Thread by @FondOfBeetles

The argument that sports categories be divided according to current testosterone completely misunderstands the biological function of testosterone.

https://twitter.com/mearnsgal/status/1363459889138438144

It assumes symmetry - that is, if T causes X, removing T will remove X.

That is intuitive and sometimes true, but not necessarily so. Any developmental biologist will tell you that the effects of a molecule on a target system are not always reversible.

Target tissues can be induced to develop in ways that are irreversible or heavily resistant to change.

Once such a developmental change is set, removing the trigger makes no difference.

Some targets of T *are* dependent on current levels of T. For example, the amount of hemoglobin (Hb; oxygen-carrying cells) in the blood is very sensitive to T, and reduction of T very quickly and reliably leads to reduction in Hb.

Some targets of T will, once triggered and set, will be insensitive to future loss of T, most obviously skeletal shape.

And some targets of T may be somewhat sensitive to loss of T, notably muscle mass.

But even within muscles, some effects of T are likely irreversible and some may be very sensitive. There’s no evidence that T changes alter fibre type (broadly, explosive v endurance types) but it seems to alter the size of fibres.

But some of these and other findings will come from studies adding T to a low T environment, and to go back to the start, we cannot assume symmetry of effect.

In terms of % change, females who supplement T gain more muscle than males suppressing T lose.

And I haven’t touched on long term effects of T on capacity to train, resistance to injury etc. ‘Muscle memory’ needs greater study.

If anyone has insight here, feel free to add thoughts!

These concepts are often referred to as a ‘legacy effect’, and that’s a phrase one often comes across in discussions of sporting policies regarding transwomen.

In short, the effects of T are, in many physiological contexts, likely to be enduring and/or permanent.

I should credit @jasonkoop for shared ponderings on assumed symmetry.

Another dev bio angle to understanding mechanisms of activity.

A molecule can induce different effects at different concentrations, but not always in an immediately understandable pattern. That is, one might observe increasing Effect X with increasing T *to a point*.

Over a threshold T, we might then see Effect X plateau (become less responsive to increasing T).

Alternatively, we might see no Effect X until a threshold of T is reached, and a 100% response thereafter. Essentially, a certain amount of T acts as switch.

Testosterone acts through receptors, and receptor availability will be a ‘limiting step’ to how much T one can process.

To grossly simplify, if you have enough T to trigger all your receptors, extra T won’t make a difference.

But we know that T receptor numbers are themselves responsive to amount of T (your body adapts) and we know that doping works in males (although is it more effective in those with lower T?). It’s complicated.

The above simplification was only to illustrate a principle.

Regardless, I have a vague idea that a linear/plateau T mechanism might underpin the premise that male performance doesn’t appear to be linked to T once a threshold has been reached but females perhaps do see improved performance with elevated T (weightlifting/PCOS).