Screening For Obesity

Although obesity should be regarded as a disease entity of its own, many of its more serious consequences are due to the strong relations that exist between obesity and some common chronic diseases. Obesity is an important risk factor for type 2 diabetes, cardiovascular disease, sleep apnoea, gallbladder disease and certain types of cancer.

Why Screen for Obesity?

The association between obesity and type 2 diabetes is perhaps the strongest of all reasons why a crusade against overweight and obesity must be initiated in the near future. The ongoing secular trends in Westernized societies are alarming. It is a healthy sign that the medical profession is showing an increasing interest in and awareness of the impact lifestyle has on health. However, many health professionals express a pessimistic view over the future with regard to many of our chronic diseases, if the 'Westernized' way of living continues to expand over the world. Headlines such as, 'obesity—a time bomb to be defused' (11) and 'non-insulin dependent diabetes mellitus—an epidemic in progress' (12), have been used. The global burden of diabetes has, as stated above, been projected to rise from about 120 million type 2 diabetic individuals in 1997 to about 215 million in 2010 (1).

The 1980s and the 1990s research in cell and molecular biology has presented a body of evidence that supports many years of past clinical experience. Sustained high levels of free fatty acids (FFAs) seem to affect both insulin secretion and insulin action in susceptible individuals by interfering with the glucose transporter mechanism in the pancreatic islet cells and in the muscle and fat cells. Caloric restriction and exercise training have, in animal experiments, been shown to counteract the suppression of glucose transporters, verifying on a cellular level over forty years of experience from observational and experimental studies in humans on the relationships between body weight, physical activity and glucose tolerance. The amount of body fat mass and the metabolic activity in different fat cells seem to regulate the production of FFAs and the consumption of FFAs is largely dependent on the level of physical activity. High levels of stress (cortisol and catecholamines) seem to be implicated both in the enlargement of the visceral fat depots and in boosting their activity. A long-term lifestyle change therefore seems to be the obvious remedy.

The Screening Test—Measuring Body Mass Index or Waist Circumference or Both

The purpose of screening for obesity must be to find individuals at high risk of developing future disease that is caused by a long-standing effect of obesity. Since 'obesity' is associated with a certain level of BMI (more than or equal to 30), the term obesity in this passage will be replaced by the term 'excess storage of body fat'. Excess storage of body fat may be measured in many different ways. Some methods measure the amount of body fat very precisely, but are not suitable for screening purposes. They are often laborious and expensive. Ideally, a screening test should be safe, valid, rapid and inexpensive. The identification of high-risk individuals based on measuring body weight seems, at least compared to many other screening tests, relatively simple and straightforward. The measurements are easy to obtain, safe to use, and reliable. What about validity? Some studies imply that BMI correlates well with percentage of body fat (13). Other studies, using modern imaging techniques, have found significant variations in the percentage of body fat across the whole range of BMI (14). Yet, it is reasonable to believe that measuring BMI would for most subjects capture the increased risk for comorbities that is linked to an excess storage of body fat. Furthermore, BMI is much more stable as a measurement than blood pressure or serum cholesterol, and consequently more suitable as a screening tool. Stress or anxiety can, within seconds, change the level of blood pressure but will have no effect on an individual's BMI.

If measuring BMI is simple and strightforward, the issue of where to draw the line between a high-risk situation and a normal situation is more complex. Body mass index, like many other risk factors, is measured on a continuous scale. However, when used as a screening test, the purpose is to divide the screened population into a minority assessed as having a high risk of developing future disease and a majority assessed as normal. A cut-off point at a low level of BMI will define our high-risk group more broadly, ensuring that practically all individuals with excess storage of body fat are classified as high-risk individuals (true positives). This is easily understood if we hypothetically choose a BMI of 20 kg/m2 as a cut-off point. The sensitivity would be high in such a situation (Figure 5.1). However, a broadly defined high-risk group would also include many individuals not having excess storage of body fat. These would be falsely regarded as high-risk individuals (false positives). Another way of expressing this is to say that the screening test does not correctly classify as negative those individuals without excess storage of body fat. The specificity of such a test would be low (Figure 5.1). Similarly, if a high level of BMI was chosen as the cut-off point, the high-risk group would be defined narrowly. This is most easily understood if we hypothetically choose a BMI of 35kg/m2 as a cut-off point. In such a situation, many individuals having excess storage of body fat would by the screening test be classified as negatives, i.e. belonging to a normal risk situation (false negatives). The sensitivity of the test would be low. However, the specificity would be high, since practically all those without excess storage of body fat would be classified as normals (true negatives).

Many different considerations have to be taken into account when deciding which cut-off point to use in a screening test. All individuals found to have a high risk are entitled to a treatment programme. If obesity is to be treated successfully on a long-term basis, a lot of professional support is needed. In this way, the amount of resources allocated to the treatment component of the screening programme will have a major impact on how broadly or narrowly the high-risk group is to be defined. Of course, a cut-off point that is already generally accepted by the medical profession, and represents a level of BMI above which the risk curve for comorbidities rises more steeply, would seem natural. A BMI of more than or equal to 30kg/m2 would define between 10 and 25% of most Western populations as high-risk individuals (15).

