Basal Metabolic Rate
This continues from the page about metabolic rate, which states that BMR is the rate of energy consumed by the body when at rest.
What is Basal Metabolic Rate (BMR) ?
Definition of Basal Metabolic Rate (BMR)
Basal Metabolic Rate (BMR) is the rate of energy expended by animal (such as human) bodies when at rest.
This is the minimum amount of energy needed by the organism to perform essential functions such as breathing and associated movements, heartbeat and blood circulation, synthesis of molecules e.g. proteins, maintenance of ion gradients across membranes, and so on. Another way to describe this is energy that is just sufficient for the functioning of the body's tissues and vital organs.
What is meant by "at rest" ?
BMR is measured:
- When the subject (e.g. a person) is lying still, i.e. not making physical movements
- When the subject (e.g. a person) is quiet, i.e. not talking or otherwise under mental stress
- In a room that is comfortably warm - so that the body does not need to adjust body temperature
- Approx. 12-18 hours after the subject's most recent meal, so that energy is not being used for digestion and absorption of food (which can require significant energy).
This is a short statement of the meaning of "at rest" re. definition of animal basal metabolic rates; different considerations apply to plants.
Basal Metabolic Rate (BMR) is stated in different units by different sources. For example, BMR is sometimes stated in kcal/day. Alternatively, BMR can be expressed in units of kJ per square metre of body surface area per hour. This can be written as kJ / m2 / h or kJ m-2 h-1. For an explanation see the "Units of Metabolic Rate" section on metabolic rate.
Always check the units used and think through their meaning before making comparisons based on values of BMR.
Factors affecting Basal Metabolic Rate (BMR)
Main Factors affecting BMR:
BMR decreases as age increases because:
- The proportion of lean body mass decreases as age increases
- The synthesis of molecules such as proteins decreases as age increases.
Males usually have a higher BMR than females (of the same age) because males tend to have a higher proportion of lean body mass than females of the same age.
Conversely, females tend to have a higher proportion of fat cells and fat cells have a lower metabolic rate than lean muscle cells.
BMR increases during pregnancy and lactation due to the high energy requirement of producing foetal tissues and then breast milk.
Other factors that increase BMR:
lncreased intake of foods
Eating large amounts of food requires the digestive system to process more material which therefore requires more energy. (However, as food is the source of the body's energy any resulting increase in BMR is likely to be less than the increase in energy intake due to the additional consumption of food.)
lncreased secretion of certain hormones
Increased physical exercise
Exercise requires and uses energy. Although BMR is energy expended at rest, exercise has both short-term effects (during the time of the exercise itself) and long-term effects (after and between exercise sessions). One of the longer term effects of frequent physical exercise is an increase in BMR. This is due to increased overall activity of the heart and vascular system, together with other body systems and tissues. See also the effects of exercise on circulation and the effects of exercise on muscles.
e.g. extremes of temperature. If/when body temperature increases above or decreases below its ideal range, mechanisms within the body act to reduce or raise its temperature to preserve health (and ultimately life - as prolonged elevated body temperature can destroy proteins within the body and prolonged depressed temperatures can cause cardiac arrhythmias - which can be fatal in both cases). The body's temperature regulation mechanisms require and use energy. If/when these mechanisms are often or continuously active, BMR increases to allow for such activity and corresponding use of energy even when the body is at rest. In general, the higher the body temperature the higher the metabolic rate, hence metabolic rate is higher when a person has a fever.
Stress is sometimes thought of as an unpleasant combination of overwhelming worry, anxiety, fear and the feelings of constraint and helplessness. Such emotions are associated with physical responses within the body which vary but may include e.g. increased heart rate or blood pressure, difficulty sleeping, nausea, abdominal discomfort, increased sweating, etc.. To the extent that these involve (e.g. increased heart rate), cause (e.g. difficulty sleeping) or are caused by (e.g. nausea, abdominal issues) increased bodily function and so increased use of energy, BMR increases accordingly.
Other factors that decrease BMR:
If the body is not supplied with sufficient energy and nutrients in the form of solid food and liquids (water, drinks, soups, etc.) it becomes unable to support the body by performing all necessary functions as effectively as if supplied with those. When the body is not sustaining itself and e.g. replenishing worn-out tissues effectively, it tends to use less energy than when supplied with the dietary resources to do so.
During fasting the digestive system does not expend energy processing foodstuffs at each stage of the alimentary canal. The body's energy requirement is therefore adjusted to be lower during fasting than at times of regular eating.
The body uses less energy during sleep than when awake. The proportion of time a person spends sleeping can affect his or her BMR.
Measurement of Basal Metabolic Rate (BMR)
Basal Metabolic Rate (BMR) is measured by calorimetry, either direct calorimetry or indirect calorimetry :
- Direct calorimetry
The heat released from the body of a person who is resting, e.g. lying still on a comfortable surface or mattress, while breathing normally is used to increase the temperature of circulating water.
The resulting increase in the temperature of the water is proportional to the BMR of the person.
- Indirect calorimetry
While a person rests lying still on a comfortable surface or mattress, a respirometer is used to measure his or her oxygen consumption. His or her BMR is then calculated from the volume of oxygen he or she used according to:
BMR (in kJ/m2/h) = [volume of oxygen gas used (in dm3)] x 18
This formula applies because it has been found that the average human adult typically releases 18 kJ of energy (heat energy) per 1x dm3 of oxygen used by the body.
See also types of sugar, carbohydrates, dietary fibre, fatty acids, fats and proteins.
More about Metabolism