cancer and diabetes are diseases of civilization and processed foods, especially Sugar and refined grains. That has been shown over and over again in the literature. And very likely why vegetarians get these SAME diseases too.
Omnivores *tend* to eat conventional and processed meats... and that is a huge problem too. This does not happen with omnivores of unprocessed, range fed, chemical free animals.
As IC mentioned it is about the soil... for both plants and the animals\humans that live *off* of it.