As mentioned earlier, intensive research in cardiovascular disease and type 2 diabetes, over more than two decades, has pointed to visceral fat accumulation and free fatty acids as key factors in the aetiology of these diseases. This clearer understanding of the mechanisms by which obesity contributes to cardiovascular disease and type 2 diabetes may also imply that waist circumference, abdominal sagittal diameter or waist-to-hip ratio are more accurate measures of the risk of future disease than BMI and should therefore be preferred as screening tests. Abdominal obesity has in prospective studies independently, after controlling for BMI, been associated with the incidence of cardiovascular disease (16) and type 2 diabetes (17). However, the picture is not entirely clear-cut. In the Health Professionals' Follow-up Study, although waist circumference was shown to be a good predictor, a high BMI was the dominant risk factor for type 2 diabetes (18).

The new techniques for measuring body fat introduced during the 1980s and the 1990s, such as computed tomography and magnetic resonance imaging (MRI), have the ability to determine accurately not only total body fat but also the body fat content in different compartments. This makes it possible to measure the amount of visceral fat in a more precise manner (14). Several studies have already been performed using these new techniques to measure the amount of visceral fat. The visceral fat content has been linked to different anthropometric indices and to the presence of cardiovascular and metabolic risk factors. Pouliot and colleagues showed, by using computed tomography, that waist circumference and abdominal sagittal diameter correlated better than the more commonly used waist-to-hip ratio with both the amount of visceral fat accumulation and the different metabolic risk factors (19). From the data in this study it was suggested that a waist circumference of above 100 cm, or an abdominal sagittal diameter of above 25 cm, would indicate an increased risk situation for the development of cardiovascular disease and/or type 2 diabetes. However, since the sagittal diameter in the study was measured indirectly using an abdominal scan obtained with computed tomography, further studies need to be performed where the abdominal sagittal diameter is measured clinically, before the method can be recommended for clinical purposes.

The use of waist circumference has also been advocated. In this context, it should be pointed out that the relationship between waist circumference (or waist-to-hip ratio) and abdominal visceral fat accumulation may be age-specific. Older subjects seem to have more visceral fat for each given value of waist circumference (or waist-to-hip ratio) than younger adults (20). From a public health point of view, it might be argued that measuring waist circumference on large subgroups of a population, and then using age-specific threshold values to identify individuals at risk, would present no major problems. In a study from Glasgow, the waist circumference was found to relate closely to both BMI and waist-to-hip ratio. By using a combination of these two measures of adiposity as a 'gold standard' and calculating sensitivity and specificity with respect to waist circumference, two action levels for waist circumference were suggested. A waist circumference of more than or equal to 94 cm in men and 80 cm in women (lower action level) identified' with a sensitivity of 96% and a specificity of 97.5%, subjects having either a BMI more than or equal to 25 kg/m2, or a waist-to-hip ratio that was high ( > 0.95 for men and > 0.80 for women). The higher action level was defined as a waist circumference of more than or equal to 102 cm for men and 88 cm for women. These levels identified, with the same degree of sensitivity and specificity, subjects with either a BMI of more than or equal to 30kg/m2, or a high waist-to-hip ratio. The conclusion was that waist circumference is a suitable tool to use in health promotion programmes to identify, i.e. screen for, individuals who might benefit from weight management programmes (21). It was also suggested that individuals identified to be above the lower action level should acknowledge being in an increased risk situation and take action to avoid weight gain, and that individuals above the higher action level should seek professional help to lose weight and maintain a lowered body weight.

From samples within the Nurses' Health Study and the Health Professionals' Follow-up Study, the validity of self-reported body weights and waist and hip circumferences has been explored. The self-reported measurement was compared to a measurement conducted by a specially trained technician in the study. An especially high degree of correlation was found for body weight (0.97) and waist circumference (0.95 in men and 0.89 in women) (22). Recently, national guidelines for the management of obesity in Scotland were presented by the Scottish Intercollegiate Guidelines Network (23). Their report recommended the use of BMI and waist circumference as screening tests for obesity.

A point of caution is warranted if using these new action levels of waist circumference on populations other than Caucasian. A study comparing the prevalence of glucose intolerance in Chinese and Europid men and women showed similar age-adjusted prevalences in men (13%) but higher prevalence of glucose intolerance in Chinese women (20%) compared to Europid women (13%), despite the fact that the study found mean BMI and waist circumference to be lower in Chinese men and women than in Europids. Furthermore, the mean waist-to-hip ratio in Chinese women was higher than in Europid women (24).

Taken together, total body fat measured as body mass index and body fat distribution measured as waist circumference seem to supplement each other in indicating a cardiovascular or metabolic risk situation for an individual. They seem both to be suitable as screening tests and should preferably be used together in health promotion programmes. The action levels that have been suggested are based on our present knowledge of using simple proxy measures to assess total and visceral fat accumulation. However, as stated earlier, the choice of cut-off point in screening programmes must be determined by the level of resources allocated to take care of those identified to be at high risk. Alerting without offering advice and support is harmful. It goes without saying that different degrees of risk may need different levels of treatment programmes. It is crucial that every high-risk individual feels that the screening programme offers something they experience as beneficial.

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