Researchers are attributing it to fatty acids. Diets that include meat and fish are higher in arachidonic acid (AA), an animal source of omega-6 fatty acids. Much of the meat Americans eat today is quite high in AA: The average omega-6 to omega-3 fatty acid profile of modern grain-fed meat is 5 times higher than grass-fed meat, like our ancestors ate. And previous research has shown high levels of AA can cause mood-disturbing brain changes.