1
Q
What does the GI tract consist of?
A
Mouth, esophagus, stomach, small intestine, large intestine, caecum, rectum
2
Q
what components are added to the GI tract to create the digestive system?
A
- salivary glands, gall-bladder, liver, pancreas
3
Q
general function of the mouth in digestion
A
- mechanical breakdown of food
4
Q
general function of the esophagus in digestion
A
- unidirectional tube that connects the mouth to the stomach
5
Q
general function of the stomach in digestion
A
- holds food
- distinguishes the 4 systems from each other
- can be monogastric, avian, or 4 compartment
6
Q
general function of the small intestine in digestion
A
- takes up nutrients into the body
- “nutrient gateway”
7
Q
general function of the large intestine in digestion
A
- flows into caecum
- majority of gut bacteria resides here
8
Q
general function of the caecum in digestion
A
- abundance of gut bacteria
- roll in breaking down indigestible carbs
9
Q
general function of the rectum in digestion
A
- things not digested are excreted from the body
10
Q
what are different ways you can classify a dietary CHO by for digestion?
A
- solubility
- digestability
- fermentability
11
Q
solubility of CHOs
A
is a CHO soluble in the aqueous environment of the digestive tract?
yes = soluble
no = insoluble
- determined by physicochemical properties of CHO
12
Q
digestability of CHOs
A
does the host organism have the enzymes necessary to digest a CHO?
yes = digestible
no = not digestible (fibre)
- determined by the host digestive enzymes
13
Q
fermentability of CHOs
A
do gut bacteria have the enzymes necessary to break down a CHO?
yes = fermentable
no = non-fermentable
- determines by gut bacteria
14
Q
4 types of digestive systems
A
- simple system (w/o caecum)
- simple system (w/ functional caecum)
- ruminant system
4.avian system
15
Q
simple system w/o caecum: key features
A
- human, cat, pig, dog
- monogastric (single stomach)
- non-functional caecum (still there but does not contribute to fermentation of digestible CHO)
- hindgut fermenter
- suited for a nutrient-dense, low fibre diet
16
Q
how does the oral cavity work in a simple system?
A
- food is chewed and mixed with saliva
- 2 enzymes get the ball rolling…
1. a-amylase: starts to break down CHO
2. lingual lipase: starts to break down dietary fats
17
Q
How does the stomach work in a simple system?
A
- includes the cardia, fundus, body and antrum
- empty = 50mL, filled = 1-1.5L (remarkable ability to expand when food is consumed)
- gastric emptying takes 2-6 hours
- pH is about 2 (acidic)
- food becomes chyme (food + acid)
- gastric glands secrete gastric juice (includes H2O, electrolytes, HCl, enzymes)
18
Q
how does the small intestine work in a simple system?
A
- includes the duodenum, jejunum and ileum
- main site for nutrient digestion and absorption
- surface area = 30m^2
- intestinal motility is controlled by longitudinal and circular muscles
- chyme acidity neutralized by pancreatic juice
- food is digested by pancreatic juice and bile acids
- soluble fibre = slows down digestion, insoluble = speed up
19
Q
how does the large intestine (aka colon) work in a simple system?
A
- by the time food reached the LI most nutrients have already been taken up
- site of fermentation - break down indigestible carbs
- production of SCFA aka volatile fatty acids
- site for water absorption
- bile acids are taken back up and recycled
20
Q
What gives the small intestine such a large surface area
A
- kercking folds
- villi (and crypts) - pertrusions
- microvilli
21
Q
What are the different nutrient transport mechanisms for molecules to pass through the intestinal lumen?
A
- Diffusion
- molecules follow a concentration gradient
- small molecules with the right chemical properties can pass through with no energy needed - Facilitated diffusion
- like diffusion but involves a transporter - Active transport
- requires energy (ATP) to move molecules against a concentration gradient
22
Q
how can we decide which transport mechanism to use to get molecules from the intestinal lumen to the cytoplams
A
- solubility of the molecule
- concentration gradient
- molecular size
23
Q
Gut bacteria
A
- region specific - not evenly distributed along the tract
- 1000:1 anaerobic to aerobic bacteria
- very important for the fermentation of non-digestible CHO
- CHO fermentation produces many compounds (e.g lactate and SCFA)
- associated with disease
24
Q
simple system w/ functional caecum: key features
A
- horse, rabbit and hamster
- pseudo-ruminant
- hindgut fermenter
- functional caecum
- all other regions of the gut function similar to the monogastric system
- suited for a diet with large amounts of fodder and foraging
25
what is the purpose of a functional caecum in a simple system?
- enormous hindgut (20-30L capacity) filled with bacteria
- SCFA provide 70% of total energy needs for host
- site for the production of vitamins
26
what are signs of an energy or nutrient deficiency for animals with a functional caecum?
- Coprophagy (eating dung or feces)
- young animals eating feces colonize their guts with bacteria
27
where are nutrients absorbed in a simple system w/ a functional caecum?
