Table 1: World Health Organisation classification of BMI
18.5-24.9 25-29.9 30-39.9 >40
Healthy weight Overweight Obese
Figure 1: Prevalence of clinical obesity in the UK
DEFINING OBESITY Obesity is a chronic condition characterised by excessive overweight together with an increase in stored fat.
Obesity has dramatically increased over recent decades, current figures show that 17% of men and 20% of women in the UK are obese. It is predicted that by the year 2010 at least 25% of British adults will be obese.
Obesity is measured using the weight-height ratio, body mass index (BMI). It is calculated using the equation: BMI = weight (kg) / height (m)2
For the majority of purposes BMI is a satisfactory indicator of fat- ness and hence obesity, however it is a simple height to weight ratio and does not differentiate between the amount of fat mass and amount of muscle mass of individuals. Neither does it indicate the distribution of fat in the body. Despite this, there is substan- tial evidence that BMI correlates well with morbidity and mortal- ity and it remains the most widely used measure of obesity. A
number of researchers are beginning to use waist measurement alone as an indicator of visceral fat and this is possibly a better predictor of morbidity and mortality.
While obesity contributes significantly to a reduced quality of life and increased risk of premature death it is also an independent risk factor for a number of other chronic conditions such as coronary heart disease, stroke and certain cancers. Clinical obesity is asso- ciated with a BMI of around 30 although risk of premature mor- tality starts to increase from a BMI of around 25 (Fig.2).
Figure 3 clearly illustrates the dramatic increase in risk of devel- oping diabetes with increasing BMI. The contributing mechanism to this increase is that excessive body fat (particularly intra- abdominal fat) decreases the sensitivity of tissues to the action of insulin. This leads not only to an increased likelihood of develop- ing non-insulin dependent (type 2) diabetes (NIDDM) but also to raised blood lipids and blood pressure which are important risk factors for cardiovascular disease and stroke.
Obese individuals are also more likely to develop respiratory disor- ders, infertility and mechanical disorders including osteoarthritis