The term nutrient density has several meanings.
Nutrient/energy ratio definition
Most commonly, nutrient density has been defined as the ratio of nutrient content to the total energy content. For example, according to the Dietary Guidelines for Americans (2005), nutrient-dense foods are those foods that provide substantial amounts of vitamins and minerals and relatively few calories.
Under this definition, nutrient-dense foods such as fruits and vegetables are the opposite of energy-dense food (also called "empty calorie" food) such as alcohol and foods high in added sugar or processed cereals.
Note that nutrient density under this definition does not imply bioavailability.
Energy types definition
Second, nutrient density has been defined[where?][by whom?] as the ratio of food energy from carbohydrate, protein or fat to the total food energy. To calculate nutrient density (in percent), divide the food energy (in calories or joules) from one particular nutrient by the total food energy in the given food.[example needed]
Third, nutrient density can be understood more holistically as the ratio of the nutrient composition of a given food to the nutrient requirements of the human body. Under this definition, a nutrient-dense food is a food that delivering a broad nutritional package. Note that this type of definition, while implying bioavailability, is exceedingly hard to define on any concrete level due to the variety of dietary requirements across human populations, environment and time.