Cortisol is your stress hormone. It's what triggers fight or flight. When your body enters fight or flight it doesn't need your digestive system to be active, it doesn't need to build muscle or burn fat, it doesn't need your immune system it just wants to keep you alive in a dangerous situation, so cortisol shuts them all down till the danger goes. If you're stressed for a long period of time them you won't digest food as well as you should so it can have an effect on your diet. Easy ways to reduce cortisol - learn slow breathing techniques and learn to meditate. If you breath long and slow your body knows it's not in danger so will stop releasing cortisol. Meditation keeps you in the calm zone and helps keep cortisol levels to a minimum.