Once enzymes are exposed to heat, they are no longer able to provide its function. The damaged enzyme would then result in chronic illness. Thus requiring us to make our own enzymes to process the food.
Digestion of cooked food demands much more energy than the digestion of raw food. While raw food is so much more easily digested when it passes through the digestive tract.
But is cooked food healthy for our body?
No. Eating cooked food places a burden on your pancreas and other organs and overworks them, which eventually exhausts these organs. Many people gradually impair their pancreas and progressively lose the ability to digest their food after a lifetime of ingesting processed foods.
Source: http://www.healingdaily.com/detoxification-diet/enzymes.htm