What determines weight and shape?
Whilst our “ideal” or “desirable” body shape is culturally determined, recent studies have shown that body weight is strongly influenced by genetic and familial patterns. This is quite at odds with the messages presented by the media and by the diet industry: that anyone can achieve any body weight. We know from studies looking at twins and adopted children that weight is much more determined by genetics than by upbringing, and whilst environment of course has an impact, it has much less of an impact than might be expected. When in treatment for anorexia, many people express the same fear - that their weight will continue increasing and they will be unable to stop it. However, we know from experience and from our knowledge of genetics and physiology that this is not the case.
Set Point Theory
Set point theory refers to the theory that body weight is genetically determined and that is is actually incredibly difficult to push the body far below, or above, its natural ‘set point’ weight. The human body has a remarkable ability to adapt to changes in food intake (both increases and decreases) in order to maintain a weight around where it wants to be. This makes sense from an evolutionary perspective, where historically one of the greatest threats to life was starvation. In the past, access to food has been very much less regular than in today’s modern society with supermarkets and ready-made foods. If weight were not regulated internally, many organisms would have died when food was scarce. What this means is that the human body has an exceptional resistance to weight gain or loss. Changes in weight are countered by metabolic adaptation designed to restore the state of body weight equilibrium.
In starved individuals various changes occur to slow down the body’s physiological processes. These include:
decreases in body temperature
decreased heart rate
decreased respiration
decreased basal metabolic rate (the amount of energy that the body requires at rest in order to maintain the functioning of all internal organs).
reducing energy levels for physical activity
reducing drive for social and sexual functioning which would be viewed by the body as unnecessary when life is threatened
Basal metabolic rate accounts for about two-thirds of the body’s total energy needs, with the remainder being used during physical activity.
Support for set point theory comes from the observations of relative long-term stability of body weight, and the resistance observed to permanent weight loss. There has been a growing recognition that, despite substantial variations in daily caloric intake, body weight remains remarkably stable over time. Many people are able to maintain a relatively constant weight without monitoring food intake against energy output. What this means is that the body is not sensitive to changes in calorie intake from day to day, and weight will not be gained by small or irregular increases.
Various studies, including Keys’ experiment on the effects of starvation in 36 conscientious objectors (See Resource Box ‘Understanding the effects of starvation’) have shown us that most people display a strong resistance to weight loss. When weight loss does occur, it is accompanied by many adverse side effects, including dramatic physical, emotional, social and sexual changes. The study also showed us that the adaptations the body makes are only reversed once normalisation of weight has occurred, and not after normalisation of eating - i.e. eating regularly is not enough to reverse the changes made by the body, but must also be accompanied by full restoration of weight.
A further study by Ethan Sims in the 1960s that explored the impact of overeating on university students, also provides support for set point theory, but illustrates the adaptations the body makes to avoid going above its set point weight. In this experiment, even though students were fed huge amounts of food, they found it extremely difficult to add even 10% to their normal body weight. This and other studies have provided evidence that a weight gain or loss cannot be explained simply by the number of calories consumed. In fact the body is extremely resistant to weight gain and for most people, it takes a huge amount of food to see weight gain occurring above their body’s genetically determined set point. When weight is pushed beyond its natural set point, it is relatively easy to restore the body to its natural weight when normal eating resumes.