, muscles and
nerves in the tract coordinate their actions to
provide motility, an automatic response to the
presence of food.
• Muscles
– Muscle tone/tonic contraction: Ensures continuous
passage of the food mass and valve control along the GI
tract
– Periodic muscle contraction and relaxation: Rhythmic
waves that mix the food mass and move it forward
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32
Mechanical Digestion, cont’d
• Nerves
– Specific nerves regulate muscle action along
the GI tract
– The intramural nerve plexus is the network of
nerves in the GI wall extending from the
esophagus to the anus
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33
Chemical Digestion
• Digestive enzymes: Break down nutrients
• Hydrochloric acid and buffer ions: Produce the correct
pH necessary for enzyme activity
• Mucus: Lubricates and protects the GI tract tissues; helps
mix the food mass
• Water : Carries and circulate the products of digestion
through the tract and into the tissues
• Electrolytes: absorbed, transported and play a role in
homeostasis
• Bile: Emulsifies fat into smaller pieces to assist fat
enzymes
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34
Digestion in the Mouth and
Esophagus
• Mechanical digestion
– Mastication breaks down food orally.
– Food is swallowed and passes down esophagus.
– Muscles at tongue base facilitate the
swallowing process.
– Gastroesophageal sphincter at stomach entrance
relaxes, allowing food to enter, then constricts
to retain food.
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35
Digestion in the Mouth and
Esophagus, cont’d
• Chemical digestion
– Salivary glands secrete fluid containing salivary
amylase or ptyalin.
– Salivary glands also secrete a mucous material to
lubricate and bind food particles, facilitating the
swallowing of the food bolus.
– Secretions from the mucous glands in the esophagus
help ease movement of food toward the stomach.
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36
Digestion in the Stomach
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Digestion in the Stomach
• Mechanical digestion
– Under sphincter control, the food enters the upper
portion of the stomach as individual bolus masses.
– Stomach muscles knead, store, mix, and propel the
food mass forward.
– By the time the food mass reaches the lower portion
of the stomach, it is a semi-liquid acid/food mix called
chyme.
– Chyme is released slowly into the first section of the
small intestine (duodenum) by the pyloric valve.
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38
Digestion in the Stomach,
cont’d
• Chemical digestion: three types of gastric
secretions
– Hydrochloric acid: Parietal cells in the stomach lining
secrete acid to promote gastric enzyme activity.
– Mucus: Secretions protect the stomach lining from the
erosive effect of the acid and also bind and mix the food
mass and help move it along.
– Pepsinogen is secreted by stomach cells and activated by
acid to become pepsin, a protein-splitting enzyme.
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39
Digestion in the Small
Intestine
• Mechanical digestion
– Peristaltic waves slowly push food mass forward.
– Pendular movements from small local muscles sweep
food back and forth stirring chime at the mucosal surface
– Segmentation rings chop food mass into successive soft
lumps and mix them with secretions.
– Longitudinal rotation rolls food in a spiral motion,
exposing new surfaces for absorption.
– Surface villi motions stir and mix chyme
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40
Digestion in Small Intestine,
cont’d
• Chemical digestion
– Pancreatic enzymes
• Carbohydrate: Pancreatic amylase converts starch
to maltose and sucrose.
• Protein: Trypsin and chymotrypsin split large
protein molecules into small peptide fragments and
eventually into single amino acids.
• Fat: Pancreatic lipase converts fat to glycerides and
fatty acids.
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41
Digestion in the Small
Intestine, cont’d
• Small Intestine enzymes
– Carbohydrate: Disaccharidases convert
disaccharides into monosaccharides.
– Protein: Enterokinase activates trypsinogen
from the pancreas to become trypsin; amino
peptidase removes end amino acids from
polypeptides; dipeptidase splits dipeptides into
amino acids.
– Fat: Intestinal lipase splits fat into glycerides
and fatty acids. (Triglycerides= 1 glyceride + 3 fatty
acids)
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42
Digestion in the Small
Intestine, cont’d
• Mucus: Large quantities of mucus protect the mucosal
lining from irritation and erosion.
– Bile: Emulsifying agent produced by the liver and stored
in the gallbladder aids fat digestion and absorption.
– Hormones
• Secretin- produced by mucosal cells in response to
acid in the duodenum releasing the alkaline pancreatic
enzymes
• Cholecystokinin-secreted by intestinal cells when fat
enters the duodenum to release bile
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43
Digestion in the Small
Intestine (cont’d)
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Factors Influencing GI Tract
Secretions
•
•
•
•
Nervous control
Conditioned reflexes
Oral reflexes
Physical contact
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45
Absorption and Transport
• At this point in the process:
– Carbohydrates: reduced to simple sugars
(glucose, fructose, galactose)
– Fats: changed into fatty acids and glycerides
– Proteins: changed into single amino acids
– Vitamins and minerals: liberated from food
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Bioavailability
• Bioavailability refers to:
– Amount of nutrient present in the GI tract
– Competition between nutrients for absorption
– Form in which the nutrient is present
• All nutrients present in a food are not absorbed
because of differing bioavailability.
