At first glance, you might think just the opposite: Stress will cause you to lose weight. You might have a friend who was stressed from a pending divorce and dropped 20 pounds (9 kilograms), or a sibling anxious about a job loss who suddenly lost his appetite and became too thin. But stress is actually the reason behind a lot of people's weight gains.
First, adrenaline floods our bodies when we're anxious and stressed out, preparing us for battle (adrenaline is known as the "fight-or-flight" hormone). That rush of adrenaline is followed by a cascade of cortisol, known as the "stress hormone."
Cortisol tells our bodies to eat, because in early human history, that extra energy from food was necessary for activities like running and physically fighting. Nowadays, our stress might come from not having enough money to pay bills rather than facing a wild animal, yet our bodies are still programmed to store fat when we're anxious. Cortisol is also the culprit behind emotional eating, that mindless shoveling of food down when we're stressed, and behind our craving for "comfort foods" like ice cream and chips when we're feeling low [source: Greenberg].
As for those people who lose weight from stress, it's likely that they've lost interest in eating or are fidgeting a lot from anxiety, which burns calories.