What is Metabolic Rate ?
Definition of Metabolic Rate
Metabolic rate is the energy used by the body in the performance of its normal functions including both maintaining the body itself (replacing tissues, etc.) and using the body to perform external functions e.g. physical work, sports and daily tasks.
= Basal Metabolic Rate
+ Rate of Energy Use for Activity
+ Rate of Heat Energy Produced by body for processing food through the digestive system
Units of Metabolic Rate:
In general, a "rate" (of anything) is an amount of something per time, e.g. packing boxes at a rate of 5 boxes packed per minute.
Metabolism and metabolic rates concern use of energy and so, in the case of metabolic rates, use of energy per time.
In the physical sciences incl. e.g. modern physics and chemistry, energy is measured in units called "Joules" (J). When the quantity of energy is of the order of thousands of Joules it is often expressed in "kilo Joules" (kJ) where 1 x kJ = 1000 J
However, in the context of diet and nutrition as well as in some older physical sciences papers and textbooks energy is stated in units called "calories" (cal) or "kilocalories" (kcal) where 1 x kcal = 1000 calories. A kilocalorie (kcal) also also known as a Calorie (written with an upper-case "C").
Definitions of Units of Energy:
- One Joule (J) is the energy expended in applying a force of one Newton (N) through a distance of one metre (m),
hence J = Nm, OR an equivalent definition is:
- One Joule (J) is the energy expended in passing an electric current of one Ampere (A) through a resistance of one Ohm (Ω) for one second (s).
- One calorie (cal) is the heat energy required to increase the temperature of 1 gram of water by one degree Celsius (oC).
Many packets of foodstuffs list the energy value of the food in Calories stating the units as "kcal". The *energy content of food and drink products is often stated in both Calories (kcal) and kJ.
Using the scientific unit of of energy (kJ), metabolic rates could be stated in units of kJ per second (kJ/s), kJ per minute (kJ/min) or kJ per hour (kJ/h). In the physical sciences it is common and often preferred to state rates or speeds "per second" but there are many exceptions e.g. road speeds stated in mph (miles per hour) in the UK and United States and kph (kilometres per hour) in many European countries. An hour is a convenient duration of time over which to consider human activity so kJ per hour may seem to be a good choice for expressing metabolic rates. kJ per hour can be written kJ/h or kJh-1.
However, people who work with values of metabolic rates, incl. human metabolic rates (especially human basal metabolic rates, BMRs) have found it convenient to scale the value of metabolic rate according to the size of the person in terms of the surface area of his or her body. In general, areas are measured in units of length squared, e.g. square metres (m2) or square centimetres (cm2) or, using the older "imperial" units, as in "dots per square inch" (dpi) as used to specify the resolution of images, electronic displays and similar.
When scaled by the surface area of the person's body, a metabolic rate can be stated in units of:
kJ per square metre of body surface area per hour,
which can be written as kJ / m2 / h or (equivalently) kJ m-2 h-1
This is why some textbooks state values of basal metabolic rate (BMR) in units of kJ / m2 / h or kJ m-2 h-1.
However, as different sources (books, websites, research papers, etc.) may use different units, it is always important to note the units stated and to state units clearly when quoting, measuring and writing about metabolic rates.
Importance of Metabolic Rate:
Metabolic Rate is one way to consider and express the overall level of function and general activity of the body.
Metabolic rate is of particular interest to professionals concerned with the management of body mass (colloquially referred to as "weight"), e.g. dieticians, nutritionists, medical specialists concerned with weight management, sports coaches and, of course, individual people who are concerned about weight management, including weight loss.
How does the body use the energy included in "Metabolic Rate" ?
As stated below the definition of metabolic rate at the top of this page, the three main parts of metabolic rate are:
- Basal metabolic rate (BMR) which is the metabolic rate at rest,
see Basal Metabolic Rate (BMR) for more information about energy use when the body is at rest.
- The rate of energy use for activity at any particular time.
Examples of typical amounts of energy required by and consumed during some common activities is listed below.
These are estimations of typical amounts of energy. Actual values in any particular case depend on only on the activity and duration but also on the person participating in the activity, e.g. if male or female, age, level of fitness, etc..
kcal per hour
kcal per min
Walking Slowly e.g. approx 2.5 mph
Light Housework e.g. dusting or using a vacuum cleaner
1 kcal = 4.1868 kJ (approx)
1 kJ = 0.2388 kcal (approx)
1 kcal per hour = 1/60 kcal per min
1 kcal per hour = 0.1667 kcal per min (approx)
1 kcal per min = 60 kcal per hour
If comparing the values in this table with other sources of similar information always note the units.
Some tables of energy usage for activities give values of energy in kcals while others use kJ.
Similarly, some give values per hour while others give values per minute, or even estimated values per unit of body weight. (Values of energy usage for activities are often rounded e.g. to the nearest 5 of the units used.)
- The energy involved in processing foodstuffs through the digestive system which is formally known as "dietary induced thermogenesis" or sometimes "diet induced thermogenesis" (DIT). It may also be called the "thermic effect of food".
Dietary induced thermogenesis (DIT) is the heat generated by the body following the digestive processes (of eating/ingestion, digestion and absorption/assimilation followed by elimination/defecation - see stages of digestion). This heat energy contributes to the body's "resting", i.e. non-physically-active metabolic rate - although not its basal metabolic rate which is measured at least 8-12 hours after eating in order to exclude the effects of DIT. The heat energy is produced in Brown Adipose Tissue (BAT) due to increased activity of the sympathetic nervous system. BAT is a special type of fat that is produced in various amounts by different types of mammals. For example, bears, rodents and other hibernating animals living in cold environments have plenty of BAT but humans usually only have small deposits throughout the body.