I want to believe that eating vegetables is necessary even when one takes vitamins, but I can't find any medical evidence for this. To make my question specific, suppose a study on twins were done in which one ate anything they wanted and the other was limited to just potatoes, chicken, vanilla ice cream, and water, but both took vitamins (with 100% RDA from the 2015 guidelines). What statistically measurable difference will you see? Will the twin with the free diet live much longer on average? Is there a published medical study (perhaps on mice) showing this benefit?