Energy Requirement in Humans

The Estimated Energy Requirement (EER) is the average dietary energy intake predicted to maintain an energy balance (that is, the calories consumed are equal to the energy expended) in a healthy adult of a defined age, gender, and height whose weight and level of physical activity are consistent with good health.
Differences in the genetics, body composition, metabolism, and behavior of individuals make it difficult to accurately predict a person’s caloric requirements. However, some simple approximations can provide useful estimates. For example, sedentary adults require ~30 kcal/kg/day to maintain body weight, moderately active adults require 35 kcal/kg/day, and very active adults require 40 kcal/kg/day.
A. Energy content of food
The energy content of food is calculated from the heat released by the total combustion of food in a calorimeter. It is expressed in kilocalories (kcal, or Cal). The standard conversion factors for determining the metabolic caloric value of fat, protein, and carbohydrate are shown in Figure 1. Note that the energy content of fat is more than twice that of carbohydrate or protein, whereas the energy content of ethanol is intermediate between those of fat and carbohydrate. [Note: The joule (J) is a unit of energy widely used in countries other than the United States. One cal = 4.2 J; 1 kcal (1 Cal, 1 food calorie) = 4.2 kJ. For uniformity, many scientists are promoting the use of joules rather than calories in the United States. However, kcal still predominates and is used throughout this text.]

Figure 1: Average energy available from the macronutrients and alcohol.
B. Use of food energy in the body
The energy generated by metabolism of the macronutrients is used for three energy-requiring processes that occur in the body: resting metabolic rate (RMR), physical activity, and the thermic effect of food. The number of kcal expended by these processes in a 24-hour period is the total energy expenditure (TEE).
1. Resting metabolic rate: RMR is the energy expended by an individual in a resting, postabsorptive state. It represents the energy required to carry out the normal body functions, such as respiration, blood flow, and ion transport. RMR can be determined by measuring oxygen (O2) consumed or carbon dioxide (CO2) produced (indirect calorimetry). [Note: The ratio of CO2 to O2 is the respiratory quotient (RQ). It reflects the substrate being oxidized for energy (Fig. 2).] RMR also can be estimated using equations that include sex and age (RMR reflects lean muscle mass, which is highest in men and the young) as well as height and weight. A commonly used rough estimate is 1 kcal/kg/hour for men and 0.9 kcal/kg/hour for women. [Note: A basal metabolic rate (BMR) can be determined if more stringent environmental conditions are used, but it is not routinely done. RMR is ~10% higher than the BMR.] In an adult, the 24-hour RMR, known as the resting energy expenditure (REE), is ~1,800 kcal for men (70 kg) and 1,300 kcal for women (50 kg). From 60%–75% of the TEE in sedentary individuals is attributable to the REE (Fig. 3). [Note: Hospitalized individuals are commonly hypercatabolic, and the RMR is multiplied by an injury factor that ranges from 1.0 (mild infection) to 2.0 (severe burns) in calculating their TEE.]

Figure 2: The respiratory quotient (RQ). [Note: For protein, the nitrogen is removed and excreted, and the α-keto acids are oxidized.]

Figure 3: Estimated total energy expenditure in a healthy 20-year-old woman, 5 ft, 4 in (165 cm) tall, weighing 110 lb (50 kg), and engaged in light activity.
2. Physical activity: Muscular activity provides the greatest variation in the TEE. The amount of energy consumed depends on the duration and intensity of the exercise. This energy cost is expressed as a multiple of the RMR (range is 1.1 to >8.0) that is referred to as the physical activity ratio (PAR) or the metabolic equivalent of the task (MET). In general, a lightly active person requires ~30%–50% more calories than the RMR (see Fig. 3), whereas a highly active individual may require ≥100% calories above the RMR.
3. Thermic effect of food: The production of heat by the body increases as much as 30% above the resting level during the digestion and absorption of food. This is called the thermic effect of food, or diet-induced thermogenesis. The thermic response to food intake may amount to 5%– 10% of the TEE.