"If you always keep this in mind: cell division -> cancer, so much will make sense." I am not so sure this is a good heuristic. An athlete, or even a moderate

"If you always keep this in mind: cell division -> cancer, so much will make sense." I am not so sure this is a good heuristic. An athlete, or even a moderately active (fit) person is inducing quite a bit of cell division due to the teardown and regrowth of muscle and bone, etc., caused by exercise. However, activity and exercise seem to be (strongly) negatively correlated with cancer. Instead, I think a better mental model is the winnowing of a population. When you subject your (muscles/bones/heart/lungs) to a highly stressful exertion, some of the cells just can't maintain that output and so they are killed off, or commit suicide by the process of apoptosis. But if you never subject your cell population to an apoptosis-inducing level of stress, you're just cultivating an increasingly weak population of cells. I highly recommend the book by Nick Lane titled: Power, Sex, Suicide which explores these topics... https://en.wikipedia.org/wiki/Power,_Sex,_Suicide https://en.wikipedia.org/wiki/Power,_Sex,_Suicide