How Much Energy Is Enough? - The Renaissance of Renewable Energy - page 41

Energy for Human Metabolism
For most of history, human energy needs consisted of food and fuel for heating and
cooking. It was only quite recently that we started to use energy for a wider variety of
applications. Now, in the early twenty-first century, we have reached a point where food
represents just 1 per cent of the energy consumed by people living in wealthy industrialised
countries. The remainder is used in transportation, by electrical appliances, for lighting and
heating, and to power industrial machinery.

Mostpeoplearefamiliarwiththeterm'calorie'astheunitusedtomeasurefoodenergy. 1
Even though the joule is the standard unit of energy, calories are often used to describe the
amount of energy in food. When we talk about the number of 'calories' in a piece of cake
or a serving of pasta, we are actually talking about kilocalories (kcal or Cal or 1,000 cal),
yet the word 'calorie' has persisted in popular usage.
Our bodily energy needs depend not only on our level of activity but also on our age and
size, as well as on environmental conditions such as the temperature of our surroundings,
which determines whether we need to expend energy to warm or cool our bodies (see Table
3.2 ) . An infant needs about 700 kilocalories (roughly 3,000 kilojoules) per day, while a
very active adult man may consume up to 4,000 kilocalories (more than 15,000 kilojoules).
Of course, energy is not the only thing we get from food. Food also provides the raw
materials to build our cells. Because of this dual requirement, our diet needs to be varied,
including foodstuffs such as fats and oils that are very high in energy, and others, such as
vegetable fibres, that are low in energy but are vital to the maintenance and repair of tissues
(see Table 3.1 ).

Table 3.1. Approximate energy content of common foodstuffs compared with fuels;
data are expressed in the international unit measure of energy and in the other two
common units, kilocalories and kilowatt-hours

Food 100 gr Fuel 100 gr kJ kcal kWh

Watermelon 63 15 0.017

Tomato 63 15 0.017

Red wine (100 ml) 251 60 0.070

Milk (100 ml) 264 63 0.073