13 · ENERGY CHANGES IN CHEMICAL REACTIONSIf a person’s body is using only carbohydrates to provide energy, the RER is about
1.0. If only fats are being used as the source of energy, the ratio is about 0.7. In both
cases, about 20 kJ of energy is being produced per dm^3 of oxygen consumed.
For an average person at rest (e.g. sitting in a chair) who lives on a mixed diet, the
RER is about 0.8. Typically, an average male at rest consumes about 0.30 dm^3 O 2 per
minute, or 0.30 60 24
430 dm^3 per day. (The man would breath in much more
oxygen gas than this, but only a small fraction of the inhaled oxygen would be actually
used up in the metabolism of foodstuffs.) Every day the typical man would produce
20 430
8600 kJ (or 2060 Cal) per day. This would also be the minimum daily
energy requirement required for such a man – if his diet was of a lower energy value
his body would have to draw upon its food reserves. If the man engaged in walking,
reading or writing, more than 8600 kJ day^1 would be required. In extreme cases,
athletes can consume over 50 000 kJ per day! To lose weight, a person persistently
ingests food with a calorific value which is less than their daily requirement – the
practice is more difficult than the theory!232
A footballer on a treadmill,
showing the equipment used
to measure the respiratory
exchange of oxygen and
carbon dioxide.
Energy consumption
Estimate the minimum energy consumption per day of an average woman. Assume that her
oxygen consumption is 270 cm^3 min^1.Exercise 13K