WEEK 5 isotopes Flashcards
What are isotopes and what makes them different?
Certain elements are composed of atoms that are chemically identical but of a slightly different weight due to different numbers of neutrons
How is isotopic enrichment expressed?
expressed as the tracer:tracee ratio (TTR), corrected for the baseline TTR or as atom % excess (APE) or mole % excess (MPE)
What is very important when working with isotopes?
Measuring the background levels is very important
Nomenclature:
[2H7] = ?
[U-13C] = ?
7 atoms of 2H
Every 13C has been replaced by a stable isotope (U = uniformly labelled)
Why is it important that when working with stable isotopes, the quantity is low?
Because stable isotopes tracers have mass:
You dont want the concentration of the molecules (or elements) being measured to significantly change
What is tracer vs tracee?
Tracer = labelled compound
Tracee = original molecule
What is the most widely used method for measuring molecules labelled with a stable isotope?
Gas chromatography mass spectronemy
What is the principle and application of isotope dilution?
Add known amount of tracer -> mixes in body pools -> reach steady state (tracer same loss rate as tracee = flux) -> sampling body pools -> measure dilution = measure of size of body pool
Application: measurement of body composition
What are assumptions for isotope dilution?
- Stable tracee
- Assumed tracer dilutes in a single body pool (e.g. plasma)
When is a priming dose added?
When you have a prolonged duration of tracer infusion due to a large body pool size relative to flux.
What is the principle of the dual isotope method?
Tracer of the molecule/element of interest is given orally with a meal and a tracer that is distinguishable from the oral tracer is given itravenously (or both meal), isotopes are analyzed: different in appearence reflects absorption
Application of dual isotope method? Requirements?
Measuring nutrient absorption. Or bioavailability
Mathematical models required
Application of precursor product method?
Application: measuring macromolecule synthesis rates.
Dual tracer technique for measuring mineral absorption (calcium) :
Method: Stable isotope of calcium in meal -> second isotope iv -> oral tracer mixes with centrum Ca pool, iv Ca normalises for variations in calcium pool mass between individuals -> urine collection: analysis of isotopic enrichment with the two tracers.
How is fractional calcium absorption calculated?
as the ratio of the oral to the iv tracer recovered in urine
Tracers are also used to measure..
Vit A bioavailability
How is muscle protein synthesis measured?
- can be measured by the incorporation of isotopically labelled amino acids into muscle protein, sampled by needle biopsy
Leucine
When is a biomarker sensitive?
> Sensitivity is high, if a biomarker captures true intake, or if a biomarker identifies an abnormal nutritional status (e.g. deficiency) accurately
When is a biomarker specific?
> Specificity is high, if biomarker is not influenced by other factors, or if the biomarker accurately
identifies a normal nutritional status
Pros of biomarkers of intake vs interview/record methods?
▪ Objective: not dependent on memory, truthfulness, or social desirable answers
▪ May be less time consuming for subject and researcher
▪ Easier quality control
▪ Not just a reflection of intake; includes bioavailability
Cons of biomarkers of intake vs interview/record methods?
▪ Several important nutrients lack good biomarkers
▪ No data on food distribution over the day or on eating behaviour
▪ Effect of food intake is mixed with effects of genome and metabolism
True or false: Biomarker is not restricted to nutrient intake. They may be affected by other factors, e.g. environment or genes
True
Measurement of biomarker in blood: what type of uptake and what do you need to know?
Fast uptake, you need to know the pharmacokinetics of the nutrient
Graph: how soon does the biomarker (nutrient) appear in the blood? =
peak
How soon is the biomarker (nutrient) eliminated?
Half-time needed: time in which nutrient drops by 50%
E.g. peak at 4 hours
50% of nutrient at 12 hours
12-4 = 8
When do you measure nutirent/biomarker in tissue? What do you measure? What does it reflect?
When you have a nutrient with a cumulative value that is storaged there. You measure concentration. May reflect long term exposure.
Is measuring in tissue a good reflection of absolute amount?
No. But you can rank people.
What value do you measure when you sample a nutrient in urine/faeces? What type of exposure?
Cumulative value (concentration or absolute amount).
Values reflect medium-term exposure (up to few days)
Recovery biomarkers are a direct reflection of..
direct reflection of nutrient intake (recover the nutrients)
Concentration biomarkers are a reflection of..
nutrient status (value about the level)
Recovery markers are measured in..
