Obesity is, quite simply, a condition characterised by an excess of body fat. There are a variety of ways in which obesity can be assessed in individuals and populations. One commonly used definition of obesity is the body mass index (BMI), which is a person’s weight in kilograms divided by the square of their height in metres. For adults, overweight is defined as a BMI of greater than 25 and obesity is defined as a BMI of greater than 30. This is not without its problems as an athlete with a large muscle bulk can have a high BMI but not be obese. Waist circumference, waist–hip ratio and skin-fold thickness are alternative assessment tools.
The standard BMI classification is:
BMI range (kg/m2) Classification
< 17 Malnourished
17–<20 Underweight
20–<25 Normal weight
25–<30 Overweight
30–<40 Obese
The terms ‘overweight’ and ‘obese’ are mutually exclusive when using BMI, so the category of ‘overweight’ does not include ‘obese’. The point, for a public health audience, is that the health risks of obesity are serious. People who are obese are more likely to develop a range of chronic conditions, including osteoarthritis, high blood pressure, diabetes, heart disease and some cancers. Children are not immune from these risks. Until recently, type 2 diabetes was only seen in adults, but now some adolescents are developing this condition. This can lead in later life to serious consequences such as heart disease, stroke, kidney failure and blindness. Nor are the effects limited to physical illnesses. For example, obese children (especially girls) are more likely to show evidence of psychological distress than children who are not obese.
Obesity is rising across the world. In 2006, the number of obese and overweight people in the world overtook the numbers who are malnourished and underweight. Figure 1 shows the rising epidemic of obesity in the United States. Where America leads, other countries will follow, as demonstrated by the Foresight Obesity Report in the UK.
Another key point is that obesity is a problem affecting the entire population. The entire weight distribution of the population is shifting upwards. So, the slim are becoming less slim while the already overweight are getting heavier and the number of people in the obese category steadily grows.
In Scotland, this shift in weight distribution translates into a one kilogram increase in weight per adult per year (on average over the adult population). Scotland has one of the highest levels of obesity in OEC countries, second only to the United States of America. The prevalence of obesity (BMI greater than 30) in Scotland has increased over the past two decades, reaching 22% in men and 26% in women in 2003. Women in Scotland are more likely to be obese than women in England. Levels of obesity in Scottish and English men are broadly the same. Obesity is now common in children. In Scotland, nearly 1 in 5 (18%) of boys and 1 in 10 (14%) of girls aged 2 to 15 years are obese.
Figure 2 shows that the entire population’s BMI distribution is shifting upwards, as argued above: there is a whole population effect.
From a purely biological point of view, whether or not each of us becomes obese depends on the balance between our energy input and output. Our bodies have sophisticated mechanisms that can regulate body weight by adjusting energy intake to match energy expenditure and vice versa. It is now widely accepted that we cannot blame the current obesity epidemic on individual behaviour and poor choice although many current approaches still focus on individuals, emphasising education about diet and exercise, and encouraging weight loss. Children, in particular, are vulnerable to the social and environmental pressures that raise the risk of obesity.
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