Answer
The metabolic rate of an organism is the amount of energy (heat) released in its body per unit of time. For example, kilocalories per hour (kcal/hr) or kilocalories per day (kcal/day).
BMR my be measured by estimating heat generation with a lorimeter or by measuring oxygen consumption by the organism using a spirometer.
Calorimetric Method
In this method the subject (person) is place in a closed chamber (the calorimeter) with water-filled walls. The water will of course absorb any heat given of by the enclosed organism. The rate of energy released is estimated by measuring the change in temperature of the water over a specified period.
Spirometric Method
The spirometers an instrument used in doing breathing tests (lung function tests). Since respiratory activity is closely related to BMR this instrument has been used to derive good estimates of metabolic rates. The spirometer measures the amount of oxygen consumed by the subject over time. Every liter of oxygen consumed equals 4.82 kilocalories of calories released from organic nutrients in the body of the subject.
The metabolic rate (MR) derived by this method varies with the types of nutrient being oxidized at the time of measurement--carbohydrates, lipids, proteins. Other factors that have significant effects on the result of MR measurements include physical activity, mental state, absorptive state, post-absorptive state, and hormones including thyroid hormones. The sex of the subject (male/female) is also an important factor in MR results.
Work Step by Step
Basal metabolic rate (BMR) and total metabolic rate (TMR) represent energy/heat production in different states of the subject. BMR is the baseline metabolic rate when the subject is awake but in a relaxed/resting in a comfortable temperature, and post-absorptive state 12-14 hours after the last meal. Total metabolic rate is usually higher than BMR. The former accounts for every expenditure of energy for voluntary activity-- especially muscle contractions.
The average BMR for an adult is about 2,000 kcal/day; it is slightly less for females. This 2,000 calories supplies the energy for muscle tone, brain activity, heart and pulmonary activities, renal function, active transport and temperature maintenance. However this is not enough calories to support hard/vigorous physical activity . A sedentary person may need much fewer calories per day, however, a very active subject may require as many as 5,000 kcals/day .
Pregnancy, anxiety, fever, nutrients ( calcium, magnesium, iron), catecholamines, and thyroid hormones all affect MR measurement results. BMR is high in children but declines with maturity. TMR is particularly sensitive to prolonged starvation and to emotional states-- apathy, depression -- but genetics, and body type (ectomorph, mesomorph, endomorph) also impact these MR measurements.
