Genetic influence

It is recognised that susceptibility to obesity is in part genetically determined but that the expression of obesity ultimately depends on environmental factors.¹³ It appears that multiple genes are involved in the control of appetite, satiety and energy expenditure and this is an ongoing area of obesity research.

Establishing the heritability of obesity is difficult and estimates provided by research studies vary depending on the study population. For example, studies involving identical twins reared apart show a high genetic contribution to BMI, i.e. approximately 70%.14 However non-twin family studies and adoption studies have generated much lower estimates of 30% or less ^15–17.

Much remains to be learned about the extent of the role of genes in the development and expression of obesity, and there are claims for the enormous potential that genetic advances could bring in terms of generating new pharmacological interventions.¹⁸ Understanding more about the genetics of obesity will undoubtedly aid prevention and treatment programmes but it is accepted that it is our environment that is exerting a much more powerful influence over the susceptibility of developing obesity.

Some rare genetic causes of obesity include:

Bardet–Biedl syndrome

This syndrome is characterised by mental retardation, pigmentary retinopathy, polydactyly and hypogonadism. Weight gain does not usually begin until after the first or second year of life and occasionally does not present until puberty. A slowed metabolism and hyperphagia are thought to account for the increased incidence of obesity.

Prader–Willi syndrome

Individuals with Prader–Willi syndrome generally feed poorly until the age of two years, after which time there is a severe risk of obesity mainly as a result of extreme hyperphagia. Learning difficulties are a feature of the syndrome and adults tend to be of small stature with small hands and feet. The complications of obesity such as type 2 diabetes are a threat to the health of individuals with Prader–Willi syndrome.

Leptin deficiency

Leptin is a hormone involved in appetite regulation. Animal work in the 1990s showing that leptin administration could reverse obesity has not been translated into human findings. Leptin deficiency is an extremely rare genetic disorder, manifesting in childhood, and is treated with leptin injections. Fewer than 20 people worldwide have been diagnosed with leptin deficiency.

Although the prevalence of these disorders is low, it is important to be mindful of the genetic influences. Hypothyroidism is also associated with obesity and should be part of preliminary investigations prior to embarking on a treatment programme. Appropriate treatment for any thyroid disorders is essential for effective weight management.

The influence of medication

Consideration should also be given to the fact that medication such as corticosteroids, some antidepressants and sedatives can induce weight gain. Further information on drugs that influence body weight is provided in the drug therapy section of the Treatment module.