Basal metabolic rate (BMR) is the minimal rate of expenditure per unit time by endothermic animals at rest. It is reported in energy units/unit time ranging from watt (joule/second) to ml Oz/min or joule/hour/kg body mass J/(h.kg). While measuring BMR, the person should be in a physically and psychologically undisturbed state. The person should be in a thermally neutral environment, post-absorptive state, and performing minimal metabolic activity. A thermally neutral environment means the external temperature is fairly constant. Since body temperature is constant at 37°C, any change in external temperature modifies metabolic rate. During digestion of food, metabolic activity increases. It remains fairly constant during the post-absorptive state. The requirement of minimal metabolic activity is to ensure that the energy demands of the body remain fairly constant and minimal.
Metabolism comprises the processes which are needed by the body for normal functioning. BMR is the amount of energy expressed in calories which a person needs to keep the body functioning at rest. Some of the processes are breathing, blood circulation, controlling body temperature, cell growth, brain and nerve function, and contraction of muscles. BMR affects the rate at which a person burns calories. It may be with respect to constant weight, losing weight, or gaining weight. BMR accounts for about 60-75% of daily calorie expenditure by individuals. It is influenced by several factors. It usually declines by 1-2% per decade after the age of 20. It is due to the loss of fat-free mass. There is a lot of individual variability in this process.
The primary organ responsible for regulating metabolism is the hypothalamus. Through regulation of the autonomic nervous system, it regulates the contraction of smooth, cardiac muscles and secretions of the thyroid gland which controls metabolic activity. Hypothalamus regulates body temperature. It also regulates food intake by controlling hunger and thirst center. It is the main regulator of activities like heart rate, movement of food through GIT, and contraction of the urinary bladder. Thus, it helps in sustaining body processes which are measures of BMR.
The energy expenditure breakdown for various organs:
- Liver: 27%
- Brain: 19%
- Skeletal muscle: 18%
- Kidneys: 10%
- Heart: 7%.
- Other organs: 19%
About 70% of human energy expenditure is due to basal life processes within organs of the body. About 20% of energy expenditure is for physical activities and the remaining 10% is for maintaining body temperature or digestion of food. All these processes require the intake of oxygen, food in the form of carbohydrates, proteins, and fats along with several enzymes and coenzymes. Major metabolic activities are through the process of the Krebs cycle. The output of metabolic activities is carbon dioxide which is expelled through respiration.
Metabolism comprises anabolism and catabolism. The breakdown of large molecules into smaller molecules is associated with the release of energy. It is termed catabolism. The synthesis of different molecules is termed as anabolism. Exergonic reactions are energy-releasing reactions and are generally catabolic in nature. Endergonic reactions require energy and are generally anabolic in nature.
For the Basal Metabolic Rate (BMR), most of the energy is consumed in maintaining fluid levels in tissues through osmoregulation. Only about 10% of energy is consumed for mechanical work like digestion, heartbeat, and breathing.
ATP is the intermediate molecule which drives the exergonic transfer of energy to switch to endergonic anabolic reactions used in muscle contraction. During metabolic activities, chemical bonds in ATP are broken down in the process of the Krebs cycle. It is the main source of energy needed for muscular contraction.
BMR was estimated by the Harris-Benedict equation in 1918/19.
Basal Metabolic Rate Formula
- BMR = 66.5 + (13.75 x Weight in kg) + (5.003 x Height in cm) – (6.755 ~ Age in years) {for men}.
- BMR = 665.1 + (9.563 x Weight in kg) + (1.850 x Height in cm) – (4.676 x Age in years) {for women
The equation was revised in 1984 as
- BMR = 88.362 + (13.397 x Weight in kg) + (4.799 x Height in cm) – (5.677 x Age in years) {for men}
- BMR = 447.593 + (9.427 x Weight in kg) + (3.098 x Height in cm) – (4.330 x Age in years) {for women}
- 95% confidence range for men is +/- 213.0 kcal/day; and for women +/- 201 kcal/day.
The equation was further modified in 1990 as
- Men: BMR = (10 x Weight in kg) + (6.25 x Height in cm) – (5 x Age in years) + 5
- Women: BMR = (10 x Weight in kg) + (6.25 x Height in cm) – (5 x Age in years) – 161
For various metabolic activities recommended daily kcal intake to maintain current weight are as follows:
- Little or no exercise: daily kcal need = 1.2 x BMR
- Light exercise (1-3 days per week): daily kcal need = 1.375 x BMR
- Moderate exercise (3-5 days per week): daily kcal need = 1.55 BMR
- Heavy exercise (6-7 days per week): daily kcal needs = 1.725 x BMR
- Very heavy exercise (twice per day): daily kcal need = 1.9 x BMR
Make sure you also check our other amazing Article on: Anatomy and physiology of the Liver
