The study followed almost 9,000 people for an average of six months; the participants had never been diagnosed with depression or taken antidepressants. About 500 were later diagnosed with depression or started antidepressant medication; all in all, the researchers found that those who regularly ate fast food were 51 percent more likely to develop depression (with the more fast food consumed leading to a higher rate of depression), and even those who ate just small amounts of fast food or commercially baked goods (like doughnuts or croissants) were susceptible to developing depression.