I hear you, and I'm glad you are a survivor! But I question that "ending up with" cancer can be tied directly to your conclusion that most supplements are worthless.
Diet is a major element of health, but it is not fully determinative. Your supplements may have helped you avoid any number of conditions that could have weakened you or taken you out before you developed the cancer. Many factors figured in to an "attack" of cancer, such as environmental toxins or genetic predisposition; and there are a wide variety of cancers. Many cases are more individual than not. My hard-drinking and roll-your-own smoking granddads both lived cancer-free and died of heart conditions. But both my non-smoking, non-drinking parents died of lung cancer. How is that possible? I suspect radon in their leaky suburban cinder-block basement and lawn chemicals from their daily golf games; but I'll never know.
Doctors from med school onward are trained to say that supplements are no good; I've heard it for years. So I challenged my latest doctor and stopped taking them for 6 months. My next lab tests showed a measurable elevation in the bad cholesterol and other markers. Then I took them again and there was a measurable improvement. Now she tolerates my supplement "habit" with a watchful eye, but has stopped scoffing.
There has been a lot of original research about supplementation that is not merely a claim by pharmaceutical companies. And it's not hard to imagine that test results that show a possible benefit lay persons at an affordable price is suppressed or condemned by pharm lobbyists and sales reps. You'd be shocked at the amount of "bribery" in the form of gifts, all-expenses-paid "medical seminars" (in locations like Las Vegas and Hawaii) and other pelf the drug reps lavish on their M.D., Pharm.D. and med school faculty clientele. I learned this while worked in marketing in the prescription drug industry for many years, with a degree in liberal arts but no scientific education beyond high school biology, like many of us in the background who promoted BigPharma's bottom line.
You can see from the recent pandemic how cheap, available medications such as hydroxychloroquine and ivermectin were viciously attacked in the media for both political and BigMed interests, in favor of expensive hospitalizations paid for by a Democrat spending bill and BigPharma drug treatments. Why would their attacks on affordable supplementation by us non-elites be any different?
My primary physician has been skeptical of my use of vitamins and minerals; yet she had to admit that during the pandemic, suddenly many credible sources started recommending supplements I'd already been taking for years: especially C, D and zinc.
Supplement-takers should be scientific about researching not only the anticipated good effects of adding or deleting a supplement from their diet, but also the possible side effects, including the combination of other elements in the diet and the timing of taking the doses, since some elements can cancel out the effects of others, or contribute to mild harms over time. They should also obtain objective measurement of the effects through blood tests at regular intervals.
“But I question that “ending up with” cancer can be tied directly to your conclusion that most supplements are worthless.”
You seem to have done your homework, which is pretty important if you plan on treating yourself. I suspect you are in the minority of people who take supplements. I don’t disagree with much of what you say which is based on your own research and discussion with your doctor.
While I agree we have a capitalist system where both drug companies and supplement manufacturers promote their product in the best way they know how I personally come closer to listening to my medical doctor, with years of training and certification (and reputation) than a supplement promotor who is not regulated. So far, that seems to have worked out pretty well.
In my mind, the key is what you described. That is look for deficiencies through your blood tests. If you have them and they can be treated with supplements then by all means do it. I mentioned I do take them still, but they are from deficiencies and actually recommended by my doctor.
My previous method of using them was to hear about a particular supplement that would improve my health and buy it and start gulping them down. I think that is what most do — not the research as you have done.
By the way, my cancer was a rare form of blood cancer with no hereditary relationship and actually no known cause — except a genetic mutation that occurs as cells are copied about a million times a day and one goes wrong.
Diet and exercise go a long way to minimizing your chance of disease, but they don’t prevent it. They also make a huge difference when it comes to the survival of both a disease and the treatment.
Good luck.