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Energy Balance and Temperature Regulation Chapter 16
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Energy Balance Energy balance is the concept that states: Body weight will remain constant if energy in (calories consumed) = energy out (calories expended). This is also called neutral energy balance. The majority of food energy is actually used in the body as heat. Ultimately, it’s all turned into heat.
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The body converts most food energy into heat. The energy of ingested food represents energy input. Energy released from the broken bonds in food molecules is used, in part, to make ATP. Energy from ingested nutrients is used either immediately to perform work or stored in molecules in the body for future use.
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Energy output has two categories: External work is expended when skeletal muscles move external objects. Internal work refers to the expenditure of energy that does not accomplish mechanical work outside the body. (metabolism) Energy from nutrients that does not perform work is converted to heat (about 60%). Heat energy cannot be used to perform work. The other 40% of nutrient energy is transferred to ATP.
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Food energy Metabolic pool in body Internal work Thermal Energy (heat) External work Energy storage
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Internal work Internal work is the energy used to keep the body alive. This includes: Pumping blood Nerve activity Cell transport Production of cellular parts This is usually referred to as the BMR (Basal Metabolic Rate) which uses the majority of our daily caloric needs (60-70% for most people).
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The metabolic rate is the rate of energy use. Most of the energy expenditure of the body eventually appears as heat. Therefore this rate is expressed as the rate of heat production in kilocalories per hour. A person’s metabolic rate is measured under standardized basal conditions. This is the basal metabolic rate (BMR).
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The BMR is measured under rather stringent conditions. The person should be: Resting (no recent physical activity) Fasting (no food for at least 8 hours) Comfortable (not too hot or cold) Supine (lying down) Healthy (no disease--fever) Awake After this rate is determined, it is compared to the normal values of people.
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The RMR (Resting Metabolic Rate) Since the BMR is measured under such controlled (and unnatural) conditions, a more realistic metabolic rate is usually established called: the RMR (Resting Metabolic Rate). The RMR establishes the minimum number of calories necessary to sustain body functioning (life) for a 24 hour period, not counting activity. The RMR is about 10% higher than the BMR. When combined with the calories used to perform external work = total daily energy expenditure.
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Energy Balance A neutral energy balance occurs if the energy in food intake equals the amount of energy expended daily.
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