## Units of energy

Energy expenditure is measured by the output of heat from the body (section 5.1). The unit of heat used in the early studies was the calorie — the amount of heat required to raise the temperature of 1 gram of water by 1 degree Celsius. The calorie is still used to some extent in nutrition; in biological systems the kilocalorie, kcal (sometimes written as Calorie with a capital C) is used. One kilocalorie is 1000 calories (103 cal), and hence the amount of heat required to raise the temperature of 1 kg of water through 1 degree Celsius.

Correctly, the joule is used as the unit of energy. The joule is an SI unit, named after James Prescott Joule (1818—89), who first showed the equivalence of heat, mechanical work and other forms of energy. In biological systems, the kilojoule (kJ = 103 J = 1000 J) and megajoule (1 MJ = 106J = 1,000,000 J) are used.

Remainder 16%

Skelet ' muscl 22%

Remainder 16%

Skelet ' muscl 22%

Kidneys 8% Liver 21%

Heart

Brain 20%

Figure 1.2 Percentage of total energy expenditure by different organs of the body.

Liver 21%

Kidneys 8%

Heart

Brain 20%

Figure 1.2 Percentage of total energy expenditure by different organs of the body.

To convert between calories and joules:

1 kcal = 4.186 kJ (normally rounded off to 4.2 kJ) 1 kJ = 0.239 kcal (normally rounded off to 0.24 kcal)

As discussed in section 5.1.3, average energy expenditure of adults is between 7.5 and 10 MJ/day for women and between 8 and 12 MJ/day for men. 