Energy requirements during development can be partitioned into components of basal metabolism, thermogenesis, physical activity, and energy cost of growth. Basal metabolism is defined as that energy expended to maintain cellular and tissue processes fundamental to the organism. Energy is needed to maintain body temperature, support the minimal work of the heart and respiratory muscles, and supply energy to other tissues at rest. The basal metabolic rate (BMR) is measured under standard conditions defined as awake and supine in a fasting, relaxed state in a thermoneutral environment. The thermic effect of feeding (TEF) refers to the energy required for the ingestion and digestion of food and for the absorption, transport and utilization of nutrients. The TEF amounts to about 10% of the daily energy expenditure . Thermoregulation can constitute an additional energy cost when exposed to temperatures below and above thermoneutrality; however, clothing and behavior usually counteract such environmental influences. Physical activity is the most variable component of energy requirements, and entails both obligatory and discretionary physical activities. The energy requirement for growth relative to maintenance is low, except for the first months of life. The energy cost of growth as a percentage of total energy requirements decreases from around 35% at 1 month to 3% at 12 months of age, and remains low until the pubertal growth spurt, at which time it increases to about 4% .
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