WEEK 5 tracers

Question 1

When do you use tracers?

Answer 1

Follow nutrients inside the body: ▪ when you are not just interested in nutrient intake or status, but want to follow their fate in the human body

Question 2

What types of tracers are used? characteristics?

Answer 2

 usually stable isotopes
> safe, non-toxic
> expensive
> measurement with mass-spectrometer
> differences in mass (#neutrons)

 less commonly: radioisotopes (unstable)
> measurement with scintillation counters

Question 3

isotopic abundance = formula

Answer 3

tracer/(tracer+tracee) *100%

Question 4

%APE = ?

Answer 4

isotopes - baseline isotopes

Question 5

Extrinsically labeled isotopes are..

Answer 5

taken in externally

Question 6

Intrinsically labelled isotopes are..

Answer 6

taken up intrinsically (e.g. through soil)

Question 7

What tracer methods are used in nutrition research?

Answer 7

▪ Isotope dilution methods
▪ Oxidation rate methods
▪ Dual isotope methods
▪ Precursor-products methods and metabolic flux methods

Question 8

What is the oxidation rate method? Applications?

Answer 8

Measuring a labeled molecule (e.g. 13CO2) in breath samples > measure oxidation rate

▪ applications: gastric emptying, gastrointestinal transit time, bacterial overgrowth in small intestine

(molecule exhaled quicker, gastric emptying quicker)

Question 9

Dual isotope measurement. Isotopes can be given..

Answer 9

1. Oral + intravenous isotope
2. Two oral isotopes

Question 10

Applications of dual isotope measurement?

Answer 10

Applications: mineral absorption, cholesterol absorption, bioavailability studies

Question 11

When do you start to think about the limits of agreement in a bland altman plot?

Answer 11

Before the experiment/analysis!!

Question 12

What are the limits of agreement theoratically?

Answer 12

2 SD, 95% of data always in between.

Question 13

What do the limits of agreement mean? What does it mean if these limits are wide apart?

Answer 13

Assume: differences follow a normal distribution

-> 95% of all data points are within those limits. (In other words, if you measure
100 students you expect that 95 students have a difference between intake and excretion within
those limits.)

This is simply because the LoA are calculated as bias ± 2SD and by definition 95% of all
data fall in that interval.

If the limits are wide apart, the means that the two methods may differ a lot for a single individual

Question 14

What is the difference between limits of agreement and 95% confidence interval around the mean?

Answer 14

a confidence interval is calculated as the mean ± 2*SE,
with the SE = SD/√n. It is a measure of the precision of the estimate of the mean

Question 15

How can you tell whether the difference between intake and excretion is dependent on the amount of X consumed?

Answer 15

If you follow the x-axis from
left to right, could you
imagine a regression-line through the data-points that is sloped?

In our case, the regression-line has a slope of about 0,5, meaning that for each 1000-mg increment in
intake (x-axis) the difference changes with 500 mg

Question 16

What do you look at when you estimate whether your method is useful on a group level?

Answer 16

Group level > look at bias
- Close to 0?
- Under- or overestimation?
- Systematic bias or depending on intake?

Question 17

What do you look at when you estimate whether your method is useful on an individual level?

Answer 17

Individual level > look at limits of agreement

• Wide? Not interchangeable. You could make a huge error
• What would be acceptable?

Question 18

 When are 2 methods interchangeable? (= use both methods in the same individual and use all data)

Answer 18

1. Means close
2. Need to correlate
3. Limits of agreement should be small (acceptable)
4. Bias (diff between the means) should be independent of the measurements