Food is any substance ingested to provide nutrition to the organisms within the gastrointestinal tract. Food is generally of animal, plant or fungi origin, and has key nutrients, including proteins, carbohydrates, vitamins, or minerals, which are required by the human body. The body uses food for energy production, storage of energy, and the regulation of homeostasis, a balance in the development of cells throughout the body. When food is not well-maintained, it can cause disease or discomfort.

Carbohydrates are found in many foods, with the majority being simple sugars. Simple sugars include glucose, sucrose, lactose, maltose, and fructose. Simple sugars are absorbed too slowly by the body, causing rapid rises in blood sugar. This causes the body to utilize other sources of energy, resulting in hunger and the desire to eat more. As food enters the body, carbohydrate levels rise, causing the appetite to increase for food. When carbohydrate amounts increase, so does the amount of calories to be eaten, resulting in weight gain.

Proteins are found in meat, fish, poultry, and some plants such as nuts and vegetables. Proteins are essential for growth and development, but excess proteins can cause weight gain as protein is hydrolyzed in the liver and kidneys. Vitamins and minerals are found in fruits, vegetables, whole grains, and certain meats. Vitamin A, B, C, and E are fat-soluble, which means that they are stored in fat tissues in the body. Some vitamins, including A, D, and K, are water-soluble, which means they are stored in the urine and feces instead of the fat tissue.