> "My conclusion was that cancer is best prevented by removing negatives from life, not necessarily adding positives."

But diet is a complex system. So while there might not be any magical "direct" benefit from eating a certain fruit or vegetable, they could still produce a significant positive effect.

Perhaps the biggest contribution to preventing cancer that fruits and veggies make is to curb negative behavior. They might replace sugary foods or regulate cravings or satiate (all interrelated). And then, this could feed back into promoting positive behavior: more energy which encourages more rigorous exercise. Which boosts the subject's mood and increases their sense of well-being. Which feeds back into their dietary choices. Etc.