What are the means by which antioxidants prevent of chronic diseases such as cancer, heart disease, and macular degeneration?

Do they?: Re: cancer -- pre-clinical evidence supported supplements such as anti-oxidants as "chemo-preventive" interventions to decrease the risk of cancer. However, the test tube (in vitro) predictions did not work in humans (in vivo) in a number of studies including vit e, beta carotene in lung cancer, various supplements in head/neck cancer & selenium in lung/prostate. Humans are complicated.

They don't: There's a whole lot of conspiracy-mongering going around these days that somehow the pharmaceutical and health care industries are suppressing some "magic bullets" that can prevent all of these diseases. If this were true, i don't know a doctor who wouldn't immediately shout it from the roof-top and get all of their patients on the stuff.