Caloric needs vary depending on age, just as do nutrient needs. Healthy full-term newborns require, on average, 120 kcal/kg/day to meet their energy needs and sustain growth. On a per kilogram basis, caloric needs decrease with age. The average 1-3 year old child requires only 100 kcal/kg/day. Once growth has ceased caloric requirements are impacted by age, activity and other health factors, but can be approximated as 1500 kcal/M2/day.
Individual health issues also impact caloric needs. For example, children with heart disease have increased caloric needs because of increased work required to circulate blood with an inefficient pump; sedentary children require fewer calories than very active children; and burned children have extremely large increases in caloric needs during the healing process.
Calories, once absorbed, are used in four ways:
- for growth,
- for work and activity,
- to support bodily functions,
- and all the rest go into storage (fat!).
Growth utilizes a huge proportion of the calories ingested by children. When growth ceases, the caloric intake must decrease in order to maintain weight. If caloric intake exceeds requirements, the extra intake will remain in body stores.
The proportion of calories used for activity varies, thus sedentary children are at risk for obesity.
Several terms must be explained in order to understand the use of calories to support life functions (e.g. circulation, respiration).
- Resting Energy Expenditure (REE) is the energy expended by a person at rest in a thermoneutral environment.
- Basal Metabolic Rate (BMR) is the energy expenditure for an individual under standard conditions after a 12-18 hour fast, recumbent, at mental rest and in a thermoneutral environment.
REE and BMR usually differ by less than 10%, but REE is higher because it includes energy expenditures for mental activity and other energy expenditures. Both are measured as kilocalories (kcals) or units of heat. Both will be affected by age, body size and composition, disease states such as febrile illnesses, malabsorption, trauma or infection.
Healthy newborns require about 55 kcal/ kg/day to satisfy needs for basal metabolism; mature humans require only 25-30 kcal/kg/day.
- Thermal effect of food is the increase in metabolism as a result of the ingestion and assimilation of food. To a large degree, the food components determine the thermal effect of food. Carbohydrates and fats are relatively sparing in their impact upon energy expenditure and increase the metabolic rate by only 4 % (for carbohydrates) to 6 % (for fats). Ingestion of proteins increases the basal metabolic rate by as much as 30% until the material is completely digested and assimilated. Thus carbohydrates can be considered as "cheap" energy sources.
Healthy infants use about 7-10% of ingested calories to support the thermal effect of food; older children apply on the average about 5% of ingested calories toward this purpose.