Part 2 Flashcards
3 point definition of indispensable amino acids
1) performs identifiable function
2) Abnormality results if omitted from the diet - and is corrected when added back in
3) Required in diet because the body cannot make it or make it fast enough to satisfy demand
How are nutritional and metabolic essentiality different?
In a strict metabolic sense only Lys, Thr and Trp cannot be synthesized
Nutritional sense:
Ex. Leucine can be produced by transaminating a keto acid
Ex. Arginine can be produced with excess citrulline in the diet
Chickens used to test essentiality of Phe and how the experiment was set up?
Results?
Algae consumed C13 and produced protein which was fed to chickens
Chickens laid eggs containing C13 which were tested, along with slaughter chickens
Over time the amount of C13 available in the egg increased and the amount of unlabelled carbon decreased
- Implies that proteins synthesized in the egg were synthesized with dietary Phe, not endogenous (indispensable AA)
- When chicken was euthanized, highest content of Phe was in liver, then kidney and muscle had the least
Chicken/egg experiment with Glx (Glu + Gln)
Glutamate is a fuel for the gut and used to transport amino groups between organs (high flux rate)
After 28 days there was a lower proportion of regular glutamate but very little labelled glutamate in the egg, and virtually no labelled protein in chicken tissues
- Gut consumed glutamate
- Endogenous was used to synthesize proteins
Thus GLX is dispensable
Definition of AA requirement
What is the response in a response-dose curve?
Minimal intake level which represents a single point on a response-dose curve and that is sufficient to maintain a specific criterion of nutritional adequacy
Nitrogen balance or oxidation as the curve approaches 0 line with increasing dose and reaches breakpoint (requirement)
Ways of measuring AA requirement?
Methods of measurement:
Growth
Plasma AA response (not a good measure)
Nitrogen balance
Direct AA oxidation
Indirect AA oxidation
24 hour AA balance
Measure of organ or system function
General guidelines:
All methods should give the same answer
Subjects should test >/= 6 intake levels above and below requirement
Endpoint should show clear response to change in test AA intake
N = 7-10 to allow for urea adaptation
Measure CO2 for hours to a couple of days
Essential AA Direct Oxidation Requirement Testing Method
Choose a label that when catabolized is released and exhaled as CO2
Ex. Leu or Phe, starts as limiting AA
As dose increases, oxidation of the AA increases (low dose —> low oxidation, high dose —> high oxidation)
- Breakpoint is when oxidation begins to increase and this is the requirement level
- Growth curve is the inverse
Clinical and metabolic limitations of direct oxidation AA methods
Analytical
Modelling
Clinical/metabolic:
Choice of AA restricted to branched AA (Leu, Ile, Val), Lys and Phe because of the carboxyl carbon released
Free pool changes with changing test AA intake (free pool increases with increasing intake)
Can’t study very low intakes - non-negligible tracer
Analytical:
Breath collection is expensive using Isotope Ratio Mass Spectrometry for CO2 enrichment and calorimetry
Blood samples evaluated using GCMS
Modelling:
requires steady nibbling of crystalline AA
Breakpoint increase in oxidation with increasing intake
Indicator AA oxidation overview
When an indispensable AA is limiting, then all other indispensable AA will be oxidized because protein cannot be synthesized
Ex. as lysine doses increase, oxidation of Phe decreases
An increasing intake of indispensable AA will decrease indicator AA oxidation - indicator AA is being used for protein synthesis
Graph: Oxidation vs. Dose (starts high and decreases until it flatlines at breakpoint)
Breakpoint is the estimated requirement (EAR)
Indicator AA used for indicator AA oxidation (IAAO) - Clinical aspects
Analytical Aspects
Modelling Aspects
Clinical Aspects:
Indicators: Phe (main), Lys and Leu
Test: can test any AA and at zero intake level
Analytical Aspects:
Breath collection via IRMS isotope ratio mass spec (CO2 enrichment) and calorimetry (CO2 production)
Blood and urine samples (GCMS for AA enrichment)
Modelling:
Steady state nibbling and 24 hour collection
Breakpoint decrease in oxidation with increasing intake
How was the nitrogen balance method reanalyzed to get the same or similar AA requirement results?
Using modern statistical approaches and incorporating the miscellaneous losses of N moved 0 threshold for Nitrogen balance by 5-10 mg/kg/day which translated to new recommendations
Former requirement, new EAR and RDA for lysine
Formerly 12 mg/kg/day
EAR: 31 mg/kg/day
RDA: 38 mg/kg/day
Total mg of IAA per g of protein needed to meet requirements?
1985 WHO: 111 mg/g protein
WHO 2002: 277 mg/g protein
Plant protein sources of protein are often low in potentially limiting AA
- Importance of combing plant-based protein sources for complementary protein
- Ex. Beans and rice, legumes and grains
- Vulnerable groups: vegans and children
AA recommendations which did not change drastically?
AA recommendations which did change drastically?
Trp: 3 —> 4 mg/kg/day
Met + Cys: 13 —> 15 mg/kg/day
Ile: 10 —> 15 mg/kg/day
The rest nearly doubled, and His gained a recommendation of 11 mg/kg/day
Histidine requirement and how it was discovered
Decreased and increased markers in blood
Role of His in hemoglobin?
11 mg/kg/day
No effect on nitrogen balance but protein turnover decreases on a 48-day His-free diet
- Studied using IAAO but didn’t fit the model - Plasma [His] levels decrease over 48 days
Decreased: hematocrit (blood volume), hemoglobin (high in His), transferrin (binds Fe for transport, low when excess Fe available) and albumin (impaired protein synthesis)
- AKA hemoglobin synthesis impaired
Increased: Ferritin (protein that binds iron for storage, more iron stores as iron not being used to make hemoglobin)
His stabilizes binding of iron in hemoglobin and is required for hemoglobin synthesis
