Relevant physiology

Obesity is a chronic condition in which there is an excess amount of body fat. Some body fat is required by the body in which to store energy, insulate the body against a cold environment and to cover and protect the bones and organs of the body. Normal body fat is generally expressed as a percentage of body mass and should be between 25–30% in women (18–23% in men) and individuals with a higher percentage are considered obese.

Body weight depends in part on the genetic make-up of the individual and in part on the balance between food intake and energy expenditure of the body. Food intake is the only source of energy for performing all the biological reactions on which body function relies. These metabolic processes occur in all the cells of the body; chemical substances are taken into the cell where under the guidance of deoxyribonucleic acid (DNA) and the influence of enzymes, new products are formed.

Anabolism is the process by which chemical reactions combine to form more complex molecules, such as proteins, e.g. hormones, and glycogen for storage in the liver (see Ch. 9). Catabolism is the opposite process. Complex molecules are broken down into simple substances to enable these substances to be readily available to body cells. A by-product of catabolism is the release of energy which is required to fuel many anabolic reactions. The molecule that participates in most energy exchanges in body cells is adenosine triphosphate (ATP) which is utilized and rebuilt constantly. Heat is another vital by-product of metabolism ensuring that the human body temperature is largely independent of external temperature (Box 11.1)

A stable body weight is dependent on a balance between food intake and energy output. Food intake is measured in calories. Should an individual’s calorific intake exceed the number of calories used in body functioning, the excess calories are stored in the body for times when food is scarce. In the developed world, this rarelyhappens. Over time, an individual who constantly takes in too many calories will increase in weight to a greater or lesser extent, depending on the degree of overeating.

Other factors involved are thought to be eating a diet high in simple carbohydrates and the frequency of eating. Simple carbohydrates are absorbed rapidly and cause large swings in blood sugar levels. This results in cravings for food which is often taken as snacks (Box 11.2). Eating large meals infrequently also does this, especially if the meals taken are also largely composed of easily digested foodstuffs.

Psychological factors play a large part in obesity; emotions influence eating habits. Many people eat excessively in response to emotions, such as boredom, sadness, stress or anger. Racial factors play a part with African-American women and Hispanic women putting on weight earlier in life than Caucasian women. The female hormone oestrogen is also thought to have some influence on weight gain – when on the contraceptive pill, during pregnancy and during the menopause. Diseases such as hypothyroidism, polycystic ovaries and Cushing’s syndrome are also contributors to obesity.

Many of the above factors however are additionally linked to an individual’s metabolic rate. Metabolic rate is the rate at which the body breaks down nutrients to liberate energy. Many factors affect metabolic rate and these may significantly influence whether or not an individual gains weight.

Obesity is more than a cosmetic consideration – it carries a greatly increased risk of premature death and disease processes. Briefly these are mentioned below.