Cardio fitness is a strong, consistent predictor of morbidity and mortality

> Or perhaps you don’t fully understand the challenges of establishing causality? Just because an intervention causes an improvement in some bio markers that are associated with lower mortality (unfortunately) does not mean that the intervention will cause lower mortality.

> The classic example is vitamin D supplements: Higher vitamin D levels are associated with lower mortality in many medical conditions. Vitamin D supplements increase vitamin D levels. But vitamin D supplements seldom lower mortality.

That is quite a simplification. The research as far as I know is that too low Vitamin D levels are associated with higher mortality, but that supplementation above a certain level is meaningless (and most caucasians can get those levels through normal sun exposure). So yes vitamin D supplements in general don't improve mortality.

Note also the situation here is completely different. The link between mortality and morbidity markers and exercise have been established in other studies. The study here establishes that this directly translates to a correlation between fitness and mortality. So in a way it's the opposite of the vitamin D case.

> Why? Probably because vitamin D is produced in the skin when we are in the sun. The more healthy subpopulation of any study will typically spend more time outside, so they will have higher vitamin D levels. But it’s (relative) health that causes higher vitamin D, not the other way around.

You realise that you are proclaiming causality here?

> The only way to reliably establish causality is really an end-to-end randomized controlled trial (RCT). Stitching together two RCTs is not sufficient.

A RCT does not establish causality. That's essentially my point. A single study/experiment never proofs causality. You need a theory to explain the causality and multiple studies that falsify other possible causality mechanisms. That has been done extensively for excercise and morbity/mortality, the current study just establishes that this also correlates in the bigger picture. So yes the study itself does not "proof" causality, it's just a piece in the bigger puzzle of causality.

> (Not saying that exercise does not lower mortality BTW, just that it’s complicated, and a study such as this is probably picking up two signals: one causal and one purely correlational.)

when i've been running i do similar. though i also do accellerate a bit and slow down, because its... more fun.

its a nice idea, but i think adults should pay for their own helmets

however, children... sure. give them one.

With 30km bike commute you can scroll the web as much as before.

"Every time we send 5 fire trucks to a fire, the damage is 10x than when we only send one fire truck. We've observed this 1000 times. And still you don't believe that the fire trucks are causing the damage."

In this case it should be absolutely clear that A (lots of trucks) aren't causing B (lots of damage), but rather a third aspect, C (size of fire) is causing both A and B. Insisting that A causes B will result in completely counterproductive interventions, like "send only one truck to all fires".

The same thing could be true for cardiovascular fitness. If people are sick, they're much less likely to running or hiking up a mountain. So rather than poor cardio fitness (A) causing high mortality (B), it could be that a third thing, sickness (C) is causing both A and B. If that is the case, then shaming people who are sick into trying to exercise, instead of making them healthy enough so that they feel like running, is likely to make things worse rather than better.

How do you tell the difference? Well the "gold standard" is randomized controlled trials. Pick 3,000 random people. Tell 1000 of them to exercise more, and 1000 of them to exercise less, and 1000 leave alone, and compare. If the "exercise more" group is healthier at the end of 10 years, that's decent evidence that "exercise more" is a useful intervention.

Failing that, you can think of other possible confounding factors and control for them. Don't just ask how much they exercise; ask how old they are, and how well they are, how stressed they are, and loads of other factors which might both cause both A and B, and use statistical methods to detect whether one of those factors is actually a better explanation than "A -> B".