1.3 Comparative Nutrition - Digestibility and Availability Flashcards
Not all nutrients in feed are available to the metabolism of animals. When do losses occur?
- during digestion
- during absorption
- when absorbed nutrients can not be metabolized
- endogenous losses
How is “digestibility” measured?
Disappearance from gut (either at the ileal level, or in the feces/excreta)
Nutritional Value: AA
Crude Protein
Total AA
Total Tract Digestible AA
Apparent Ileal Digestible AA
Standardized Ileal Digestible AA
True Ileal Digestible AA
Available AA
How is the digestion coefficient and digestibility expressed?
Digestion coefficient: expressed as a proportion of 1
- ex. for every 1g of nutrient put into the animal, 0.92 (coefficient) is used
Digestibility: expressed as a percentage
- ex. 92%
Total tract digestibility (digestion coefficient)
(nutrient intake - fecal excretion)/ nutrient intake
- considers nutrient losses during digestion and absorption
What are 2 disadvantages of total tract digestibility (digestion coefficient)?
- Endogenous losses ignored (underestimate)
- LI fermentation included; interferes with digestibility measurements, esp AA
Ileal digestibility (digestion coefficient)
(nutrient intake - nutrient in ileal digesta)/ nutrient intake
- collection of digesta at the terminal ileum
What are 2 disadvantages of ileal digestibility?
- Need to surgically modify (cannula at terminal ileum) or euthanize animals
- No info on post-absorptive nutrient utilization
Gut endogenous nutrient losses occur as:
- salivary, gastric, pancreatic and biliary secretions
- sloughed-off cells
- bacterial protein
- mucin
Gut endogenous nutrient losses are affected by:
- type of feed
- feed processing
- nutrient composition of feed
- protein and fiber levels, and ANFs
3 types of ileal digestibility
- Apparent = (ingested - excreted)/ ingested
- does not account for gut endogenous losses - Standardized = ingested - (excreted + basal endogenous AA losses)/ ingested
- correction for basal endogenous losses which occur naturally as a process of the animal being alive - True = ingested - (excreted + total endogenous AA losses)/ ingested
- correction for ALL endogenous losses
Why is it important to measure endogenous losses?
If we don’t have an understanding, we’ll underestimate the amount of AA available to the animal from the diet
2 types of endogenous AA losses
- Basal: independent of feed ingredient composition
- used to estimate standardized (ileal) digestibility (SID)
- widely used in swine, poultry moving towards SID from AID - Specific: induced by feed ingredient characteristics
- levels and types of fibre
- ANFs
- used to estimate true (ileal) digestibility (TID)
What are basal endogenous losses?
Minimum quantities inevitably lost by the animal
- constant, regardless of diet effect
- as protein increases in the diet, basal losses do NOT change
What are specific endogenous losses?
- Induced by specific feed ingredient characteristics such as fibre and ANFs
- Increases linearly with increasing protein from ingredient
What are dietary losses?
- Undigested AA from ingredient
- Increases linearly with AA provided as ingredient
3 techniques for collection of ileal digesta
- Slaughter technique
- Cannulas
- Ileo-rectal anastomosis
Ileal digestibility flow measurement
Use indigestible markers
- longer in digestive tract indicates a higher rate of absorption
Definition of availability
Proportion of ingested nutrient (eg. AA) that is absorbed in a chemical form that is potentially suitable for metabolism
- to support bodily fxns (maintenance, growth, etc.)
What is availability commonly applied to?
- AA
- Minerals
- Vitamins
How is availability estimated?
Slope-ratio assay
- estimate for digestive and post-absorptive utilization of nutrients
- costly and time-consuming
- uses reference substance (ex. L-lysine-HCl); expresses availability relative to reference
Digestibility vs availability
Digestibility
- does not include inefficiencies that result from poor utilization of absorbed nutrients for its required role in metabolism
Bioavailability (availability)
- includes inefficiencies in both absorption and metabolic utilization
*This difference can result in a large divergence btw a nutrient’s digestibility and bioavailability values
What does heat create?
Maillard products
- absorbed, but excreted in urine
- with acid hydrolysis before AA analyses, reverted back into intact lysine = overestimation of intact Lys in ingredient and thus overestimation of Lys digestibility
- Need to go beyond simple AA analyses for heat-treated ingredients: slope ratio
Explain how D- and L-lysine have similar digestibilities but very different bioavailabilities
Both are absorbed efficiently
- Only L-lysine can charge tRNA for use in protein synthesis
- D-lysine is absorbed but cannot be converted to L-lysine for use in protein synthesis; instead it is oxidized or excreted, and thereby of limited nutritional value to the bird
- Thus, D- and L-lysine have similar digestibilities but very different bioavailabilities
Not all of a particular nutrient present in a feed is available for use by the animal. What can “losses” occur from?
- The digestive tract loses some of that nutrient through secretions- cell losses, enzymes, etc. (basal and specific endogenous losses)
- Not all of a particular nutrient is liberated from protein and absorbed from the feed (ingredient specific)
- Not all of a particular nutrient that is absorbed can be used metabolically by the animal (ex. Maillard products cannot be used in protein synthesis)
