module 2 companion animal nutrition

Pre-gastric fermentation

Fermentation proximal to (before) H/E digestion Species • Ruminants & pseudoruminants (e.g., camels) • Hippopotamus • Marsupials • Herbivorous monkeys

Post-gastric fermentation

Fermentation distal to (after) H/E digestion Species • Pigs, dogs, cats, and humans • Rodents • Rabbits • Large herbivores (e.g., horse, rhinoceros)

simple non-ruminant

Cat, dog, mink (carnivores, some omnivores

Very short GIT Large stomach with respect to entire GIT *Lack of or very small cecum

Non-ruminant with limited postgastric fermentation

Rat, pig, human (omnivores)

Sacculated colon and (or) enlarged cecum 5 to 15 x body length

non-ruminant herbivore

Horse, rabbit

Sacculated colon and very large cecum 10-15 x body length *Both examples practice coprophagy

Pre-gastric fermentors

Ruminants, kangaroo

Very long GIT (20-25 x body length) Pre-gastric fermentors

Hydrolytic / enzymatic digestion (auto) vs. fermentation (allo)

Hydrolytic / enzymatic digestion (auto-enzymatic)

• Uses the animal’s own enzymes and gastric acid

• Occurs mainly in the stomach and small intestine

• Breaks food into monomers (monosaccharides, amino acids/small peptides, long-chain fatty acids)

• Primary way protein and fat are digested

• Important in all animals

  slides_3 (2)

Fermentation (allo-enzymatic)

• Uses microbial enzymes, not the animal’s

• Location depends on species

• Produces short-chain fatty acids (acetate, propionate, butyrate), gases, microbial protein, B vitamins, vitamin K

• Importance varies by species and diet

• Critical for herbivores

Factors that affect the importance of fermentation to the animal

• Diet composition

  • More fiber → more reliance on fermentation

• GI tract structure & functional adaptations

  • Size of cecum, colon, or forestomach

• Site of fermentation

  • Pre-gastric vs post-gastric

• Ability to recover microbial nutrients

  • Example: coprophagy in rabbits and horses

Difference between pre- and post-gastric fermentation

Pre-gastric fermentation

• Fermentation occurs before hydrolytic/enzymatic digestion

• Microbial products are efficiently used by the host

• Seen in ruminants, pseudoruminants, kangaroos, camels

• Large stomach capacity dedicated to fermentation

Post-gastric fermentation

• Fermentation occurs after stomach and small intestine

• Some microbial protein and vitamins are lost unless coprophagy occurs

• Seen in horses, rabbits, pigs, rodents, humans, dogs, cats

• Relies on enlarged cecum and/or colon

GI tract changes from carnivore → omnivore → herbivore

Carnivores (dog, cat)

• Very short GI tract

• Large stomach, minimal cecum

• Little fermentative capacity

• Digestion dominated by enzymatic processes

Omnivores (pig, human)

• Moderate GI length

• Sacculated colon and/or enlarged cecum

• Some post-gastric fermentation

• Balanced enzymatic + limited fermentative digestion

Herbivores

• Very long GI tract

• Large fermentative organs

  • Ruminants: massive stomach (up to ~70% of capacity in cows)

  • Hindgut fermenters (horse, rabbit): huge cecum and colon

• Fermentation provides a major energy source via SCFAs

• Often practice coprophagy if fermentation is post-gastric

cow/sheep dental formulas

rabbit dental formula

horse dental formula

pig dental formula

dog dental formula

cat dental formula

human dental formula

carnivore teeth have

Lower number or lack molars

Presence of large canine teeth

Presence of “carnassial” teeth (last upper premolar, first lower molar)

• Act like scissors to slice flesh Sheep Skull Dog Skull Gripping and tearing, not chewing and grinding

herbivore teeth have

Usually greater number of teeth

Canines rudimentary or nonexistent

Large grinding premolars and molars

saliva primary function

Primary function Moisten and lubricate food

↑ production by sight of smell of food Gustatory response Pavlov – Nobel prize in 1904

saliva dog important function

Important function is evaporative cooling

dog and cat saliva

Secreted by 4 pairs of salivary glands

• Zygomatic

• Parotid

• Sublingual

• Mandibular

Composition:

• Inorganic ions (Na, K, Cl)

• Buffers (bicarbonate)

• No α-amylase

Composition affected by:

