[s1artibartfast]

There is a huge conflation of cause and effect with respect to muscle mass and longevity.

Low muscle mass is associated a broad swath of illnesses, low activity, and generally poor health.

Muscle mass's power as a predictor is not the same as it's utility as an intervention.

[naasking]

As long as you're not using exogenous hormones, muscle mass can only be achieved with exercise that builds or preserves muscle. I'd say that's a pretty good predictor against frailty, which is strongly associated with mortality among the elderly.

[inglor_cz]

True, but frailty in the old age has a reason that cannot fully be mitigated with exercise: depletion of stem cells. The same mechanism will make our blood vessels thin and prone to bursting etc.

[s1artibartfast]

Exactly my point! you have a chain of 3 associations right there. One is nearly tautological and another has backwards causality. Correlation =/= causation.

Something being a good a good predicative indicator does not mean it is an effective intervention.

[naasking]

There is no backwards causality in the implication that building or preserving muscle that would otherwise be lost can prevent or delay frailty, nor is it backwards to imply that frailty can lead to death (from falls, disease, etc.). I really don't understand what you're trying to say.

[sn9]

"Muscle" isn't an intervention.

"Building muscle" is an intervention and has extremely well-documented mechanisms that have a causal role in improving health.

[cthalupa]

Muscle actually is an intervention for one of the biggest risk factors in metabolic syndrome - insulin resistance.

Muscles use glycogen. They use more glycogen when you're doing the sort of thing that builds muscle, so of course it's even better there, but someone who just genetically puts on more muscle at the same level of activity as someone who puts on less will still use more blood sugar for their muscles, and thus be less likely to increase their insulin resistance.

[sn9]

Ok how do you "intervene" with muscle? Do you surgically transplant it?

It's a thing that exists. Genetic differences are not interventions. Those are also simply things that exist.

An intervention is a change in treatment or behavior that induces a change with effects we want.

Differences between individuals are not interventions.

The intervention of interest is building muscle through strength training.

[cthalupa]

There's a significant number of posts in this larger thread saying that it is basically just the benefits of exercise that is providing the positive health outcome and not just having the muscle, and I interpreted intervention in a broader manner based on that context.

My point is simply that muscle in and of itself has positive benefits, even if you didn't need to do another healthy activity to maintain or gain it.