OK, so it's not a food, but it is one of the most important nutrients (yes, water is considered an essential nutrient). With two-thirds of our body made of water, it is important to stay hydrated. Water helps control body temperature, digestion, and acts as a medium in many metabolic processes in the body, including energy production. Although 8 cups of water per day is the classic recommendation, the amount of water you need varies from person to person. Find out how much water you should drink each day.