We often hear about ‘ Metabolism ’ when we think about health. It simply means ‘various chemical reactions happening inside the body when it is converting the food we eat into energy’. These reactions are either breaking down a complex substance into smaller ones (catabolism) or building a complex substance from smaller ones (anabolism). Energy is also used or spent during these reactions.
When metabolism is all about energy, ‘metabolic rate’ is the energy that is utilized for the body's basic functions such as digestion, respiration, circulation, regulation of body temperature calculated over a given period of time. Basal metabolic rate (BMR) is defined as the minimum amount of energy that is utilized by the body at rest. How do we measure or calculate BMR? Will it be the same or different for all people? Does it change based on various factors? Let’s quickly learn about it in simple terms.
Measuring Basal Metabolic Rate (BMR)
Basal metabolic rate is a measure of the heat produced by the body to maintain the basic functions over a particular period of time. The heat produced (heat energy) is typically measured in ‘ calories ’ and in humans, it is ‘ kilocalories ’.
BMR can be calculated under strict conditions that include being in a lying position after 8 hours of sleep with the environmental temperature 22-26 C (Why? Because, beyond these values the body spends energy to keep the body in a balanced state – ‘Homeostasis’), fasted (not eaten in the past 12 hours), to be at rest without any physical activity. Because of these rigid conditions, researchers started measuring resting metabolic rate (RMR) where the person is under rest for a period of time (not as soon as waking up) and the period of fasting is under 12 hours.
Both BMR and RMR are often used interchangeably, as there is only very little difference between them (RMR value appears little more than BMR value). The difference is around 10-20% higher than the BMR, either due to intake of food ( see Thermic effect of Food , TEF ) or recent physical activity. BMR is usually calculated using the various anthropometric measurements in the formulas. It is also measured in laboratory settings by two different ways, direct calorimetry and indirect calorimetry. These are not practical for the general population.
Basal metabolic rate will differ for every individual based on various other factors. Two persons might have same BMR (calculated with formulas) but the actual metabolic rate differ with their age, body mass (weight), gender, muscle mass, nutritional status, phases of life (such as pregnancy, menopause for women), hormonal status, cold/hot temperature regions and also if any, disease.
To understand better, let's look at one of the factors, nutritional status. If a person's nutritional status (macro or micro) is insufficient or deficient, the body, in an effort to conserve energy, reduces BMR and the person will show less progress with either a health or fitness goal.
Calculation of BMR calories
To know the minimum amount of calories that the body needs for basal functions, we use variables of the body (anthropometric measurements) such as age, height, and weight.
The numbers are used in a formula devised by Harris-Benedict (various revised formulas exist today but can follow any one of them).
The revised Harris-Benedict equation by Mifflin et.al., that is used to calculate BMR calories is,
| BMR (Men) = (10 * weight in kg) + (6.25 * height in cm) - (5 * age in years) + 5 |
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| BMR (Women) = (10 * weight in kg) + (6.25 * height in cm) - (5 * age in years) - 161 |
We will understand more about energy expenditure apart from for basal functions in the next topic ‘ Total Daily Energy Expenditure ’ (TDEE).
References
- Piperata, B. A. (2018) ‘Basal metabolic rate (human)’, The International Encyclopedia of Biological Anthropology, pp. 1–5. doi: 10.1002/9781118584538.ieba0045.