Measured in bodily excretions (mainly 24-hour urine)
Assumptions recovery marker? Less suitable for?
homeostasis (steady state) e.g. constant weight, no accumulation or depletion.
less suitable for growing children, elderly, pregnant women, kidney disease, some medication
Name three examples of recovery markers
protein (N), Na (sodium), K (potassium)
Concentration biomarkers: relation with intake?
Not consistent
> cannot be translated to absolute intake without further references
What is are concentration biomarkers measured in?
▪ Measured in urine samples, plasma and tissues
Examples of concentration biomarkers?
Examples: fatty acids, vitamins, Fe
When does concentration of a nutrient no longer reflect intake?
When it is under homeostatic control
Plasma nutrient level depends on which factors?
Intake: pharmacokinetics
Body needs: homeostatic control
Plasma values usually highly time-dependent & difficult to model/predict
What technique is used for measuring protein intake?
Urinary nitrogen excretion: measure for protein intake at preceding days, because protein is the major dietary N-source
Pros of urinary nitrogen intake?
▪ Validity high
▪ Analysis simple (Kjeldahl)
▪ Samples are stable
Cons of utrinary nitrogen intake?
▪ Requires 24-hour urine collections
- for group mean, one day is enough
- due to large fluctuations within subjects → multiple collections for determination of individual intake
▪ Only for total protein, not for specific proteins or amino acids, or plant vs animal protein (stable isotopes needed)
No biomarkers available for. …. then what should you do?
total fat and carbohydrates
Need FFQ or food records
You do not have a biomarker for total fat, but you do have it for specific types of fat.
What period of time do the following biomarkers reflect?
- Fatty acids in serum triglycerides
- Fatty acids in erythrocyte- or trombocyte membranes
- Fatty acids in fat tissue
- Fatty acids in serum cholesteryl esters
- Fatty acids in serum triglycerides
Reflects intake previous day - Fatty acids in serum cholesteryl esters
Reflects intake previous few weeks - Fatty acids in erythrocyte- or trombocyte membranes
half-life ~60 days
Reflects intake previous few months - Fatty acids in fat tissue
tissue half-life ~1-2 years
Reflects intake previous few years
How is alcohol measured short- and long term?
- Short term:
Exhaled air breath test - Long term:
Mean corpuscular volume of red cells (elevated in alcoholism)
Gamma-glutamyltransferase (GGT) (elevated in liver disease)
Carbohydrate-Deficient Transferrin (CDT) in plasma (elevated in alcoholism)
Sodium in plasma/urine is a measure for intake
Urine! Not plasma
Urinary excretion of sodium equals …% of intake. What about the validity and reproducability?
Urinary excretion equals ~90% of intake.
- Validity: high, but reproducibility is poor. You need the average of 10-20 repeated 24-hour urine collections to get a valid estimate of individual intake
Pro’s of sodium excretion measurement?
Pro
▪ Much more valid than questionnaire data
▪ Simple analysis
Cons of sodium excretion method?
▪ Large error
▪ Only suitable for short-term intake
Is potassium in urine a good measurement for intake?
Potassium in urine is a reasonable measure for intake, BUT error is larger than for sodium because of high K excretion in faeces
How do you measure calcium intake? What are its properties?
Calcium in serum is tightly controlled
in faeces plus urine is doubtful measure for intake
But not really necessary because FFQs work quite well (bias with food diaries close to zero, 95% limits of agreement ± 200 mg)
How is selenium measured? Confounder?
Long term selenium intake can be estimated accurately
from nail or erythrocyte content of Se. ● CONFOUNDER: Dandruff shampoos
Serum iron is too variable to detect intake. How is iron status regulated in the body?
▪ Iron intake
▪ Bioavailability of dietary iron (heam, non-heam)
▪ Blood losses (e.g. menstruation; helminths)
How to measure zink?
No good indicator available. Only used to monitor populations.
Vit A: how is it regulated? How is it measured?
Concentrations tightly controlled by the liver
▪ Serum and plasma vitamin A are a useful measure for long-term low intakes.
- Not good to monitor changes in intake at high intakes
Name four good biomarkers
- Energy
- Nitrogen
- fatty acid compo
- sodium
Name four fair biomarkers
Alcohol
Potassium
Some trace elements
Most vitamins
Is usually only dietary questionnaires used or biomarkers?
Combo
