“God comes to the hungry in the form of food.”

-Mahatma Gandhi

After air and water, the only thing more critical to human life is food. Food provides energy that the human body needs to maintain both its involuntary (like breathing, blood circulation, heartbeat) and voluntary (walking, cooking, speaking, etc.) functions.

The energy content of food is measured in calories, and every person needs a certain amount of calories each day to stay healthy. An average 30-year-old man who’s active for less than 30 minutes a day, for example, needs about 2,400 calories daily. The average 30-year-old woman with a similar level of exercise needs about 1,800 calories per day. In general, older adults and young children will need fewer calories a day to maintain health.

One food calorie is equal to 1,000 calories when you’re discussing a measure of energy used in physics. The physics term refers to the amount of heat energy needed to increase the temperature of one gram of water by 1 degree Celsius.

People need food for more than just energy, though. Food also provides a variety of nutrients that help sustain the body’s health. These nutrients include carbohydrates, fats, fiber, minerals, proteins and vitamins.

There are two primary sources of food: plants and animals. Both derive their food energy from the sun, either directly (plants using photosynthesis) or indirectly (herbivores and omnivores eating plants, omnivores and carnivores eating other animals). Without the sun, there would be no food.

Throughout much of history, humans got most of their food by either hunting or gathering wild plants and animals. With the advent of agriculture, though, people began actively cultivating certain types of plants and raising certain types of animals as domesticated food sources.