stomach: nothing
small intestine: glucose, amino acids, fatty acids
large intestine/caecum: lactic acid, amino acids, SCFA
- indigestible fibre does not get absorbed
- few of the fat, proteins and CHO are not digested and are excreted from the body
28
lactic acid in horses
- lactic acid is produced in a diet high in carbs for horses
- horses cannot vomit
- colic will develop if there is too much lactic acid produced
29
Ruminant (multiple) system: key features
- cattle, sheep, goats
- stomach is divided into 4 regions: reticulum, rumen, omasum, abomasum
- system highly suited for animals that eat a high quantity of fodder and forage plant materials
- all other regions of the gut function similar to the monogastric system
30
ruminant digestion: reticulum
- honeycomb appearance serves as a filter: can capture and trap foreign materials that are accidentally swallowed
- can cause hardware diease
- rich in bacteria (fermentation vat)
31
ruminant digestion: rumen
- the largest section of the stomach
- rich in bacteria (fermentation vat)
- rumen papillae: similar to vili, increase surface area for absorption
- food is mixed and partially broken down and stored temporarily
- 60-80% of total energy produced here as SCFA
32
ruminant digestion: omasum
- resorption of water and some electrolytes
- filters large particles
33
ruminant digestion: Abomasum
digestive enzymes secreted from gastric glasnds (HCl, mucin, pepsinogen, lipase, etc)
34
fermentation in the ruminant system
- fermentation takes place before entering the intestine (foregut digestion)
- nutrients produced by bacteria are available for digestion and absorption by the ruminant
2 ways to get rid of products of fermentation...
1. rumination: partially digested food called cud is regurgitated and chewed further then re-swallowed
2. eructation: periodic belching where methane gas is released from the stomach - can be bad because energy is lost
35
pros and cons of the ruminant system
pros
- vitamin synthesis (B and K)
- non-protein nitrogen used for making protein
cons
- carbohydrates degraded into gases and lost through eructation
- heat production (large methane released)
36
avian system: key features
- chickens, turkeys
- beaks and claws break up foods into smaller pieces that birds can swallow (don't have teeth)
- rapid digestion (can starve if deprived of food for even a few hours)
37
unique components of the avian system
- crop
- two chambered stomach
- small intestine
- ceca
- large intestine (slight different function)
- cloaca
38
avian system: crop
- enlarged area of the esophagus
- well developed in most species but not in all
- temporary storage location for food
- food is softened here and often regurgitated to feed offspring
39
avian system: two-chambered stomach
Glandular portion: proventriculus
- chemical digestion, gastric enzymes and HCl secreted
- most comparable with monogastric stomach
Muscular portion: Gizzard
- has stone-like objects used for mechanical breakdown
- grind and digest tough food
40
avian system: ceca
- minor site of bacterial fermentation
- fermentation is not critical for avians but happens because bacteria are here
41
avian system: large intestine
- very short and serves predominantly to connect the small intestine and cloaca
- a bit of storage of undigested material
- water absorption
42
avian system: cloaca
- very unique
- where the digestive, urinary and reproductive system meets
- makes it difficult to understand how much is actually absorbed in birds
43
What is digestibility?
- measure of the fraction of a specific nutrient (or of energy) that is extracted by the GI tract
-how much nutrient actually gets absorbed by the body
- calculated from the amount of nutrient consumed and the amount appearing in feces
- represents a combination of nutrient release from the food matrix, microbial fermentation and absorption
44
why is determining digestibility important?
- can prevent deficiency and ensure essential nutrients are available to the organism ( to support their lifestyle)
45
measuring digestibility: total collection method
- measures everything that goes in (food) and comes out (feces)
- allow the animal to adapt to a diet over 7-21 days
- isolate the animal for quantitative analyses
- measure intake over a 3-10 day period
- collect and weight all feces
- analyze for nutrient
46
how do you calculate the apparent digestibility coefficient?
apparent digestibility coefficient = (total intake - total feces) / total intake
the closer to 100%, = almost complete absorption of the nutrient
47
limitations of the total collection method
- accuracy in measuring food intake (e.g. crumbs)
- metabolic cages create anxiety in animals, causing them to behave abnormally
- labour intensive
- animals confined in costly equipment
- not feasible for captive wild animals (not much control)
48
what do metabolic cages do
collect and analyze urine and feces
49
measuring digestibility: Indicator method
- incorporates a marker into the food and tracks it
- internal marker: a natural component of the feed
- external marker: a component added to the feed
- adapt the animal to test diet containing the marker
- collect a feed and fecal sample
- analyze each for the marker and nutrient of interest relative to your indicator
50
what is the difference between the total collection method and the indicator method
- indicator method just requires a sample since the marker will appear in both the feed and feces
51
what are characteristics of a marker for the total collection method?
1. non-absorbable
2. must not affect / be affected by the GI tract (not upset stomach)
3. must mix easily with the food
4. easily and accurately measured in samples
52
apparent digestibility coefficient for indicator method
(A - B) / A
A = ratio of nutrient/marker in the feed
B = ratio of nutrient/marker in feces
53
advantages of indicator method
- less labour intensive
- ideal for wild animals
54
What is apparent digestibility?
- under-estimates the true digestibility
doesn't take into account...
1. endogenous secretions
- epithelial cells (e.g. fatty actids released from dying intestinal cells)
- secreted into lumen and appear in feces
2. Bacterial growth in gut
- nutrient synthesis takes place (e.g. biotin produced by gut bacteria)
3. Digestive enzymes
- protein secretion (e.g. digestive enzymes released by cells)
55
What is true digestibility?
- perform a digestibility study using a test diet
- switch to diet containing none of the nutrient of interest
- analyze the feces after test diet is cleared
- subtract the level of nutrient in feces of animals fed the zero nutrient diet from a test diet
56
how can you calculate the true digestibility coefficient?
use indicator marker method
(A - (B -C)) / A
A = ratio of nutrient/marker in TEST DIET
B = ratio of nutrient/marker in feces
C = ratio of nutrient/marker in feces after ZERO NUTRIENT DIET
57
what are some factors that affect digestibility?
- feed intake (what are they eating?)
- particle size (smaller particles have faster transit time)
- chemical composition (what nutrients?)
- climate (hotter temps have different digestibility than colder)
- age (younger can have different efficiency as older)
