Lecture 7- VFAs and digestion Flashcards
What is the main fuel the rumen makes?
VFAs
-ruminants do not use glucose like we do
What are the three main VFAs?
- Acetic= mainly as energy source
- Propionic = for lactate production
- Butyric= same as propionic
- Acetic, propionic and butyric are the predominant forms
- Generally ~ 75:15:10 to 40:40:20
- Weak acids (pK
What are the VFAs?
• Volatile Fatty Acids (VFA’s) • Also called short chain fatty acids (SCFA’s) • Produced during microbial fermentation • Acetic • Propionic • Butyric • Formic • Isobutyric • Valeric • Isovaleric • 2-methylbutyric • Hexanoic • heptanoic) -all produced in the rumen
What is the main energy source for ruminants?
• In the 1940’s researchers (mainly from Cambridge) demonstrated that most
of the feed consumed by ruminants is fermented
• Continuing work demonstrated that VFA’s are used by many tissues
• VFA’s are the main source of energy for ruminants
• Except for starch etc. that are digested in the small intestine • Acetate mainly energy
• Propionate contributes to energy, lactate and CO2
• Butyrate contributes to 3-hydroxyl butyrate, CO2 and energy
-the animal use VFAs in
PIC1What are the concentrations of VFAs in different parts of the animal in different animals?
- sheep and cow have it in the rumen
- hindgut fermenters like a horse have it caecum
What does the concentration of the VFAs depend on?
- on the species and the diet
- particularly starch content depends
How are VFAs metabolised?
- VFA’s are transported into the blood via the rumen epithelium • Some VFA’s are metabolised during this process
- ~88% of VFA’s are directly absorbed by the rumen • ~12% make it into the omasum
- VFA’s not absorbed by the rumen are metabolised by the liver
- REMEMBER glucose is NOT absorbed in the digestive tract of ruminants although tissue requirements for glucose are similar to that of other species
- The gut itself is the major consumer of glucose (and the mammary in lactating animals)
What is the liver’s and portal blood flow’s function in digestion?
- the portal vein supplies the liver (from the digestive tract)
- here the VFA’s are made into glucose and can be used
- when fed, right after get an increase in portal blood flow
What is special about the digestion of starch?
-can get glucose from starch directly in the small intestine
• Starch is the exception and glucose can be directly absorbed from starch sources by the small intestine
• E.g. maize or other grains
• Manipulations of starch to ensure it bypasses the rumen is possible
• Commercial products are available
• This increases the glucose supply and can prevent acidocis
-if you can manipulate the starch so it bypasses the rumen can control how much glucose the animal has
What is propionate turned into and where?
- glucose in the liver
- B12 coenzyme is very important for this
- the conversion is energetically costly
What is the acetate metabolism in ruminants?
• Ruminants absorb more acetate than glucose into the blood
• Therefore there are differences in ruminants and non-ruminants in
how acetate and glucose are used as precursors for acetyl-CoA • For later use in oxidation or lipogenesis etc.
• In ruminants, only a small amount of acetate uptake in the liver is low • But the liver does produce acetate
• In ruminants (& pigs) the majority of lipogenesis occurs in adipose tissue
• Unlike humans and birds where it occurs in the liver
• This is related to the metabolism of acetate as Acetyl-CoA is readily used for fatty acid synthesis
-glycolisis happens in the adipose tissue as opposed to humans
What is the propionate and butyrate metabolism like in ruminants?
• Metabolised by both the rumen & gut epithelium and the liver
• Little propionate & butyrate is in the general circulation (unlike
acetate)
• Propionate is the main glucose precursor in the liver
• Liver is glucose factory with little need for or capacity to absorb glucose
• Butyrate → butyryl-CoA → acetyl-CoA → long chain fatty acids or
ketone bodies
• In peripheral tissues butyrate is rapidly oxidized or used in lipogenesis
• Or removed by the mammary for milk fat synthesis
• Propionate → 5 steps → glucose (TCA cycle)
What is the main glucose precursor in the liver?
• Propionate is the main glucose precursor in the liver
What are some of the reactions that happen in the rumen?
-production of VFA and methane as well as many other substances (acetate, lactate etc,)
What are the functions of acetyl co-A?
-used differently in the liver, muscle and adipose tissue
What is the importance of digestive kinetics in ruminants?
• When a feed particle enters a site of digestion/degradation (e.g. the rumen), it can only leave by one of two mechanisms
• Fermentative digestion (ranges from 61 to 85% of OM)
• Passage from the site
• These two processes compete with each other – rapid rate of passage = lower potential degradation
-if something passes through rapidly then the animal will have little time to ferment it than if have more time in the rumen
What are the factors influencing the rate of passage (ie VFA production)?
• Increasing the rumen passage rate will decrease total VFA production
• Associated with reduced DM digestion
• On a given diet, increasing the liquid dilution rate will:
• Increase the production and concentration of acetic acid, butyric acid and methane
• Decrease the production and concentration of propionic acid
-propionic is the most important one for glucose!
What is BMR (basal metabolic rate) important for and how can you establish it?
- Limitations:
- Requires fasting
- This is difficult in ruminants (humans = overnight, birds = 2 days, ruminants & pigs = 4 days).
- Indicators of fasting include a decline in heat production to a steady state, respiratory quotient (0.7 which indicates energy is being obtained from body reserves)
- Needs a state of complete relaxation
- Hard to obtain in animals
- Standing Vs lying can increase heat production
- May be more accurate to call ‘fasting metabolism’
-fasting takes 4 days
-get heat production from breathing
-
What is heat production related to?
