I think it far more likely that McDonald's spikes their food with something (it might be the salt) that increases the appetite. After all, they are in the business of selling food, and if they can get you to come back for more, that helps their business.
Or perhaps they (also) try to avoid ingredients that help reduce feelings of hunger? McD have such a dodgy history, that I expect their reputation to eventually be on a par with the cigarette industry.
Always remember: QUIDQUID LATINE DICTUM SIT ALTUM VIDITUR
Science flies you into space; religion flies you into buildings.