we have enzymes to prevent oxygen radicals from forming. such as Superoxide dismutase, which is present in nearly every cell in our body, and we have 3 different version. most oxygen dependant organisms have similar enzymes.

another fun fact, our immune system can produce the same superoxide as a defense against bacterial infections, reactive oxygen species including radical oxygen. but bacteria can produce the enzyme to neutralize that too.

i think the dietary source, is somewhat alittle homeopathic/ pseudosciency, because our stomach acid would probably neutralize any beneficial effect before we can absorb it. of course there are some foods that provide nutrional benefit to certain cells(if they a chemical compound). but they dont have like anti-oxidant effect of something like superoxide dismutase. also because of this effect, people with chronic illness as a result of the dysfunction.