- Fasting heat production is more closely related to the surface area of an animal than its weight
• This is difficult to measure
• Thus animals are compared by expressing them in relation to their surface
area (SA)
• SA is proportional to 2/3 bodyweight, Thus W0.67
• In reality it is closer to W0.73
• Rounded to W0.75
What is digestibility?
-• Nutrient analysis of feeds does not tell us:
1. Efficiency of utilisation of nutrients 2. Chemical integrity of nutrients
3. Acceptability (intake)
• Not all the nutrients present in food are available for metabolism.
• Digestibility most important determinant of nutrient availability, or feeding value of feeds
• Measure of efficiency of digestion and absorption of nutrients present in food
• Expressed as either as a percentage (digestibility coefficient), a proportion or g/kg DM
• Dry matter (DM) digestibility is calculated as:
-DM dig (%) = (DM intake - DM faecal/ DM intake) x100
What are the nutrients in facese?
- Nutrient excreted in faeces; • Undigested food residues
- Body tissue residues (enzymes, damaged cells from GI tract lining, gut microorganisms)
- Types of digestibility
- Apparent digestibility not corrected for endogenous losses
- True digestibility corrected
What is an apparent and true digestibility?
- Appdig (%) =(Nutrient intake - Nutrient output/ Nutrient intake) x100
- True dig (%) = (Nutr. Int - (Faecal Nutr. Endog. Nutr.)/ Nutr. Int.) x100
- Endogenous nutrients =body tissue residues
- True digestibility > Apparent digestibility
What are the animal factors influencing digestibility?
- Age of animal
- Underdeveloped enzyme systems or digestive organs in young animals
- Loss of competence with age e.g Lactose intolerance • Species of animals
- Ruminants better able to digest fibre than non ruminants
- Sheep digest grains more efficiently than cattle, while the reverse is true for forages
How does food composition affect digestibility?
- Digestibility is related to chemical composition
- Cellulose less digestible than starch
- Lignin content impinges on digestion
- Deficiencies or excesses of other nutrients in the diets
- N & S deficiencies, restrict microbial growth in the rumen and reduce fibre digestibility
- Excess dietary fat in the diet (> 5%) inhibits rumen micro-organisms and fibre digestion
What is meant by associative effects on digestibility?
- Some foods are less digestible when consumed in conjunction with other foods than on their own
- Associative effects are negative (digestibility of mixed rations is less than expected)
- In ruminants, digestibility of forages is poorer when they are fed in conjunction with starchy feeds
- Starch is rapidly fermented producing lots of VFA that lower pH inhibiting cellulolytic bacteria
How does preparation of food affect digestibility?
- Heat treatment
- Boiling potatoes increases starch digestibility
- Destroys trypsin inhibitors e.g. soyabeans • Maillard product formation
- Chemical treatment of forages
- Forages = grinding reduced rumen digestibility by 20%
- increase intake and outflow, reduces exposure time to enzymes • Grains crush for cattle, grind for pigs
How can you affect digestibility with enzyme supplementation?
• Non-ruminants are not well equipped with enzymes to digest many constituents of foods
• Enzyme preparations added to diets e.g.
• beta-glucanase = destroys glucans poultry in feeds (they interfere with digestion
of other nutrients)
• Phytase = destroys phytic acid, increases availability of P • Cellulases =-linked glucose from cellulose
• Proteases = breaks down proteins
How does level of feeding affect digestibility?
- Feeding levels
- Maintenance = no gain or loss in body weight
- Describe as multiples of maintenance
- increase quantity of food consumed, increase rate of passage through the gut
- reduced digestibility is due to shorter duration of exposure of ingested food to enzymes
What are some anti-nutritional factors?
- Substances that inhibit enzymes or impair to nutrient utilisation • Tannins = bind to proteins and amino acids
- Trypsin inhibitors = impair trypsin activity
- Maillard products = reduced protein & CHO digestion
- Aflatoxins = toxic, also erode gut lining
How can pig’s digestion be affected by time?
- wheat is rapidly digested
- others digested more slowly
- Diet composition can alter transit time in the gut in the pig
What drives & limits intake in ruminants?
- Extremely complex!
- We have discussed some of them • Distention
- Gut fill
- Metabolic signals are less understood in ruminants
- Fuel types differ between monogastrics and ruminants (who consume less fat and more fibre)
- Fermentation means that the type and supply of fuels is different in ruminants
- Some satiety factors are independent of feeding behaviour& others are integrated through their effects on metabolism
- Growing consensus that fuel-based sensing of tissues contributes to the control of intake in ruminants
- Temporal mechanisms in control of intake can have effects that last from minutes to months
- Long term mechanisms that influence maintenance include hormones and cytokines like leptin & TNFα
- Fuel oxidation influences feeding behaviour
- Signal from the liver can be stimulatory or inhibitory
What are some inhibitory signals and stimulatory signals?
- in ruminants these are as follows:
- nhibitory signals (distention etc.) wane &stimulatory signals increase following meals
What is the hepatic oxidation theory?
-theory that liver drives the satiety and digestion in ruminants, contentious
What are some other theories?
- Direct nutrient sensing by peripheral and metabolic tissues?
- Different influences of protein vs. fat. vs. carbohydrate
- Specific receptors
- Particularly in splanchnic tissues
- E.g. G-coupled receptors & glucose receptors
- Role of the CNS?
- Role of the small intestine?
