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. 27.6).] 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. 27.7). [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.] 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. 27.7), 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.
IV. ACCEPTABLE MACRONUTRIENT DISTRIBUTION RANGES
Acceptable Macronutrient Distribution Ranges (AMDR) are defined as a range of intakes for a particular macronutrient that is associated with reduced risk of chronic disease while providing adequate amounts of essential nutrients. The AMDR for adults is 45%–65% of their total calories from carbohydrates, 20%– 35% from fat, and 10%–35% from protein (Fig. 27.8). The biologic properties of dietary fat, carbohydrate, and protein are described below.
V. DIETARY FATS
The incidence of a number of chronic diseases is significantly influenced by the kinds and amounts of nutrients consumed (Fig. 27.9). Dietary fats most strongly influence the incidence of coronary heart disease (CHD), but evidence linking dietary fat and the risk for cancer or obesity is much weaker.
Earlier recommendations emphasized decreasing the total amount of dietary fat. Unfortunately, this resulted in increased consumption of refined grains and added sugars. Data now show that the type of fat is a more important risk factor than the total amount of fat.
A. Plasma lipids and coronary heart disease