Food is a basic need for all living creatures. We consume, or we die. Life on Ashenfall is no different.
Your Sustenance meter tracks how hungry you are. It is filled by eating food and it will deplete over time as you exert yourself trying to survive the wilds of Ashenfall.
A high Sustenance meter causes your health to regenerate more quickly. However, should you run out of Sustenance, your health will start depleting as your body canibalises itself in an attempt to survive.
Keep an eye on how hungry you are. Higher tier foods will provide more Sustenance than lower tier foods, but berries will help you survive in a pinch.