Carbohydrates are the body's primary source of fuel. Like proteins, they can be broken down into two basic categories - simple and complex. Simple carbs are digested quickly and cause a rapid spike in blood sugar. This leads to a large release of insulin from the pancreas. The insulin removes sugar from the blood. This, in turn, leads to low energy, weakness and hunger. Complex carbs usually contain fiber and provide the body with sustained energy, avoiding the highs and lows associated with simple carbs. They make you feel fuller and contain lots of nutrients. Complex carbs can be found in grains, green and fibrous vegetables.