The macronutrients (carbohydrates, proteins, fats and oils) we consume in our diet help to supply the energy needed by the body to keep it working. This energy is used to drive the complex chemical, mechanical and electrical systems of the body. The rate of energy release from macronutrients by chemical processes occurring in the body is known as metabolic rate.
The amount of energy needed to keep the body functioning in a resting, awake and fasting state in a comfortably warm environment is called the resting metabolic rate (RMR).
RMR varies from one person to another due to the following influences:
- Sex – males release energy from their food 5–7% faster than females.
- Size – the more surface area a person has, the greater is the RMR.
- Body composition – the higher the ratio of lean tissue to fat tissue, the higher is the RMR.
- Thyroid hormone – this hormone stimulates resting metabolism. Too little and the RMR drops.
- Age – the younger the individual, the higher is the RMR for a given size and sex.
- Other factors – fever, certain drugs like caffeine, pregnancy, lactation and emotions all increase the RMR.
A healthy boy in the 12–14 year old age group, of average height and weight, has an RMR of about 6,500 kilojoules per day. For a girl of similar age, the figure would be about 5,500 kilojoules per day.
The total metabolic rate is the amount of energy used by the body in carrying out all of its activities. It is often expressed in kilojoules per day.
The factors that determine the total metabolic rate are:
- resting metabolic rate
- the energy needed to do all kinds of muscular work, from sitting up in bed to very strenuous physical labour or exercise
- the thermic effect of food.
It takes energy to metabolise foods. For example, carbohydrates and fats have a thermic effect of about 5% whereas protein is about 30%. This means that, to metabolise 100 kilojoules of carbohydrate, 5 kilojoules is needed, leaving the body with 95 kilojoules of useful energy. In comparison, to metabolise 100 kilojoules of protein, 30 kilojoules is needed, leaving the body with 70 kilojoules.
On average for a mildly active person, about 60% of the body’s total daily energy expenditure is due to resting metabolism, about 32% of the body’s energy expenditure comes from physical activity and another 8% comes from the thermic effect.
Using the RMR figures given above, this means that the boy’s total metabolic rate is about 11,000 kilojoules per day and the girl’s is 9,200 kilojoules per day.
Equation for calculating RMR
A relatively simple way of determining resting metabolic rate is to use an equation known as the Mifflin equation. It is based on body mass (w), height (h), age (a) and sex.
Although it does not take ethnicity or amount of fat tissue to lean tissue into account, it is still a good predictive measure.
The formula is:
- Males: RMR = (10 x w) + (6.25 x h) - (5 x a) + 5
- Females: RMR = (10 x w) + (6.25 x h) - (5 x a) - 161
For a teenage boy of body mass 60kg, height 165cm, age 14 years:
= (10 x 60) + (6.25 x 165) - (5 x 14) + 5
= 600 + 1,031.25 - 70 + 5
= 1,566.25 calories x 4.18 to convert to kilojoules
= 6,546.9kJ per day
For a teenage girl of body mass 50kg, height 163cm, age 14 years:
= (10 x 60) + (6.25 x 165) - (5 x 14) + 5
= (10 x 50) + (6.25 x 163) - (5 x 14) - 161
= 1,287.75 calories x 4.18 to convert to kilojoules
= 5,382.8 kJ per day
Balancing energy in and energy out
Balancing energy intake by eating food with output through resting metabolism, growth, exercise and digesting food is one of the key components of maintaining health and wellbeing.
A simplified energy balance equation looks like this: energy in = energy out.
- If insufficient food is eaten, there is a negative energy balance, resulting in weight loss.
- If too much food is eaten, there is a positive energy balance, and the excess energy is stored as body fat resulting in weight gain.
Healthy children and adolescents (until they stop growing) are in a state of positive energy balance – weight gain. The extra kilojoules are used primarily to increase the amount of important body tissues such as bonemuscle, blood and body organs. Some of this extra may also be stored as body fat, which can be used later as a source of energy. A negative energy balance will not allow for proper growth and development, limiting the normal weight gains associated with children and adolescents.
For adults, the situation is different since growth and development have reached a peak. To maintain health and wellbeing, it is recommended that there is a balance between energy in and energy out.
Read more about the debate around energy in, energy out.