A condition in which the energy stores of the body (mainly fat) are too large. It is a prevalent nutritional disorder in prosperous countries – increasingly so among children and young people. The Quetelet Index or BODY MASS INDEX, which relates weight in kilograms (W) to height2 in metres (H2), is a widely accepted way of classifying obesity in adults according to severity. For example:

Grade of obesity

BMI (W/H2) III >40 II 30–40 I 25–29·9 not obese <25

Causes Whatever the causes of obesity, the fact remains that energy intake (in the form of food and drink) must exceed energy output (in the form of activity and exercise) over a suf?ciently long period of time.

Obesity tends to aggregate in families. This has led to the suggestion that some people inherit a ‘thrifty’ gene which predisposes them to obesity in later life by lowering their energy output. Indeed, patients often attribute their obesity to such a metabolic defect. Total energy output is made up of the resting metabolic rate (RMR), which represents about 70 per cent of the total; the energy cost of physical activity; and thermogenesis, i.e. the increase in energy output in response to food intake, cold exposure, some drugs and psychological in?uences. In general, obese people are consistently found to have a higher RMR and total energy output, per person – and also when expressed against fat-free mass – than do their lean counterparts. Most obese people do not appear to have a reduced capacity for thermogenesis. Although a genetic component to obesity remains a possibility, it is unlikely to be great or to prevent weight loss from being possible in most patients by reducing energy intake. Environmental in?uences are believed to be more important in explaining the familial association in obesity.

An inactive lifestyle plays a minor role in the development of obesity, but it is unclear whether people are obese because they are inactive or are inactive because they are obese. For the majority of obese people, the explanation must lie in an excessive energy intake. Unfortunately, it is di?cult to demonstrate this directly since the methods used to assess how much people eat are unreliable. For most obese people it seems likely that the defect lies in their failure to regulate energy intake in response to a variety of cognitive factors (e.g. ease of ?tting of clothes) in the long term.

Unfortunately, it can be possible to identify by the time of their ?rst birthday, many of the children destined to be obese.

Rarely, obesity has an endocrine basis and is caused by hypothyroidism (see under THYROID GLAND, DISEASES OF), HYPOPITUITARISM, HYPOGONADISM or CUSHING’S SYNDROME.

Symptoms Obesity has adverse e?ects on MORBIDITY and mortality (see DEATH RATE) which are greatest in young adults and increase with the severity of obesity. It is associated with an increased mortality and/or morbidity from cardiovascular disease, non-insulin-dependent diabetes mellitus, diseases of the gall-bladder, osteoarthritis, hernia, gout and possibly certain cancers (i.e. colon, rectum and prostate in men, and breast, ovary, endometrium and cervix in women). Menstrual irregularities and ovulatory failure are often experienced by obese women. Obese people are also at greater risk when they undergo surgery. With the exception of gallstone formation, weight loss will reduce these health risks.

Treatment Creation of an energy de?cit is essential for weight loss to occur, so the initial line of treatment is a slimming diet. An average de?cit of 1,000 kcal/day (see CALORIE) will produce a loss of 1 kg of fat/week and should be aimed for. Theoretically, this can be achieved by increasing energy expenditure or reducing energy intake. In practice, a low-energy diet is the usual form of treatment since attempts to increase energy expenditure, either by physical exercise or a thermogenic drug, are relatively ine?ective.

Anorectic drugs, gastric stapling and jaw-wiring are sometimes used to treat severe obesity. They are said to aid compliance with a low-energy diet by either reducing hunger (anorectic drugs) or limiting the amount of food the patient can eat. Unfortunately, the long-term e?ectiveness of gastric stapling is not known, and it is debatable whether the modest reduction in weight achieved by use of anorectic drugs is worthwhile – although a new drug, ORLISTAT, is becoming available that reduces the amount of fat absorbed from food in the gastrointestinal tract. For some grossly obese patients, jaw-wiring can be helpful, but a regain of weight once the wires are removed must be prevented. These procedures carry a risk, so should be done only if an individual’s health is in danger.

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