Junk food doesn't contain the nutrients your body needs to stay healthy. As a result, you may feel chronically fatigued and lack the energy you need to complete daily tasks. The high levels of sugar in junk food puts your metabolism under stress; when you eat refined sugar, your pancreas secretes high amounts of insulin to prevent a dangerous spike in blood sugar levels.
Because fast food and junk food don't contain adequate amounts of protein and good carbohydrates, your blood sugar levels will drop suddenly after eating, leaving you feeling grumpy, fatigued and craving sugar.