# Basal metabolic rate

### Basal metabolic rate

Basal metabolic rate (BMR) is the minimal rate of energy expenditure per unit time by endothermic animals at rest. (McNab, B. K. 1997). On the Utility of Uniformity in the Definition of Basal Rate of Metabolism. Physiol. Zool. Vol.70; Metabolism refers to the processes that the body needs to function. Basal Metabolic Rate is the amount of energy expressed in calories that a person needs to keep the body functioning at rest. Some of those processes are breathing, blood circulation, controlling body temperature, cell growth, brain and nerve function, and contraction of muscles. Basal metabolic rate (BMR) affects the rate that a person burns calories and ultimately whether you maintain, gain, or lose weight. Your basal metabolic rate accounts for about 60 to 75% of the calories you burn every day. It is influenced by several factors. [1]

## Contents

• Description 1
• Nutrition and dietary considerations 2
• Physiology 3
• BMR estimation formulas 3.1
• Causes of individual differences in BMR 3.2
• Biochemistry 4
• Glucose 4.1
• Fats 4.2
• Proteins 4.3
• Aerobic vs. anaerobic exercise 4.4
• Longevity 5
• Organism longevity and basal metabolic rate 5.1
• Medical considerations 6
• Cardiovascular implications 7
• Notes 9
• References 10

## Description

The body's generation of heat is known as thermogenesis and it can be measured to determine the amount of energy expended. BMR generally decreases with age and with the decrease in lean body mass (as may happen with aging). Increasing muscle mass has the effect of increasing BMR. Aerobic fitness level, a product of cardiovascular exercise, while previously thought to have effect on BMR, has been shown in the 1990s not to correlate with BMR when adjusted for fat-free body mass. New research has, however, come to light that suggests anaerobic exercise does increase resting energy consumption (see "Aerobic vs. anaerobic exercise"). Illness, previously consumed food and beverages, environmental temperature, and stress levels can affect one's overall energy expenditure as well as one's BMR.

Indirect calorimetry laboratory with canopy hood (dilution technique)

BMR is measured under very restrictive circumstances when a person is awake. An accurate BMR measurement requires that the person's sympathetic nervous system not be stimulated, a condition which requires complete rest. A more common measurement, which uses less strict criteria, is resting metabolic rate (RMR).[2]

BMR may be measured by gas analysis through either direct or Indirect Calorimetry, though a rough estimation can be acquired through an equation using age, sex, height, and weight. Studies of energy metabolism using both methods provide convincing evidence for the validity of the respiratory quotient (R.Q.), which measures the inherent composition and utilization of carbohydrates, fats and proteins as they are converted to energy substrate units that can be used by the body as energy.

## Nutrition and dietary considerations

Basal metabolism is usually by far the largest component of total caloric expenditure. However, the Harris–Benedict equations are only approximate and variation in BMR (reflecting varying body composition), in physical activity levels, and in energy expended in thermogenesis make it difficult to estimate the dietary consumption any particular individual needs in order to maintain body weight.

## Physiology

The early work of the scientists J. Arthur Harris and Francis G. Benedict showed that approximate values for BMR could be derived using body surface area (computed from height and weight), age, and sex, along with the oxygen and carbon dioxide measures taken from calorimetry. Studies also showed that by eliminating the sex differences that occur with the accumulation of adipose tissue by expressing metabolic rate per unit of "fat-free" or lean body mass, the values between sexes for basal metabolism are essentially the same. Exercise physiology textbooks have tables to show the conversion of height and body surface area as they relate to weight and basal metabolic values.

The primary hypothalamus. The hypothalamus is located on the diencephalon and forms the floor and part of the lateral walls of the third ventricle of the cerebrum. The chief functions of the hypothalamus are:

1. control and integration of activities of the autonomic nervous system (ANS)
• The ANS regulates contraction of smooth muscle and cardiac muscle, along with secretions of many endocrine organs such as the thyroid gland (associated with many metabolic disorders).
• Through the ANS, the hypothalamus is the main regulator of visceral activities, such as heart rate, movement of food through the gastrointestinal tract, and contraction of the urinary bladder.
2. production and regulation of feelings of rage and aggression
3. regulation of body temperature
4. regulation of food intake, through two centers:
• The feeding center or hunger center is responsible for the sensations that cause us to seek food. When sufficient food or substrates have been received and leptin is high, then the satiety center is stimulated and sends impulses that inhibit the feeding center. When insufficient food is present in the stomach and ghrelin levels are high, receptors in the hypothalamus initiate the sense of hunger.
• The thirst center operates similarly when certain cells in the hypothalamus are stimulated by the rising osmotic pressure of the extracellular fluid. If thirst is satisfied, osmotic pressure decreases.