• Gland producing

• Diet type

• Moisture content of food

trigeminal system

detects chemicals responsible for flavor qualities described as “mouthfeel” Creaminess, burning, cooling, pain

taste in cats

Lack ability to detect sweet flavors

• Receptor not functional

Less sensitive to bitter compounds

taste in dogs

• Most abundant receptor – sweet

• 2nd most abundant receptor – acids abundant in meat and meat products

• Less sensitive to some bitter compounds • Example: caffeine

number of scent receptors

Dogs: >200 million

Cats: ~200 million

Humans: 5 million

Scent detection threshold of dogs is 1 million times lower than that of humans

1. How are dog and cat teeth different? How does this relate to differences in their diets?

Dog vs. cat

• More teeth than cats

• Enlarged molars for grinding

• Both have enlarged canines

• Both have carnassiate teeth

• 
  Dentition of dog indicates an omnivorous diet

1. What is palatability and why is it important in companion animal nutrition?

• If a food isn’t palatable, the animal won’t eat enough, even if it’s perfectly balanced.

• Adequate intake is required to meet energy and nutrient requirements.

• Especially critical for:

  • Cats (very selective, strong smell-driven preferences)

  • Sick, stressed, or elderly animals with reduced appetite

• Affects owner compliance. If the pet refuses the food, the owner won’t keep buying it.

• Impacts body weight, health, and long-term outcomes. A nutritionally ideal food is useless if it stays in the bowl.

functions of the stomach

• Temporary food storage • Mixing of food • Start of hydrolytic/enzymatic digestion

parietal cell secretes what

HCl

chief cell secretes what

pepsinogen

non-ruminant stomach secretions

HCl – parietal cell

Pepsinogen – chief cell

Mucus

Hormones • Ghrelin

4 regions of non-ruminant stomach

Esophageal

• Stratified squamous nonglandular epithelia

• NO secretory glands

Cardiac

• Mucus-secreting glands

• No parietal cells (HCl) • No chief cells (pepsinogen)

Fundic

• Also known as “proper gastric mucosa”

• Contains most of the glands

– Parietal cells (HCl) – Chief cells (pepsinogen)

Pyloric

• No parietal cells • Some mucus and peptic activity, but minimal

ruminant stomach four chambers

Four chambers

Rumen

Reticulum (rumen and reticulum = Reticulorumen)

– Fermentative digestion; short-chain fatty acids account for 60-70% of ME

– pH = 6-7, buffered by saliva (HCO3 ; HPO4 )

Omasum

Abomasum

• Analagous to non-ruminant stomach • Only glandular section (“gastric” stomach)

ruminant retention time

Usually longer in compartments in which fermentation occurs

• Particle size

• Mean retention time = MRT

Reticulorumen: 16-48 hr MRT

Non-ruminant stomach: 2-8 hr MRT

• Dog stomach begins to empty in 1 hr

↑ MRT allows more extensive fiber digestion by microbes

small intestine

Joins stomach at pyloric sphincter

3 segments

• Duodenum (Adjacent to stomach, Shortest region, Common bile duct enters , 1st 12 inches of dog and cat)

• Jejunum – midsection

• Ileum – Distal portion of SI

small intestine morphology

• Lumen

• Mucosa

• Epithelium

• Lamina propina

• Muscularus mucosae

• Submucosa

• Mascularis

• Serosa

small intestinal epithelium

Epithelia covered by villi (Microvilli)

Large population of epithelial stem cells at crypt base
2 populations of daughter cells produced

cell types - absorptive

Enterocytes (absorptive) ~90% of cells in SI epithelia Small intestine is primary organ of absorption

cell types - secretory

Goblet

• Secrete mucus • Few in small intestine (~5% of cells)

Enteroendocrine

• Over 15 hormones produced
  

Paneth

• Innate immunity

cellular differentiation

• Acquire function

• Most cell types differentiate as they migrate up villus

• Cells renewed every 3-5 days

Paneth cells complete differentiation at base of crypt (Slower turnover)

cecum

From ileo-cecal junction to blind end

Fermentation vat

Almost nonexistent in carnivores

Herbivores: vary in size and importance to animal

large intestine

Distal to ileo-cecal junction

Fermentation • Short-chain fatty acid production

H2O and electrolyte absorption (Na, Cl)

canine GIT

• Canidae: omnivorous

• Dog (Canis familiaris)

– Simple stomach

• ~60% digestive capacity

– Intestine ~5X body length

• Small intestine: 85%

• Large intestine: 15%

– Small, rudimentary cecum

– Straight, tubular colon

• Unsacculated

• Low fermentative capacity

feline GIT

• Felidae: carnivorous

• Cat (Felis catus)

– Simple stomach

• ~70% digestive capacity

– Intestine ~4X body length

• Small intestine: 80%

• Colon: 20%

– Small cecum

– Short, tubular colon

• Unsacculated

• Fermentative capacity?

