Energy Balance

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.

Energy intake, 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.

People differ 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 short.

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.

CONTROLLING BODY WEIGHT

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.

APPETITE 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.

ENERGY DENSITY

A high-energy-dense 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 Figure 15.

The more water and dietary fibre, and the less fat and alcohol in a food or drink, the less energy dense it is.

NUTRIENT DENSITY

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.