The human body needs food and water in order to function correctly. We eat food to gain energy. 

Carbohydrates (such as starch and sugar), proteins and fats contain energy. A wide-ranging diet also gives us many important vitamins and other substances. 

Proteins consist of amino acids. The human body needs approximately 20 different amino acids in order to build new cells. This is why a wide-ranging diet is essential for a healthy lifestyle. Proteins and amino acids are found in foods like fish, meat, milk, beans and eggs. 

Fats contain a lot of energy. This is why people who are trying to lose weight often try to stay away from fatty foods. Despite this, fats are important substances for our cells. 

Our cells also need water in order to function properly. This is why our brain tells us when we are thirsty.