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I. Sadaf Farooqi


May 29, 2014: This chapter has been re-evaluated and remains up-to-date. No changes have been necessary.

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date: 24 April 2017

Obesity is defined as an excess of body fat that is sufficient to affect health adversely. It is associated with an increased risk of type 2 diabetes, cardiovascular disease, and some forms of cancer and is a serious medical disorder. In routine practice, body mass index (BMI) is most often used to define obesity in population studies and in the clinic: overweight, BMI 25 to 29.9 kg/m2; obese, BMI 30.0 to 39.9 kg/m2; morbid obesity, BMI >40 kg/m2. By this definition about 20% of men and 25% of women in the United States of America and Europe are obese.

Causes of obesity

The rising global prevalence of obesity is driven by environmental factors including the increased availability of palatable energy-dense foods and the reduced requirement for physical exertion during working and domestic life.

The heritability of body weight and fat mass is very high and genetic variation determines the inter-individual differences in susceptibility or resistance to the ‘obesogenic’ dietary environment. Studies of genetic obesity syndromes have revealed mutations that all arise in molecules involved in the leptin–melanocortin pathway, which plays a key role in the regulation of body weight. Genome wide association studies, which have proved to be an extremely valuable tool for unravelling the aetiology of complex diseases, have shown that variants in the FTO gene are strongly associated with increased BMI.

Management of obesity

Management of patients with severe obesity is a challenge, but success is enhanced by a sympathetic approach from the physician, with realistic weight loss goals and monitoring of the effects of treatment. Interventions include (1) low-calorie diets, energy-deficit diets and diets that are low in fat, which should initially provide a 600 kcal/day (2.5 MJ/day) energy deficit, based on estimated energy requirements; (2) behavioural approaches to help subjects to implement and sustain changes to their eating and activity behaviour; (3) drug treatment—which should always be regarded as a therapeutic trial and stopped if weight loss is not apparent after one to two months—with agents used including pancreatic lipase inhibitors (orlistat); (4) surgery—an option for carefully selected patients with morbid obesity, with procedures including laparoscopic gastric banding, gastric bypass, and duodenal switch.

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