Thread by @Gonzalez_JT

How can carbohydrate metabolism be measured during exercise?

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New #openaccess review by @AndyJKing & I in @IJSNEMJournal

Hopefully a useful resource for people researching & reading about exercise metabolism 👩‍🔬📖

A summary🧵1/12

doi.org/10.1123/ijsnem…

What are isotope tracers?

Molecules where certain elements (e.g. Carbon) differ in some ways (mass) but less so in others (function)

This means we can distinguish between "label" and naturally occurring molecules, whilst the label behaves similarly to the natural molecule
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Molecules (e.g. glucose) can be labelled in different ways.

e.g. just one carbon could be labelled (13C) in a specific position

or all the carbons could be labelled (13C), known as uniformly or universally labelled (U-13C)
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Tracers are important because they can reveal underlying flux

This cannot be discerned from concentrations alone

With exercise vs rest, plasma glucose concentrations may not change, but flux (diss/appearance) can be higher

(2-fold ⬆️ in this e.g.doi.org/10.1152/ajpend…)
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To measure rates of appearance (Ra) & disappearance (Rd), we need to infuse a known about of tracer into a vein.

Then measure changes in concentration and tracer enrichment over time.

The relationship between these two can tell us a lot (based on the dilution principle).
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There are potential issues during non-steady-state (e.g. rest-to-exercise or fasted-to-fed transition)

Due to the time taken to equilibrate between different pools (for glucose, plasma & interstitial fluid)

Meaning what we measure might not reflect what we think is it
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This needs to be considered in study design, and can be somewhat overcome by using specific equations
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If we combine the infusion of tracer with ingestion of tracer, we can understand what happens to endogenous and exogenous (ingested) carbohydrates

E.g. do we burn the carbohydrate or store it?
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This has been used in many studies during exercise to assess how much of sports drinks/gels are used as fuel during the exercise

doi.org/10.1152/japplp…
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There are some considerations when using these methods including

Limiting foods naturally high in some isotopes (13C) to increase signal-to-noise ratio

Maintaining steady-state before measures
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Check out the paper for more detail on these methods and things to consider

doi.org/10.1123/ijsnem…
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Finally, thanks @LouiseMBurke @MckayAlannah & @DrBSteamjets for the invitation to write this review, and to collaborators on tracers @gareth_wallis @Hodson_Group

Dedicated to @proftipper who encouraged me to apply for my first grant using tracers and supported the setup
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