Basal metabolic rate
- Basal energy expenditure (BEE), BMR, metabolic rate, metabolism, resting metabolic rate, slowing metabolism.
- Basal metabolic rate (BMR), also called the resting metabolic rate, is the amount of energy needed to support the body's most basic functions when at rest in a neutral, or non-stressful, environment. The BMR is the number of calories a specific human needs per day to stay alive. BMR can be responsible for 70% of the calories burned per day.
- The body's most basic functions are those that are immediately necessary for a person to stay alive. These functions include pumping blood, breathing and producing heat. Nonessential functions, such as digestion and standing are not included in this calculation.
- The BMR was originally created as a tool for measuring the thyroid status of humans by comparing the metabolic rate of animals to that of humans. Today, the BMR is used as a measure of an individual's metabolism, which changes with age, weight, height, and a variety of medical conditions. A person's exercise habits may affect their BMR.
- Both basal metabolic rate and resting metabolic rate are usually expressed in terms of daily rates of energy expenditure. The early work of the scientists J. Arthur Harris and Francis G. Benedict showed that average values could be derived using body surface area (calculated from height and weight), age, and gender along with the oxygen and carbon dioxide measures taken from calorimetry. Studies also showed that by eliminating the gender differences that occur with the accumulation of adipose tissue by expressing metabolic rate per unit of "fat-free" or lean body weight, the values between genders for basal metabolism are essentially the same. Today there are less time intensive tests that are often used in place of the BMR test.
- BMR calculations are increasingly being used by the public to formulate weight loss plans that proponents claim can be tailored to the individual needs of the individual.
- Today, the BMR is under preliminary investigation to estimate everything from reproductive success to diabetes risk. The results of such inquires remain unknown.
Theory / Evidence
- The primary organ responsible for regulating metabolism is the hypothalamus. The hypothalamus is located on the brain stem and forms the floor and part of the lateral walls of the third ventricle of the cerebrum. The chief functions of the hypothalamus are: control and integration of activities of the autonomic nervous system (ANS); production and regulation of feelings of rage and aggression; regulation of body temperature; and regulation of food intake. These functions form a survival mechanism that sustains the processes that BMR measures.
- BMR decreases with age and with the loss of lean body mass. Increased muscle mass and cardiovascular exercise can increase BMR, even when the body is at rest. Measured in calories, metabolic rates vary with exertion, recent food ingestion, muscle exertion, environmental temperature, emotional state, body temperature, pregnancy, menstruation, level of thyroid hormones stress hormones (epinephrine and norepinephrine), fear and illness.
- The human body requires energy to merely stay alive and basal metabolic rate (BMR) attempts to measure this energy. As individuals age or lose muscle mass, their basal metabolic rate decreases, which means that less energy is needed to maintain the body's functions. Often, this phenomenon is referred to as the metabolism slowing down.
- BMR is measured in the calories required per day to maintain the body. Because of this, BMR can be very useful to compare caloric intake to the actual amount of calories burned by the body. If caloric intake is lower than the calories used per day, burning fat or muscle will make up the difference. A regular routine of cardiovascular exercise can increase an individual's BMR, improving health and fitness when the body's ability to burn energy gradually slows down.
- BMR does not take into account the calories needed for exercise, so it is not representative of the amount of calories burned per day. There are several factors which may affect BMR, including:
- Age: BMR reduces with age. After age 20, it drops about 2% per decade.
- Body fat percentage: The lower the body fat percentage, the higher the BMR.
- Body surface area: The greater the body surface area factor, the higher the BMR.
- Diet: Starvation, eating disorders or serious abrupt calorie-reduction can dramatically reduce BMR up to 30%. Restrictive low-calorie weight loss diets may cause an individual's BMR to drop by as much as 20%.
- Exercise: Since physical exercise burns calories, it influences body weight and helps raise an individual's BMR by building extra lean tissue (lean tissue is more metabolically demanding than fat tissue).
- Gender: Men have a greater muscle mass and a lower body fat percentage. This means they have a higher basal metabolic rate than women.
- Genes: Genetic factors play a role in the speed of a person's metabolism.
- Medications: Some drugs slow down the BMR dramatically
- Weight: BMR increases with weight.
- Other factors: Other factors include: body temperature, health, hormones, external temperature and glands/glandular function.
- This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).
- American Thyroid Association. 11 June 2006.
- BJC Behavioral Health. Calorie Needs: Calculate Your Basal Metabolic Rate. 8 May 2006.
- Hulbert AJ, Else PL. Basal metabolic rate: history, composition, regulation, and usefulness. Physiol Biochem Zool. 2004 Nov-Dec;77(6):869-76.
- World Health Organization. 11 June 2006.
- An individual's BMR is typically measured after a full night's sleep in a laboratory under optimal fasting conditions of quiet, rest and relaxation.
- In order to calculate an individual's approximate basal metabolic rate, the following questions may be answered: what is the gender of the individual; what is the weight in pounds of the individual; what is the height of the individual in inches; and what is the age of the individual?
- Men: BMR = 66 + (6.22 x weight in pounds) + (1.96 x height in inches) - (6.8 x age in years)
- Women: BMR = 655 + (4.36 x weight in pounds) + (0.71 x height in inches) - (4.7 x age in years)
- Note: Keep in mind that the BMR calculator is an estimation of the calories needed for the body to perform basic functions. An individual's total daily caloric requirements are affected by muscle mass, illness, stress, food digestion and most importantly, exercise.
- Only an overnight laboratory test can accurately measure an individual's actual BMR.
Copyright © 2011 Natural Standard (www.naturalstandard.com)
The information in this monograph is intended for informational purposes only, and is meant to help users better understand health concerns. Information is based on review of scientific research data, historical practice patterns, and clinical experience. This information should not be interpreted as specific medical advice. Users should consult with a qualified healthcare provider for specific questions regarding therapies, diagnosis and/or health conditions, prior to making therapeutic decisions.