Energy Requirement in Humans

ENERGY REQUIREMENT IN HUMANS

The Estimated Energy Requirement (EER) is the average dietary energy intake predicted to maintain an energy balance (that is, when 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 about 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. [Note: The EER that is listed on food labels is either 2,000 or 2,500 kcal/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 27.5 . 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 Cal (1 kcal, 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 27.5 Average energy available from the major food components.

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 thermic effect of food (formerly termed specific dynamic action). The number of calories 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. [Note: A basal metabolic rate (BMR) is determined under more stringent environmental conditions. BMR can be determined by measuring O₂ consumed or CO₂ produced (indirect calorimetry). It also can be estimated using equations that include sex and age (BMR 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. RMR is about 10% higher than the BMR.] In an adult, the 24-hour RMR, known as the resting energy expenditure (REE), is about 1,800 kcal for men (70 kg) and 1,300 kcal for women (50 kg). From 50%–70% of the TEE in sedentary individuals is attributable to the REE (Figure 27.6 ). [Note: Hospitalized individuals are commonly hypercatabolic, and an “injury factor” is included in calculating their TEE.]

Figure 27.6 Estimated total energy expenditure in a healthy 20-year-old woman, 165 cm (5 feet, 4 inches) tall, weighing 50 kg (110 lb), 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. The daily expenditure of energy can be estimated by carefully recording the type and duration of all activities to determine a physical activity factor. In general, a sedentary person requires about 30%–50% more calories than the RMR (see Figure 27.6), whereas a highly active individual may require 100% or more 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.