We remain at constant body weight only if we achieve a balance between energy intake and energy output. Energy intake results from the ingestion of food, there is no other route for energy absorption. Energy output is achieved through a variety of mechanisms. Everyone who is alive, whether asleep or even unconscious, is using energy in all the chemical processes which maintain and repair the body tissues, and permit the regular functioning of the lungs, heart and circulation.
The millions of chemical reactions which work towards maintaining the body’s health result in heat loss, which is called the metabolic rate. Some people have a high metabolic rate, as their body chemistry seems to turn over at a fast rate.
Differences in metabolic rate account for the fact that some people can eat vast quantities of food without gaining weight whilst others have to be much more careful. There is no safe or reliable way of changing metabolic rate. The energy output which the individual can directly influence is voluntary movement. Posture changes, extra movements during the course of normal daily activity, and physical exercise, result in more chemical reactions in the body and more heat production. Metabolic rate rises and energy output is increased.
Eating more energy in the form of food than is expended will result in an increase in stores of fat in the body. The balance between weight stability and weight gain is a very fine one. An underestimation of intake leading to a positive balance of 150 kcals daily could lead to a gain in weight of over six kilogrammes (one stone) in one year. Each individual has to ascertain by trial and error his own optimum level of eating and activity.
It is often harder to try to increase body weight. Usually thin people cannot put up with high energy diets for periods long enough to cause a permanent effect, nor can they reduce their activity.
Most people wishing to change weight, want to lose it. This is most effectively done by decreasing energy input, that is by eating less. Effective regular increases in activity also help. When weight loss is needed it is only body fat that needs to be reduced. Slimming diets must provide sufficient nutrients to allow the remainder of the body to keep healthy.
Undernutrition in the form of anorexia nervosa afflicts some adolescent girls or more rarely boys. This is a psychological disturbance brought on by overzealous or obsessional slimming routines. The motivation to achieve thinness and a dread of obesity leads to extreme wasting and death. Sufferers seem unable to see their bodies changing shape and wasting away. In severe cases hospital admission is needed in order to establish proper feeding.