What's the mechanism of this? Cancer is caused by bad-luck random cell division creating a bad cell, I can see how chemicals influence this process, but how does physical activity?
It doesn’t necessarily have an easy to understand soundbite explanation. Biology is like spagetti code. Everything depends on everything in difficult to comprehend ways.
That is only the first necessary step towards cancer. Our bodies have various mechanisms to kill suspicious cells before they start being a systemic problem. Apoptosis, or the immune system attacking them if they can be recognized. These mechanisms are pretty efficient; in the animal world, animals with really strong immune systems, like bats, very rarely get cancer. And most of our current progress against cancer involves ... very careful stimulation of the immune system against targets specific for that particular patient's tumor.
Physical activity can probably influence these anti-cancer mechanisms, as does aging etc.
There's also a high likelihood that people who get regular exercise are also eating healthier food that hasn't been highly processed and loaded up with pesticide residue, biocides that slow fungal and bacterial growth, animal growth hormones, etc.
The fundamental equation is something like:
(body load of toxins) = (ingestion rate) - (excretion rate)
"Specifically, we posit that in carcinogenesis aberrant cell signaling due to exaggerated and continually high lactate levels yields an (inappropriate) positive feedback loop"
[1] https://academic.oup.com/carcin/article/38/2/119/2709442
I'm oversimplifying massively, but essentially, the theory is that if you don't give your body an outlet to burn with, it will burn the calories on things that aren't good for you. Eg, increasing inflammation.
It wouldn't surprise me if some of these effects are contributing factors.
Probably putting some "stress" on the cells is beneficial, is like forcing them to work a little once in a while instead of letting them function in the "just survive" mode.
Maybe blood circulation? It may clean toxins from where the blood may flow slower?
That's my headcanon
But of course we've evolved mechanisms to compensate. For that, there's a corresponding release of antioxidants and cellular repair to deal with the oxidation events of exercise.
Exercise in general raises your immune system as well. Your immune system cleans up a lot of rogue mutation cells. It may also be the physical activity increases massive amounts of blood flow through your entire system and helps at a macro level with this cleanup of deviant cells.
But I'm not an oncologist nor am I a physiological biologist.
https://www.msnbc.com/msnbc-podcast/examining-energy-evoluti...
"[Y]ou can think about all these processes as competing for calories over the course of the day. And normally, low priority things like having a really high inflammation response or having a high stress response, those get tamped down in a highly active population like we've all evolved in. But now, you move to these weird zoos that we've built ourselves, where you don't have to be active, food's available all over the place, energy supply is really easy. And now, these low priority activities which used to probably only happen very occasionally in the past, in the deep past, now are happening all the time, chronically, at these super high levels, and it's actually really bad for you."
> If you take somebody who is really sedentary and you get them exercising, what happens? Things like inflammation, which is your immune system being overactive, goes down. [Also cortisol level when under stress, and reproductive hormone levels, go down.]
So that puts a small hole in the "exercise helps the immune system clear up cancer" theory. Maybe it in fact stops the immune system causing cancer. Or maybe it does both things, since biology is messy and the immune system might have multiple energy budgets for all I know.
So in a way my simplified assumption was more or less right, my ego grew a little bit and i think that's a bad thing
e: okay, not the pathway - but we do know that additional weight increases cancer risk
Believe it or not, professional researchers probably tried to think of "obvious pathways" too.
i guess the mechanism correlating weight and cancer is more complex than “more cells more cancer”, but the correlation is still well measured and i assume a reason why physical exercise would anti-correlate as well
e: also my understanding was non-adipose cell count was also correlated as weight
I guess the main thing I was trying to say was that we know obesity is correlated with cancer and I thought we had some understanding of the mechanism behind that, so my guess is that physical exercise is just the inverse of that mechanism?
If the parent's hypothesis is:
"less exercises == higher weight == more cells == (more mitosis / mutagenic potential) == more cancer"
Which tissue type is accounting for most of the weight difference (in equal height people) who do and don't exercise?
You could even reverse this and look at body builders with greater muscularity than average. The number of cells itself is clearly not the cause of increased cancer risk, because neither fat nor muscle cells cause the most common cancers.