The simplest and most widely recognised definition of obesity is in terms of BMI. A detailed description of BMI measurements is given in the assessment section.

At its simplest level, obesity is a disease of energy imbalance. To maintain a healthy weight, energy from fuel taken into the body as food needs to balance with the amount of energy expended by the body in daily activities. Obesity can develop when the energy scales are tipped in favour of available energy. In reality, obesity is a much more complex and multifactorial disease, arising through the interaction of metabolic, genetic, behavioural and environmental factors.

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.

The direct and indirect health care costs associated with obesity are enormous. The National Audit Office estimated that the direct costs of treating obesity and its consequences in England alone are approximately £500 million.¹⁹ These are 1998 figures and it is generally agreed that the costs continue to escalate as obesity increases. The costs indirectly attributable to obesity are more difficult to calculate, but the National Audit Office gave a cautious estimate of £2.1 billion in 1998. This estimate tries to take account of the costs associated with loss of productivity in the workforce due to illness, disability or premature death.

There is good evidence to suggest that a moderate weight loss of 5–10% of body weight will have a major impact on the physical and metabolic complications of obesity.²¹ Weight reductions of 5–10 kg have been shown to improve back and joint pain, symptoms of breathlessness and sleep apnoea.²² Marked improvements in blood pressure and other risk factors for coronary heart disease have also been observed in individuals successful at losing weight.²³ Two recent studies have demonstrated that a substantial number of cases of diabetes could be prevented through modest weight loss (i.e. <5 kg).^24,25

There are many different stages in life when obesity can develop and in fact very few individuals evade risk completely. Our vulnerability is increased in a number of situations.

When exercise is limited or reduced

The opportunities for exercise may be more limited for those with a physical disability. A sudden reduction in activity (for example when athletes or army personnel retire or sustain injury) can also lead to weight gain. For individuals confined to a wheelchair, every care should be taken to ensure that there is access to facilities that allow exercise. Guidance on how to match energy intake to energy needs should also be provided.

The prevention of obesity can be addressed on two levels:

• Individual strategies
• Public health strategies

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