All of these functions taken together form a survival mechanism that causes us to sustain the body processes that BMR measures.

### BMR estimation formulas

Several prediction equations exist. Historically, the most notable one was the Harris-Benedict equation, which was created in 1919.

The Original Harris-Benedict Equation:

• for men, P = \left ( \frac {13.7516 m} {1 ~ \mbox {kg}} + \frac {5.0033 h} {1 ~ \mbox {cm}} - \frac {6.7550 a} {1 ~ \mbox {year}} + 66.4730 \right ) \frac {\mbox {kcal}} {\mbox {day}}
• for women, P = \left ( \frac {9.5634 m} {1 ~ \mbox {kg}} + \frac {1.8496 h} {1 ~ \mbox {cm}} - \frac {4.6756 a} {1 ~ \mbox {year}} + 655.0955 \right ) \frac {\mbox {kcal}} {\mbox {day}}

where P is total heat production at complete rest, m is mass (kg), h is height (cm), and a is age (years), and with the difference in BMR for men and women being mainly due to differences in body weight.[3] For example, a 55-year-old woman weighing 130 lb (59 kg) and 5 feet 6 inches (168 cm) tall would have a BMR of 1272 kcal per day or 53 kcal/h (61.3 watts).

In 1984, the original Harris-Benedict equations were revised[4] using new data. In comparisons with actual expenditure, the revised equations were found to be more accurate.[5]

The Revised Harris-Benedict Equation:

• for men, P = \left ( \frac {13.397 m} {1 ~ \mbox {kg}} + \frac {4.799 h} {1 ~ \mbox {cm}} - \frac {5.677 a} {1 ~ \mbox {year}} + 88.362 \right ) \frac {\mbox {kcal}} {\mbox {day}}
• for women, P = \left ( \frac {9.247 m} {1 ~ \mbox {kg}} + \frac {3.098 h} {1 ~ \mbox {cm}} - \frac {4.330 a} {1 ~ \mbox {year}} + 447.593 \right ) \frac {\mbox {kcal}} {\mbox {day}}

It was the best prediction equation until 1990, when Mifflin et al.[6] introduced the equation:

The Mifflin St Jeor Equation:

• P = \left ( \frac {10.0 m} {1 ~ \mbox {kg}} + \frac {6.25 h} {1 ~ \mbox {cm}} - \frac {5.0 a} {1 ~ \mbox {year}} + s \right ) \frac {\mbox {kcal}} {\mbox {day}}, where s is +5 for males and −161 for females.

According to this formula, the woman in the example above has a BMR of 1204 kcal per day. During the last 100 years, lifestyles have changed and Frankenfield et al.[7][8] showed it to be about 5% more accurate.

These formulas are based on body weight, which does not take into account the difference in metabolic activity between lean body mass and body fat. Other formulas exist which take into account lean body mass, two of which are the Katch-McArdle formula, and Cunningham formula. The Katch-McArdle formula is used to predict Resting Daily Energy Expenditure (RDEE).[9] The Cunningham formula is used to predict RMR instead of BMR.[10]

The Katch-McArdle Formula (Resting Daily Energy Expenditure):

• P = 370 + \left( {21.6 \cdot LBM} \right), where LBM is the lean body mass in kg.

According to this formula, if the woman in the example has a body fat percentage of 30%, her RDEE (the authors use the term of basal and resting metabolism interchangeably) would be 1263 kcal per day.

### Causes of individual differences in BMR

The basal metabolic rate varies between individuals. One study of 150 adults representative of the population in Scotland reported basal metabolic rates from as low as 1027 kcal per day (4301 kJ/day) to as high as 2499 kcal/day (10455 kJ/day); with a mean BMR of 1500 kcal/day (6279 kJ/day). Statistically, the researchers calculated that 62.3% of this variation was explained by differences in

• Harris, JA; Benedict, FG (1918). "A Biometric Study of Human Basal Metabolism". Proceedings of the National Academy of Sciences of the United States of America 4 (12): 370–3.
• Harris-Benedict study. Detailed discussion of antecedents, data, measurements, statistics (Published by The Carnegie Institution of Washington 1919)
• BMR as affected by alcohol
• BMR and personality