Which region of the stomach produces gastric secretions? Which secrete mucus?

• Fundic region (aka proper gastric mucosa) produces gastric secretions: HCl from parietal cells and pepsinogen from chief cells.

• Cardiac region mainly secretes mucus.

• Pyloric region has some mucus and minimal peptic activity.

• Esophageal region has no secretory glands

Why does food remain longer in the ruminant stomach than in a carnivore like the dog?

• Ruminants rely on microbial fermentation in the reticulorumen to digest fiber.

• This requires a long mean retention time (16–48 hours) so microbes can break down plant material.

• Dogs digest mostly protein and fat enzymatically, so food passes quickly (2–8 hours; starts emptying ~1 hour)

Why have villi and microvilli instead of a smooth small intestine?

• Villi and microvilli dramatically increase surface area.

• More surface area = more efficient nutrient absorption, which is the small intestine’s primary role

Are there more secretory or absorptive cells in the small intestine, and why?

• Absorptive cells dominate.

• ~90% of small intestinal epithelial cells are enterocytes.

• This makes sense because the small intestine is the main site of nutrient absorption, not secretion

What is the function of the cecum and large intestine? How does importance vary by diet?

• Cecum: fermentation vat for microbial digestion (especially fiber).

• Large intestine: fermentation, short-chain fatty acid production, and water/electrolyte absorption.

• Importance by diet:

  • Carnivores: very small or nearly absent cecum, low fermentative capacity.

  • Omnivores: moderate fermentation.

  • Herbivores: large, highly developed cecum and/or colon; critical for energy extraction from fiber

GI function

Digestion

– Luminal • Lumen of gut • GI enzymes • Microbes

– Epithelial • Enzymes on or within cells of brush border membrane (BBM)

Secretion

– Mucus, enzymes, etc.

– Produced by • Gut mucosa • Accessory organs

– Salivary glands – Liver – Pancreas

Absorption

– Nutrients

– Reabsorption of endogenous compounds • Water • Electrolytes • Bile salts

Transport food

– Facilitated by motor activity of GI organs

Protection

– Microbes – Toxins – Gut is “open” to environment – Epithelia only barrier between “outside” and blood

Immune function

– Gut contains more lymphoid tissue than entire rest of body

Excretion

– Undigested food residues – Metabolic wastes

function of mouth/saliva

– ↓ particle size of food

– Moisten food during chewing and swallowing

function of stomach

– Storage (food reservoir) • 2-8 hr in non-ruminants • Allows nutrients to reach lower GI at rate that can be handled

– Mixing • Food, water, gastric juice = chyme/digesta

– ↓ particle size

– Beginning of H/E digestion

– Fermentation (pre-gastric fermentors) • SCFA absorption

small intestine function

– Regulate digesta flow • Peristalsis – waves of contractions

– Chemical and enzymatic digestion • Major site of H/E digestion

– Absorption • Main site of absorption – Monosaccharides – Amino acids and di- and tri-peptides – Long-chain fatty acids – Vitamins – Most minerals

cecum/large intestine function

– Fermentative digestion • Importance varies across species • Large non-ruminant herbivores

– Absorption • Water • Some minerals – Na, Cl • SCFA • Ammonia

digestion accomplished by

– Physical action • ↑ exposure of food particles to digestive secretions and gut mucosa

– Acid hydrolysis and enzymatic activity • Gastric stomach and small intestine • “Auto-enzymatic”

– Fermentation • Location dependent upon species • “Allo-enzymatic”

HCl function

• Protein coagulation (denaturation)

• Activation of pepsinogen
pepsin

• Maintains acidic pH

• Tightly regulated – Neural factors – Hormonal factors

pepsinogen secretion

• Chief cells in gastric mucosa

• Activated (converted to pepsin) by H+

• Self-activating (positive feedback)

Secreted as a zymogen: inactive precursor of an enzyme (pepsin

pepsin function

– Initiates gastric protein digestion • Large peptides
small peptides

– Broad specificity • Attacks many AA linkages

– Optimal pH: 1.8 to 3.5

mucus secretion

• Cardiac, fundic, and pyloric regions

• Made of glycoproteins

• Hydrophilic – “Water-loving”

– Insoluble=Continuously secreted

– Soluble=Pepsin degradation of insoluble form

mucus function

– Protects epithelia from gastric acid and pepsin

– Lubricates gastric “chyme” during motility

mucus stimulation and inhibition

Stimulation

– Cholinergic neurons

• Same as HCl

– Prostaglandins

Inhibition

– Aspirin

• ↓ prostaglandins
↓ mucus
ulcers

– H.pylori • Also associated with ↑ ulcer incidence

What triggers the cephalic, gastric, and intestinal phases of HCl regulation?

