Food is any substance consumed to provide nutritional support and energy to an organism. It can be raw, processed or formulated and is consumed orally by animals for growth, health or pleasure. Food is mainly composed of water, lipids, proteins and carbohydrates. Minerals (e.g. salts) and organic substances (e.g. vitamins) can also be found in food. Plants, algae and some microorganisms use photosynthesis to make their own food molecules. Water is found in many foods and has been defined as a food by itself. Water and fiber have low energy densities, or calories, while fat is the most energy dense component. Some inorganic (non-food) elements are also essential for plant and animal functioning.
Human food can be classified in various ways, either by related content or by how the food is processed. The number and composition of food groups can vary. Most systems include four basic groups that describe their origin and relative nutritional function: Vegetables and Fruit, Cereals and Bread, Dairy, and Meat. Studies that look into diet quality group food into whole grains/cereals, refined grains/cereals, vegetables, fruits, nuts, legumes, eggs, dairy products, fish, red meat, processed meat, and sugar-sweetened beverages. The Food and Agriculture Organization and World Health Organization use a system with nineteen food classifications: cereals, roots, pulses and nuts, milk, eggs, fish and shellfish, meat, insects, vegetables, fruits, fats and oils, sweets and sugars, spices and condiments, beverages, foods for nutritional uses, food additives, composite dishes and savoury snacks.