Basal Metabolic Rate

1. Basal Metabolic Rate

Definition

BMR is the rate of amount of heat given out by a subject per square meter of body per hour who should be awake, preferably lying on bed after a good night’s sleep, should be in a state of complete physical and mental rest, should be in post absorptive phase i.e. 12 hours after the last meal, should be in optimal environment i.e. normal room temperature, pressure and humidity.

In short BMR is the rate of heat generation per sq. meter of body surface per hour under basal metabolic conditions. BMR is a very important parameter of physiology of human body.

BMR Values-

adult females	37 calories per sq. meter of body surface per hour
adult males	40 calories per sq. of body surface meter per hour

Factors affecting BMR- BMR can vary from person to person under certain conditions which are as follows-

1.     Age-The BMR of children is much higher than the adults. It is roughly inversely proportional to the age. BMR is progressively reduced with the advancing age in children. It is because children possess greater surface area in proportion to their weight. Exceptions are new born or premature babies where BMR is 25 calories per sq. of body surface meter per hour or even lower.

2.     Gender-BMR is higher in males then in females due to more muscles and larger surface area.

3.     Surface Area- Larger the surface area of a person more will be the heat loss and consequently more will be heat production and more will be BMR.

4.     Body Temperature- For every 1℃ rise in body temperature as in fever BMR is increased by about 12% due to more heat production in the body.

5.     Exercise- Light exercise like sitting or standing can increase metabolism above 40%, moderate exercise like walking with normal pace can increase it up to 60%. Severe exercise like running and sports can increase it up to 100%.

6.     Mental work- As brain already uses 10% of total O₂ consumption of body so its basal metabolism is high therefore mental work like studying increases BMR by 3-4%.

7.     Climate- In cold climate areas BMR is higher due to more heat production in the body. Reverse is the case in hot climate areas.

8.     Habits- Persons with active life styles like sportsmen, trained athletes, soldiers and laborers doing hard physical work have higher BMR due to increased metabolism compared to persons with sedentary life style.

9.     Diet- Under nutrition, if prolonged, may cause lower BMR due to lack of molecules to be metabolized.

10.                        Hormones- Circulating levels of certain hormones affect BMR of a person. Increase in Thyroxin (T4), Thyroxin stimulating hormone (TSH), Growth Hormone and Testosterone levels in blood increase BMR. Increase in Estrogen and Progesterone levels in blood increase BMR marginally.

11.                        Atmospheric Pressure-Moderate reduction in atmospheric pressure does not change BMR however a fall of half atmospheric pressure as seen in mountain climbers does increase BMR. Rise in atmospheric pressure does not affect BMR.

12.                        Pregnancy- After six months of gestation BMR rises as a sum total of BMRs of mother and fetus but actual BMR of mother does not change. Hence pregnancy does not affect BMR.

13.                        Medicines- Certain pharmacological molecules like Caffeine, Resveratrol,

L-Carnitine etc increase BMR while anesthetics decrease it.

14.                        Racial variation- Africans and South eastern people have lower BMR then white people. So there is some difference in BMR of various races.

15.                        Diseases- Certain diseases like Hyperthyroidism, fever, cardio renal diseases, polycythemia and Leukemia etc increase metabolic rate while Hypothyroidism, Addison’s disease and chronic malnutrition reduce it.

Measurement of BMR- BMR is the amount of heat production in calories therefore it can be measured by Direct or Indirect Calorimetry as described earlier. There are several formulas also to calculate a person’s BMR as described below-

Harris–Benedict equations-The original Harris–Benedict equations were published in 1918 and 1919 as given below-

Men	BMR = 66.5 + ( 13.75 × weight in kg ) + ( 5.003 × height in cm ) – ( 6.755 × age in years )
Women	BMR = 655 + ( 9.563 × weight in kg ) + ( 1.850 × height in cm ) – ( 4.676 × age in years )

 

The Harris–Benedict equations were revised by Roza and Shizgal in 1984 which is given below-

Men	BMR = 88.362 + (13.397 × weight in kg) + (4.799 × height in cm) - (5.677 × age in years)
Women	BMR = 447.593 + (9.247 × weight in kg) + (3.098 × height in cm) - (4.330 × age in years)

 

The Harris–Benedict equations were revised by Mifflin and St Jeor in 1990 which is given below-

Men	BMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age in years) + 5
Women	BMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age in years) - 161

 

Read’s Formula- This formula is often used in clinical setting and is ±10% accurate. It is very simple to use as seen below-

BMR=0.75(Pulse Rate+0.74xPulse Pressure)-72

Significance of BMR-

• 1.    For the diagnosis of certain diseases
• 2.     For finding out type of food being oxidized
• 3.     For finding out causative relationship between certain conditions and metabolism
• 4.     For prescribing appropriate diet for weight loss or weight gain
• 5.     For prescribing appropriate exercises to sports persons