Cephalic phase

• Triggered by sight, smell, taste, and thought of food

• Mediated by neural (vagal) stimulation

• Happens before food enters the stomach

• Contributes ~30–40% of total HCl secretion

  slides_3b

Gastric phase

• Triggered by food in the stomach

  • Stomach distension

  • Peptides and amino acids in the lumen

• Involves both neural and hormonal signals (especially gastrin)

• This is the major phase, accounting for >50% of HCl secretion

  slides_3b

Intestinal phase

• Triggered when chyme enters the small intestine

• Provides minor stimulation and then mostly inhibitory feedback

• Contributes <10% of HCl secretion overall

pancreatic secretions

Bicarbonate (HCO3 ): Buffers gastric contents Produced by “duct” cells

Enzymes Luminal digestion

• Fat

• Protein

• Carbohydrates

Produced by “acinar” cells

pancreatic secretions regulation cephalic and gastric phase

pancreatic secretions reg7ulation intestinal phase

pancreatic secretions - proteolytic chymotrypsin enzyme

Chymotrypsin

• Trypsin activates

– Chymotrypsinogen → chymotrypsin

• Endopeptidase • Cleaves peptides with aromatic AA in –COOH position – Phenylalanine (Phe) – Tryptophan (Trp) – Tyrosine (Tyr)

pancreatic secretions - proteolytic trypsin enzyme

Trypsin

• Enterokinase activates

– Trypsinogen → trypsin

• Endopeptidase

• Cleaves peptides with basic AA in –COOH position – Arginine (Arg) – Lysine (Lys)

pancreatic secretions - proteolytic elastase enzyme

Elastase

• Trypsin activates

– Proelastase → elastase

• Endopeptidase

• Cleaves peptides with aliphatic AA in –COOH position – Glycine (Gly) – Alanine (Ala) – Valine (Val) – Leucine (Leu) – Isoleucine (Ile)

pancreatic secretions - proteolytic carboxypeptidase enzyme

Carboxypeptidases

• Trypsin activates

– Procarboxypeptidase → carboxypeptidase

• Exopeptidases (-COOH terminus)

– Peptide peptide + free AA

• 2 forms – Cleave aromatic and aliphatic AA – Cleave basic AA

pancreatic secretions - amylolytic a-amylase enzyme

a-amylase

• 2 types of starch – Amylose – Amylopectin

• Cleave a-1,4 glycosidic linkages

• End-products – Maltose, maltotriose, isomaltose

High dietary starch ↑ secretion

• Dog: up to 6X

• Cat: up to 2X

– Tolerate less starch than dogs

– Diarrhea when intake is too high

pancreatic secretions - lipolytic lipase enzyme

Pancreatic lipase

• Breaks down triglycerides into:

– Monoglycerides – Diglycerides – Free fatty acids (FFA) – Glycerol

• Ester linkages #1 and #3 preferred

Requires colipase and bile to function Colipase

• Trypsin activates –> Procolipase colipase

• Changes configuration of lipase – Opens active site

bile

• Not an enzyme

• Produced by liver

• Stored in gallbladder

Composition

– Electrolytes (Na, Cl)

– Buffers (HCO3 - )

– Pigments

» Bilirubin (reddish-brown) in omnivores and carnivores

» Bilivardin (green) in herbivores

» Excretory products (hemoglobin breakdown)

bile acids

Synthesized from cholesterol

• Cholic acid

• Chenodeoxycholic acid

Function

• Emulsifies lipids

– Disperses into small droplets (micelles)

– ↑ surface area • ↑ long-chain fatty acid solubility

conjugate vs nonconjugated bile acids

Conjugation is crucial for micelle formation

Simple oil (TG) unstable in water emulsions

Surface active agents stabilize emulsion particles

• ↑ concentration of bile acids at oil/water interface

• ↓ energy required for emulsification

why is conjugation required?

Need ionized (BA- ) bile acids to be effective

How do we determine how much will be in ionized vs. nonionized forms?

pH of gut vs. pK of substance

small intestinal mucosa enzymes

Disaccharidases

Disaccharides → monosaccharides

Peptidases

• Aminopeptidases

• Dipeptidases

• Result: small peptides and free AA

Enterokinase

• Stimulates trypsinogen

terminal ileum

End of H/E digestion in dogs and cats

Virtually all starch digested

90-95% fat digested

80-85% protein digested

1. What are proteolytic enzymes? Why are they secreted as proenzymes?

• Proteolytic enzymes digest proteins by breaking peptide bonds (ex: trypsin, chymotrypsin, elastase).

