Energy requirements

Energy requirements

As is generally known, the main role of food is to provide the energy needed for normal functioning of the body. When you are eating food, you get chemical energy, which could be converted into the following:

  • Mechanical energy (kinetic energy) - necessary for muscle contractions.
  • Electrical energy - necessary for the passage of ions (sodium, potassium, magnesium, calcium and others) through channels in cell membrane.
  • Thermal energy - necessary in order to maintain body temperature.
  • The remaining part of chemical energy is necessary in order to create new molecules necessary to our body.

The largest part of the chemical energy gained from food (about 70 %) is converted into thermal energy, and regulates the body temperature, thus enabling maintenance of all vital functions.

How much energy does human body actually need?

Nutrition experts have given a definition of individual energy needs. Namely, according to this definition, individual energy needs are actually the energy supplied by food which must be sufficient to ensure an optimal body weight and physical activity and to be balanced with energy expenditure.

The energy expenditure of the body can be determined in several ways:

  • Direct calorimetry - based on the principle of measuring the heat that our body releases when idle (not physically active). This method of determining energy expenditure requires specific conditions - specially equipped rooms with the walls made of special materials and without any electrical installations. The advantage of this method is that it can determine your energy expenditure very accurately. It can determine exactly how much energy (calories) you spend while you are in the idle state, when running, walking or playing tennis, for example. This method can even determine how much energy your body releases when you are eating certain food. The biggest disadvantage of this method is that it is expensive, therefore it is rarely available.
  • Indirect calorimetry - based on the principle of measuring of the acquittal of carbon dioxide (CO2) and oxygen consumption. When eating food our body uses oxygen and releases carbon dioxide, and this method is based on that fact. However, it is not as reliable as direct calorimetry, but does not require special conditions and it is cheaper and more readily available. This method can determine the energy expenditure while in the idle state and while physically active. It is quite simple to perform. It measures the concentration of oxygen and carbon dioxide in exhaled air. You must be connected to a closed breathing system (special face masks) all the time in order to precisely determine the parameters. The concentration of oxygen or carbon dioxide is directly proportional to the amount of energy that your body releases when performing an activity.

However, the energy requirements are commonly calculated using following formula:

E = EB + EF

E - Total energy requirement.

EB - Energy required for basal metabolism (energy expended while at rest and motionless).

EF - Energy needed for physical activity.

What is basal metabolism?

It is the minimal amount of energy required for performing vital body functions in the state of rest and motionless state. This is actually the energy that our body needs to maintain breathing, circulation and other vital functions. So, even when you are not doing anything, your body burns a certain number of calories. Moreover, energy needed for basal metabolism makes up 60 % of total energy requirements.

When you sum up the energy required for basal metabolism and energy required for physical activity that we have during the day, we get the total energy requirements for our body.

Factors affecting your basal metabolism

The amount of energy required for basal metabolism depends on the following:

  • Body weight
  • Body composition
  • Age
  • Height
  • Genetic factors
  • Postprandial thermogenesis
  • Energy necessary for thermoregulation (heat production).

People who have a higher body mass have larger energy needs.

Example: If two men are the same height and age, but one of them weighs 22 lbs. (10 kg) more than the other one, then his basal metabolism is greater for 120 calories.

People who have a lower percentage of body fat and more muscles have higher energy requirements for basal metabolism. Therefore, it is very important to exercise regularly, because not only you will burn more calories while exercising but you will also lose more calories when idling.

Elderly people have less energy needs than younger people. The energy requirements for basal metabolism declines with age.

Men have higher energy needs because they have more muscles than women.

Post - prandial thermogenesis is the energy that is released while eating food. A more precise definition would be that post - prandial thermogenesis is the energy required for food intake, its digestion and utilization of nutrients from it. This part of the energy makes up about 7 % of total energy needs and depends on the kind of foods. If you are eating sweets or other foods rich in carbs, your body will burn less calories than when you are eating foods rich in protein (meat, for example).

It is very difficult to determine the precise amount of energy that is released during a meal because studies have shown that even repeated measurements for the same person while eating the same food give different results.

Some studies have also shown that climatic factors may have an impact on energy consumption. Some data indicate that if the temperature is lower for 10oC than the energy requirements are a bit higher (for 0.7 %). However, these data are still not reliable and should not be considered as an essential factor that influences the energy needs.

Exact measurement of the amount of energy required for basal metabolism is not easy and it is expensive. Therefore, the amount of energy required for the basal metabolism is approximately determined using formulas based on the body weight, age and height.

How to calculate the energy required for the basal metabolism?

Energy needed for basal metabolism can be calculated by using formulas in the tables below:


Age Formula
0-3 27.72 * TM - 51
3-10 10.23 * TM + 499
10-18 5.55 * TM + 746
18-30 6.68 * TM + 496
30-60 3.95 * TM + 829
>60 4.77 * TM + 596

TM - Body weight expressed in pounds (lbs.)


If a woman aged 35 years weighs 143 pounds, then her energy needs for basal metabolism should be calculated as follows:

From the abovementioned table take the formula for women aged 30 - 60 years:

3.95 * TM + 829, as follows:

3.95 * 143 + 829 = 546.85 + 829 = 1393.85 calories

So, women aged 35 years and weighing 143 pounds has the basal metabolism energy needs of 1393.85 calories per day. Of course, this amount refers to the maintenance at rest and does not include energy needed for physical activity.


Age Formula
0-3 27.68 * TM - 54
3-10 10.32 * TM + 495
10-18 7.95 * TM + 651
18-30 6.95 * TM + 679
30-60 5.27 * TM + 879
>60 6.14 * TM + 487

TM - Body weight expressed in pounds (lbs.)

Example: If a man aged 35 years weighs 180 pounds, then his energy needs for basal metabolism should be calculated as follows:

From the abovementioned table take the formula for men aged 30-60 years:

5.27 * TM + 679, as follows:

5.27 * 180 + 679 = 948.6 + 679 = 1627.6 calories

So, a man aged 35 years weighing 180 pounds needs 1979.5 calories per day. Of course, he would need this amount of calories just to maintain the basic body functions. This amount does not include energy for physical activity.


Therefore, in order to determine your energy needs, you will need to use the following formula:

E = EB + EF

EB - (energy requirements for basal metabolism) which you can calculate with the help of the abovementioned table and then sum up the calories you are burning during physical activity.

When you sum up these two values you will get the total amount of energy (calories) you need during the day. If you consume more food (energy) than you need, you will gain weight, but if you consume less food than you need, you will lose weight.

If you know your energy needs and if you are familiar with the caloric value of the food you're eating, you can plan your diet so that you intake only as much energy (food) as you need.