THE DIAGNOSIS OF OBESITY

The term obesity means having an excessive amount of body fat, where 'excessive' indicates an amount that significantly worsens health. To be overweight is to have more body fat than is needed for healthy living. These concepts have grown out of observations that people who have a large excess of body fat tend to be more likely to develop a number of serious illnesses, such as diabetes, and to die earlier than people of a healthy weight.

Clearly, this way of looking at overweight and obesity is subjective and imprecise. Fat is located in many parts of the body and is not easy to measure directly. Judging what is significantly worse health is a statistical matter, and entails comparing different groups of people with one another. What kind of impact on health is meant? How much worse is 'significant'? And compared with which group of healthy people? To identify people who are at greater risk of poor health (and to estimate to what extent), it is necessary to have an agreed measure of overweight.

To work out your Body Mass Index (BMI), measure your height (without shoes) and weight (in only light clothing). Working in metric units, your BMI is then given by the formula: e.g. For someone who weighs 57.5kg and is 1m 60 tall:

Overweight in adults

The most commonly used measure of overweight is the Body Mass Index (BMI). First developed between 1830 and 1850, this index is calculated from a person's weight divided by the square of their height. It makes no distinction between fat and muscle or bone. However, it has been shown that, except in people with markedly unusual body shape or amount of muscles, such as body-builders or athletes, BMI can be used to calculate the percentage of body fat fairly accurately. It is used in combination with waist circumference in guidelines issued by the National Institute for Health and Clinical Excellence (NICE).

A generally accepted classification has established BMI 'bands' that give an indication of the health risk posed by excess body fat. A BMI range of 18.5-24.9 is considered to represent a normal, healthy or desirable range of body composition, while a BMI above 25 is taken to show overweight, and a BMI of 30 or more is rated obese. Official guidelines further subdivide the obese category into grades I, II and III, indicating an increasing health risk as BMI increases.

Body fat deposits have different compositions and different influences on health, depending on where they are located. BMI measurement does not distinguish between the fat in different body locations, and thus does not always give a good guide to health risks. It is now thought that body fat deposited under the skin (subcutaneous fat) is less harmful to health than fat located deep in the abdomen, around organs such as the liver and intestines (visceral fat). This is despite the fact that, in men at least, subcutaneous fat makes up about 80 per cent of total body fat. There is therefore a need for other measures that are better indicators of visceral fat than BMI.

Waist circumference is easily measured and correlates with the presence of visceral fat. It is used in considering what initial approach is appropriate for achieving weight loss. It is at least as accurate as another measure of visceral fat, the ratio of body circumference at the waist and hips (waist-to-hip ratio), which is known to indicate increased health risks even in people who are not overweight.

Visceral fat can be measured directly by the procedure known as Computer-Aided Tomography (CAT scanning). This is usually only done in clinical studies, not for diagnosing overweight. However, such measurements have been important in advancing understanding of the changes in body chemistry occurring in and contributing to the development of overweight.

There are a range of other methods for measuring body fat. These include measuring the electrical conductance of the body (bioimpedance), or the thickness of skin folds at various body sites. They are less often used, as they are insufficiently accurate. For the great majority of situations, BMI, which is simple and quick to determine, is used to quantify overweight.

Overweight in children

The BMI values used to define overweight (BMI greater than 25) and obesity (BMI greater than 30) in adults cannot be used for children. Average BMI for healthy children shows a U-shaped curve from about the age of 2 onwards, with a minimum at age 5-6, increasing thereafter to adult values. Hence, it is necessary for doctors to use ageadjusted BMI criteria to define overweight and obesity in children and adolescents. However, there is still some disagreement as to which measure is the most appropriate to use. Moreover, as there is much less evidence of direct harm to health caused by overweight in children, the adoption of any particular cut-off threshold for defining overweight and obesity is more arbitrary than in the case of adults.

Compared with adults, children have much less visceral fat and, unlike in adults, official guidelines do not recommend the use of waist circumference alone as a measure of overweight in children. Others have, however, suggested that it may be useful as a predictor of cardiovascular risk factors (raised blood pressure, blood lipids and insulin) that may influence health later in life. Thus, BMI and waist circumference are less reliable than in adults for classifying children into 'overweight' or 'obese' categories, but they are still useful for preventing adult disease.