– This is considered when determining dietary intake
standards
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47
Absorption in the Small
Intestine
• Three absorbing structures
– Mucosal folds: Surface of small intestine piles
into folds
– Villi: Small, finger-like projections cover the
mucosal folds, increasing the area of exposed
intestinal surface
– Microvilli: Smaller projections cover each villi
(look like bristles on a brush)
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48
Small Intestine (cont’d)
49
Absorption Processes
• Simple diffusion: The force by which particles move
outward in all directions—from areas of greater to lesser
concentration.
• Facilitated diffusion: Similar to simple diffusion but
uses a protein channel to carry larger items.
• Active transport: The force by which particles move
from areas of lesser to greater concentration using a carrier
to “ferry” particles.
• Endocytosis: Penetration of larger materials by attaching
to the cell membrane and being engulfed by the cell.
• Pinocytosis: liquids are engulfed by a cell
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50
Absorption in Large Intestine
• Bacteria helps make Vitamin K
• Water is taken up by the large intestine
– Most water in chyme is absorbed in the first half of the
colon
– Only a small amount remains to form and eliminate feces
• Dietary fiber is not digested
– Contributes bulk to food mass
– Helps form feces
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51
Transport
• Nutrients must be transported to cells
• Vascular (blood circulatory) system
– Veins and arteries
– Transports waste, such as carbon dioxide and nitrogen,
to lungs and kidneys for removal
– Portal circulation carries nutrients to the liver which
filters blood.
• Lymphatic system
– Route for fatty materials, which are not water soluble
– Fat molecules pass into lymph capillaries near the villi
called Lacteals
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52
Metabolism
• Mitochondria of cells is where metabolism
takes place
• Two metabolic processes
– Catabolism: Breaking down of large
substances into smaller units (e.g., breaking
down a protein chain into amino acids)
– Anabolism: Building of larger substances from
smaller particles (e.g., building a complex
protein from single amino acids)
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53
Metabolism, cont’d
• Metabolic processes ensure that the body has
energy in the form of adenosine triphosphate
(ATP).
• Metabolism of glucose (4 kcal/kg) from
carbohydrates yields less energy than metabolism
of fat (9 kcal/kg). Still, glucose is the body’s
primary source of energy.
• Protein can be an energy source, but it is relatively
inefficient.
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54
Energy Storage:
Glycogenesis
• Glycogenesis: Anabolic process of converting
extra glucose into glycogen
• Glycogen is stored in the liver and muscles for
quick energy to be used at a later time
• Gluconeogensis: Making glucose from non
carbohydrate sources
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55
Energy Storage: Lipogenesis
• When glycogen reserves are full, additional excess
energy from carbohydrates, fat, or protein is stored
as fat in adipose tissue
• Lipogenesis: The building up of triglycerides for
storage in adipose tissue
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56
Errors in Digestion and
Metabolism
• Genetic defects: cell is missing enzyme
controlling metabolism of a specific nutrient
– Phenylketonuria (PKU)
• Enzyme responsible for metabolizing essential
amino acid phenylalanine is missing
• Untreated, causes permanent mental retardation and
CNS damage
• With proper treatment, affected children may have
normal and healthy lives
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57
Errors in Digestion and
Metabolism (cont’d)
• Galactosemia
– Enzyme responsible for metabolizing galactose to
glucose is missing
– All sources of lactose must be eliminated from diet
– Untreated, can cause brain and liver damage
– Screening and treatment can enable normal life
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58
Lactose Intolerance
• Most common disaccharidase deficiency
• Lactase is not present in sufficient amounts,
it is not absent
• Causes abdominal cramping and diarrhea
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59
Summary
• Nutrients from food must be changed,
released, regrouped, and rerouted into forms
the body can use.
• The activities of digestion, absorption, and
transport ensure that key nutrients are
delivered to the body’s cells so metabolic
tasks can be completed.
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60
Summary, cont’d
• Mechanical digestion consists of spontaneous
muscular activity responsible for initial mechanical
breakdown and the movement of the food mass
along the GI tract by the motion of peristalsis.
• Chemical digestion involves the enzymatic action
that breaks food down into smaller components and
releases nutrients for absorption.
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61
Summary, cont’d
• Absorption involves the passage of food nutrients
from the intestines into the mucosal lining of the
intestinal wall.
• Nutrients absorbed are transported throughout the
body by the circulatory system.
• Metabolism is the sum of the body processes that
change food energy taken in (carbohydrate,
protein, and fat) into various forms of energy
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62