[carbocation]

OK so the extremes of this are accepted but I’m genuinely not aware of the evidence that sodium concentration varies with minor shifts in “hydration” (an odd term to see in a medical article).

For example, we know that destroying the ion gradient in the kidneys through massive overconsumption of fluid and underconsumption of solute can lead to hyponatremia (“potomania”).

But this mechanism is not relevant in normal physiology. So I’m again wondering whether it is appropriate to be attributing sodium shifts to hydration. In the Intro, the authors really dance around this and do not address it. So I assume that there is not actually good evidence for their assertion.

You could, in contrast, easily sell me on the notion that sodium set points change with age, but that could have nothing to do with volume intake.

[psychphysic]

They don't dance around it at all. It's just a poor HN title.

The result is an association between high plasma sodium and aging.

I might have missed your point though.

But to your point in hydration and sodium.

Sodium ion concentration dominates plasma osmolality which makes it not a bad way to think of hydration.[0]

[0] https://en.m.wikipedia.org/wiki/Plasma_osmolality

[carbocation]

I don't think of plasma sodium concentration as being a good indicator of hydration because it's aggressively homeostatically controlled. As I mentioned, we usually think of sodium shifts as a pathological state. It would be interesting to me if small variations in fluid consumption could also influence sodium concentration to a lesser degree, because it's not really aligned with what I was taught. But I acknowledge that it could be, I'd just like to see data.

The entire premise of the article's interpretation—that hydration status is reflected in sodium concentration—rests on this being true.

Don't get me wrong, the finding that you pointed out (that there is an association between high plasma sodium and aging) is interesting even if the authors' premise is not true! But if small variations in daily water consumption (for example) don't actually change sodium concentration, then the observation could be 100% true and yet their interpretation and proposed follow-up studies could make no sense.

So when I said they dance around whether it is appropriate to be attributing sodium shifts to hydration, that's what I meant.

[carbocation]

Looks like others had the same issue with respect to the interpretation[1]:

> However, “it would have been nice to combine their definition of hydration, based on serum sodium levels only, with actual fluid intake data from the ARIC cohort,” Sesso [an associate professor of medicine at Harvard Medical School and associate epidemiologist at Brigham and Women’s Hospital in Boston] added.

1 = https://www.cnn.com/2023/01/02/health/hydration-disease-agin...

[Mistletoe]

Don't we know that old people "dry out" already because they don't drink enough water?

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2625510/

[carbocation]

That paper doesn’t say anything about a relationship between routine variation in fluid intake and sodium concentration, or did I miss it?

It’s undisputed that in pathological states, hyper- or hyponatremia can occur. And the formula in the paper you linked gives a solution to calculating the free water deficit (for example). But it’s not the same as knowing that modest variation in fluid intake can be seen in sodium concentration (because it’s homeostatically regulated).

[napier]

Sodium concentration is understood partially (but not entirely) be a surrogate marker for other factors associated with inadequate hydration and electrolyte imbalance.