Omega-3 fatty acid is key to a healthy diet, but Americans get far too little of it. There are many food sources rich in omega-3; fish, nuts, seeds, etc., but the average American diet is not high in any of those foods. Not eating foods high in omega-3 is the obvious part of the problem though, another factor may actually come from a very unlikely source. Have you ever wondered what the difference between grass fed cows and corn fed cows was?
"We've come to think of "corn-fed" as some kind of old-fashioned virtue, which it may well be when you're referring to Midwestern children, but feeding large quantities of corn to cows for the greater part of their lives is a practice neither particularly old nor virtuous. Its chief advantage is that cows fed corn, a compact source of caloric energy, get fat quickly; their flesh also marbles well, giving it a taste and texture American consumers have come to like. Yet this corn-fed meat is demonstrably less healthy for us, since it contains more saturated fat and less omega-3 fatty acids than the meat of animals fed grass. A growing body of research suggests that many of the health problems associated with eating beef are really problems with con-fed beef."
The Omnivore's Dilemma by Michael Pollan