If you look at the sequence of events that happen to trigger a heart attack, it becomes really clear how big a role luck is, but still you can mitigate each step. Studying this stuff also makes your body seem like a walking time bomb.
However it's irrefutable that exercising, sleeping and nutrition improves your health.
Will it prevent you from ever getting cancer? no, but it sure helps.
My mother passed away from cancer, she always exercised and took care of herself, it made the quality of her life much better. Looking back, she would have suffered much more had she not done that.
Honestly, given the extent to which many people's diets consist primarily of bleached and re-enriched wheat separated from the germ or simply refined corn, I think there are many more people who are slowly poisoned by their diet than realize it.
Yet there's plenty of hyperbole in my statement too. I don't think you could murder someone by making them eat your diet, unless it consisted of bags of broken glass.