I can’t see how a healthy mixed diet, regular exercise, a bit of meditative thinking (or singing or theatre or anything else that keeps some creative juices going) can be anything but useful in keeping body and soul as healthy as possible. I take a few (doctor endorsed) supplements myself.
It’s just not a guarantee. Plenty of healthy people got cancer in the first place. While cancer research and treatment cracks on amazingly well, a high proportion of cases are still in the ‘don’t know/bad luck’ category. It’s always seemed to me that combining the best of western medicine with everything we know about good nutrition and healthy living may be the best option for surviving cancer - or anything else, given a cancer diagnosis is no protection against getting something else undesirable. In regard to alternative therapies, one of the best tests is survival rate. And Big Pharma suspicions aside, a proven success rate would never be a secret.