• They are secreted as inactive proenzymes (zymogens) to prevent self-digestion of the pancreas and intestinal tissues.

• They are activated only once they reach the small intestine (e.g., trypsinogen → trypsin by enterokinase)

1. What 3 places do secretions that are active in the small intestine come from?

• Pancreas

• Liver (bile)

• Small intestinal mucosa

1. What are the 3 phases of pancreas secretion regulation? What triggers each phase?

• Cephalic phase: sight, smell, taste, thought of food (neural)

• Gastric phase: food in stomach and stomach distension

• Intestinal phase: chyme entering the small intestine (major phase)

1. What are endopeptidases? (Break down proteins at certain amino acids within the protein)

• Enzymes that break peptide bonds within the protein chain

• Target specific amino acids inside the protein (not the ends)

• Examples: trypsin, chymotrypsin, elastase

1. What are exopeptidases?

• Enzymes that remove amino acids from the ends of proteins

• Break down proteins by removing specific amino acids from the C-terminus end of the protein

• Pancreatic carboxypeptidases remove amino acids from the C-terminus

• Release free amino acids one at a time

1. What does amylase digest?

• Starch

• Cleaves α-1,4 glycosidic bonds in amylose and amylopectin

• Produces maltose, maltotriose, and isomaltose

1. What does pancreatic lipase digest? What other compounds are required for it to function?

• Digests triglycerides into:

  • Monoglycerides

  • Diglycerides

  • Free fatty acids

  • Glycerol

• Requires:

  • Colipase (activated by trypsin)

  • Bile (not an enzyme)

1. What is the function of bile salts?

• Emulsify fats by forming micelles

• Increase surface area for lipase

• Increase solubility of long-chain fatty acids

1. What are bile salts conjugated with? Why is this important?

• Conjugated with:

  • Glycine (glycocholic acids)

  • Taurine (taurocholic acids; only form in dogs and cats)

• Conjugation:

  • Lowers pK

  • Keeps bile salts ionized at intestinal pH

  • Makes micelle formation efficient and stable

fermentation vs respiration

Respiration

Presence of oxygen and Substrate completely oxidized

• 673 kcal/mole

Fermentation

Absence of oxygen (anaerobic)

Substantial energy retained in endproducts

• Can usually be broken down further by host

3 Examples:

• Glycolysis

• Alcoholic fermentation (yeast)

• Mixed GI fermentation – SCFA + gas are major end-products

H/E vs Fermentation

Advantages: H/E

No loss of energy or protein

Advantages: fermentation
Able to convert indigestible substrates into energy High quality essential nutrients produced by microbes • Protein; B vitamins N recycling

Degree of importance depends on preor post-gastric fermentation

Ruminants: 70% total ME derived from SCFA in rumen Horse: 40-60% of ME (colon;cecum)

Pig: 10-75% (colon; cecum)

Dog: 10-25% (colon only)

Cat: probably less than 10% (colon)

Human: less than 10% (colon)

pre-gastric fermentation advantages and disadvantages

Advantages

Use of microbial cells

Detoxification of plant compounds

More extensive digestion of fibrous material

Disadvantages

Susceptible to certain diseases

• Acidosis Limited consumption of fibrous, poorquality forage

post-gastric fermentation advantages and disadvantages

Advantages

Lower loss of energy in form of heat and CH4

• High quality nutrients degraded and absorbed prior to fermentation

Disadvantages

Unable to consume/use microbial cells

• Unless coprophagy is practiced

fermentation substrates - exogenous

Nonstarch polysaccharides (NSP)

Resistant starch

Unabsorbed sugars and sugar alcohols

Oligosaccharides

Dietary protein

fermentation substrates - endogenous

GI secretions

Mucus

Urea

Bacterial cells

Sloughed epithelial cells

colonic microbiota

Highly complex ecosystem
Dense population 1011 microbes/g digesta Entire body: 1013 cells Microbes in colon: 1014 cells

luminal colonic microbiota

associated with food particles

mucosal colonic microbiota

associated with gut epithelia

types of bacteria present in colonic microbiota

Good: bifidobacteria, lactobacilli

Bad: salmonella

Neutral: most E. coli

carbohydrate fermentation

Short-chain fatty acids (SCFA)

• 3 Main SCFA – Acetate (~60%) – Propionate (~25%) – Butyrate (~15%)

• Trophic effects on gut

• Preferred energy of colon cells • Reduce pH – limit pathogens

• Ethanol

• Lactate

• Succinate

• Bacterial cells

• Gases • CO2 , H2 , CH4