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The reason you need digestive enzymes is because, when you eat your body sometimes has problems digesting everything you've put into your stomach. This is especially the case if you're over 40 due to the fact that most people who have reached their fourth decade of life usually don't produce enough HCL (hydrochloric acid) and, therefore, the foods you eat can stagnate in your stomach and intestines.
Undigested foods can cause many different problems including malnourishment (which occurs when your body doesn't absorb enough nutrients from whatever it is you eat) and even immune system issues. The reason your immune system can be compromised if you don't digest the foods you eat properly is because your body is designed to keep you alive. In order to do this it automatically defends against undigested food particles that find their way into your bloodstream (undigested protein is probably one of the most prevalent issues that Americans have to contend with). Due to the action taken by your body (i.e., sending white blood cells to attack and 'kill' undigested food particles) your immune system is weakened because it is having to attack 'invaders' that it was never intended to defend against (food) instead of being able to concentrate on true threats to your system (i.e., viruses, bacteria, etc.).
Of equal importance is the fact that while your body can convert metabolic enzymes into digestive enzymes it CANNOT do the reverse. Since your body has a limited supply of metabolic enzymes (the enzymes that allow everything from your brain to function and your heart to beat), it's especially important to keep it from 'using up' these enzymes for digestion rather than for keeping your body alive and functioning properly.
In summary:
Digestive enzymes help your body digest foods better.
Having more digestive enzymes available enables your body to spare metabolic enzymes.
Extra digestive enzymes help to keep your immune system functioning properly.
If you eat anything except for raw foods you NEED digestive enzymes.