If you eat more
than your body needs to keep it working efficiently, you will get
fat. Many slimming diets prefer not to emphasize this displeasing
but basic fact.
When energy intake
equals energy needs, our bodies are in 'energy balance'. This means
that if you are fully grown, or are growing normally, and your weight
is within certain limits (those that conform to longest life expectancy
and least illness), you have achieved energy balance. You achieve
it by balancing, on the one hand, your total energy intake, and, on
the other, your level of activity. The more active you are, the more
energy you need.
which used to be measured in calories or kilocalories, is now measured
in kilojoules (kJ). To convert calories or kilocalories to kilojoules
multiply by 4.2 (approximately 4). Although the number of kilojoules
in a particular food is numerically larger than the number of calories
or kilocalories, the amount of energy is the same. It is rather like
measuring time in minutes or seconds. One hour is still the same amount
of time whether it is expressed as 60 minutes or 3600 seconds.
in the efficiency of their energy expenditure. This may be due to
differences in efficiency of movement, or because some people are
not as efficient as others in burning up their 'fuel' supplies (that
is, their 'metabolic efficiency' is lower). Thus, one person may eat
much more than another, with equivalent levels of activity, and yet
remain similar in weight. The more 'energy-efficient' person needs
to be more careful about eating too much to avoid becoming overweight.
However, the same person would survive better if food supplies were
Energy needs vary
with your age and weight, as well as with your level of activity.
Figure 11 and Figure
12 show this.
Energy is also
used, with varying efficiency, to maintain body temperature, to store
fuels after a meal and to form bodily wastes, as Figure
13 illustrates. So even when you are not being active your body
needs energy for vital processes. This minimum level of energy expenditure
is known as the 'basal metabolic rate'. It is what keeps you alive
even if you are completely at rest, fasting and at a comfortable temperature.
When our bodies
are not in energy balance, excess energy is stored as fat (in adipose tissues). Women normally have more body fat than men, and this should
be taken into account when considering whether to conform to the fashion
to be very slim. Desirable weights for adults of varying heights are
shown in Figure 14. Those
shown were obtained in 1959. Although 1979 figures are available,
there is controversy about their use.
If you are on
a weight-reduction programme, remember that body water is the main
constituent lost during the first few days. It is only after 2 to
3 weeks of dieting that the loss is mainly fat. At this stage, a daily
energy intake that is about 500 kilocalories (2000 kilojoules) less
than the intake actually needed at a particular level of physical
activity (see Figure 11 and
Figure 12) will lead to a
weight loss of approximately 1 kilogram per fortnight. Remember also
that not all people expend energy with the same efficiency.
CAN CONTROL HOW MUCH ENERGY (FOOD) WE CONSUME
Appetite has an
important role in controlling energy intake. When we are physically
active, appetite is more correctly related to energy need than when
we are inactive. At low levels of physical activity, we are more likely
to feel hungry and to eat more than we need.
Also, a diet that
is low in fat and high in carbohydrate and dietary fibre seems to
allow appetite to be more correctly attuned to energy need. Because
fatty foods are so palatable, more of them tend to be eaten than our
bodies really need. They are also more energy dense than high-carbohydrate,
high-dietary fibre foods.
food has more kilojoules (or kilocalories) than the same amount of
a low-energy-dense food.
The more energy
dense a food is, the less of it we can eat to provide a given amount
of energy. Conversely, the less energy dense a food, the more we can
eat to provide us with the same amount of energy, as we can see in
The more water
and dietary fibre, and the less fat and alcohol in a food or drink,
the less energy dense it is.
The greater the
number of essential nutrients, and the larger the amount of them a
food contains, the more 'nutrient dense' it is. Often foods with low
energy density are quite nutrient dense, as, for example, wholegrain
cereals and leguminous vegetables (like peas and beans). Some foods
can be both nutrient dense and energy dense, such as a piece of steak.