That's such a big disparity I'm very suspicious of that data, but there seems to be plenty of evidence that grossly excessive cardiovascular exercise is bad for you in various ways.

If people enjoy it and really get a lot out of it then I wouldn't judge them for doing it, but let's not pretend it's healthy, because all the evidence is that it isn't.

In terms of cardio being able to run a half decent 5k a couple of times a week is probably a good idea, any more volume than that is really not necessary and at some point becomes harmful

I can find only a single paper making this claim about marathoners. And that study has attracted a lot of criticism.

Can you provide a link? I looked and all I found was one study that looked for pre-cancerous adenomas.

I remember when that paper hit HN the thinking was actually to do with reduced bloodflow to the colon while running for long periods of time.

Dehydration? Overconsumption of carbohydrates?

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