Board Thread:Warhammer 40k General Discussion/@comment-92.97.77.182-20130321145115/@comment-3186669-20141007125949

Orks don't feel the emotions towards their fighting and looting the way humans do. Orks do it because it's what they are, but it doesn't give them a presence in the warp. In the rare events that one of Da Boyz ever does get possessed, the other Orks will immediately turn on them and kill them, knowing instinctually that something is "wrong".

So no, Orks don't "feed" the Chaos gods - they don't feel "sentient emotion" at that level, and since their system works based on their own belief system (everyone knows red goes faster, therefore with Orks, it actually DOES go faster), the Chaos powers aren't involved.

'Nids aren't sentient as such, so again have no warp presence or emotional input. The Eldar "path" means that they're controlled enough to avoid feeling strong emotions, and those who stick to a single path are generally seen as outcasts in their society (then there's the Harlequins, who have their own rules). Necron are blank automatons who, again, have no emotion. Basically the only race in 40k which can "fuel" the Chaos Gods to any real level are humans.