12.1.1 Obesity as a health problem

Obesity is defined as an excess of body fat that is sufficient to adversely affect health. The prevalence of obesity has been difficult to study because many countries have had their own specific criteria for the classification of different degrees of overweight. However, during the 1990s, the body mass index (weight in kg/height in metres squared), or BMI, became a universally accepted measure of the degree of overweight and now identical limits are recommended. The most frequently accepted classification of overweight and obesity in adults by the WHO is shown in Table 12.1.1.1 (1).

In many community studies in affluent societies this scheme has been simplified and cut-off points of 25 and 30 kg/m² are used for descriptive purposes of overweight and obesity. Both the prevalence of very low BMI (below 18.5 kg/m²) and very high BMI (40 kg/m² or higher) are usually low, in the order of 1–2% or less. There are some indications that the limits used to designate obesity or overweight in Asian populations may be lowered by several units of BMI; this would greatly affect estimates of the prevalence of obesity. In countries such as China and India with each over a billion inhabitants, small changes in the criteria for overweight or obesity potentially increase the world estimate of obesity by several hundred million (currently estimates are about 250 million worldwide).

The distribution of abdominal fat should be considered for an accurate classification of overweight and obesity with respect to the health risks (Table 12.1.1.2). Traditionally this has been indicated by a relatively high waist-to-hip circumference ratio; however, the waist circumference alone may be a better and simpler measure of abdominal fatness (2). In 1998 the National Institutes of Health adopted the BMI classification and combined this with limits for waist measurement (3). This classification proposes that the combination of overweight (BMI between 25 and 30 kg/m²) and moderate obesity (BMI between 30 and 35 kg/m²) with a large waist circumference (greater than or equal to 102 cm in men or greater than or equal to 88 cm in women) carries additional risk (3).

In many reviews, obesity (defined as a BMI of 30 kg/m² or higher) is a prevalent condition in most countries with established market economies (4, 5). There is a wide variation in prevalence of obesity between and within these countries. Usually, obesity is more frequent among those with relative low socioeconomic status and the prevalence increases with age (5). In most of these established market economies the prevalence is increasing over time (5). A recent report by the WHO has stated that worldwide 1.6 billion people are overweight, defined as a BMI of 25–30 kg/m², and 400 million people are obese, defined as a BMI of greater than 30 kg/m², with this latter figure projected to rise to 1.12 billion by 2030. In the UK, the incidence and prevalence of obesity is rising rapidly; in 1980, 8% of women and 6% of men were classified as obese, which rose to over half of women and two-thirds of men classified overweight or obese in 2001.

Obesity is uncommon in sub-Saharan Africa, China, and India, although in all regions the prevalence seems to be increasing particularly among the affluent parts of the population in the larger cities (6). In these countries, the paradoxical state of symptoms of increasing undernutrition and overnutrition are occurring simultaneously due to growing inequalities in income and access to food in these regions. In addition, the classification criteria based on white populations might not be appropriate for other populations. A prospective epidemiological study of more than one million individuals has established that obesity is an independent risk factor for increased mortality (7).

The exact nature of the relationship between BMI and mortality is still unclear. In many studies a U-shaped or J-shaped relationship has been observed between BMI and mortality but increased mortality at low levels of BMI may be confounded by effects of smoking and smoking-related diseases or other health conditions causing weight loss and thinness. The relationship between BMI and mortality in healthy nonsmokers is linear but in many studies a curvilinear relationship remains. An alternative interpretation for this curvilinear association is that it is the result of the combination of two linear functions: one increasing by increasing fat mass, and one decreasing by increasing lean mass (8). In addition, there are many factors that mediate or modify the relation between the degree of overweight and the incidence of morbidity and mortality (8). The effect of age and smoking are now well recognized but the nature and magnitude of the effect of other factors on the risks of overweight and obesity are still poorly understood. With increasing age, a high BMI is still associated with all-cause mortality but the relative risks are less pronounced than at younger ages.

One of the most important contributors to increased mortality in obese people is cardiovascular disease (coronary heart disease and cerebrovascular accidents). This is due to the impact of excess body weight or body fat on a wide spectrum of risk factors for cardiovascular disease. Overweight and obesity may be responsible for about 15–30% of the incidence of fatal coronary heart disease in Caucasian people (4). The disease most strongly associated with overweight and obesity is type 2 diabetes mellitus. Two American prospective studies suggested that 65–75% of new cases of diabetes mellitus can be attributed to overweight and obesity. In addition, increasing degrees of overweight are associated with an increased incidence of arthritis of hands and the weight-bearing joints, gallbladder disease, sleep apnoea, and several types of cancer (breast and endometrial cancer in women, colon cancer in men). Obesity is also associated with nonfatal chronic conditions that may have a profound effect on the quality of life and costs of medical care (Table 12.1.1.3). These include symptoms of respiratory dysfunction, chronic low back pain, and difficulties in physical functioning as well as major psychosocial morbidity, especially in the severely obese (9).

There is general agreement that our modern lifestyle is a main driver for the increasing prevalence of overweight and obesity. The human genome has not changed over the last hundreds of thousands of years, whereas the environment has been greatly altered, in particular during the last decades. Access to cheap, palatable food rich in fat and sugar but low in dietary fibre has resulted in so-called passive overconsumption of an energy-dense diet. The human appetite regulation is asymmetrical—there are strong forces to seek food at hunger, but weak forces to stop overconsumption, and hence people consume more energy than needed for a weight-stable energy balance.

Likewise the need for energy expenditure has been remarkably reduced. Very little physical activity is in fact needed to carry out the ordinary everyday tasks that have been taken over by mechanical devices. Physically demanding jobs are less common and most voluntary energy expenditure is leisure time activity. The threatening combination of an increased energy intake and reduced energy expenditure can well explain the development of overweight and obesity. A minimal daily positive energy balance, below the detection threshold of modern respirator chambers, may in the long run result in a small, but continuous weight increase.

The positive side to this is that just as minor continuous positive energy balance will result in obesity, even small but maintained lifestyle changes reversing this process will be important. Modest adjustment in eating behaviour and physical activity may result in a change in energy balance counteracting the obesogenic risk pattern of modern society.

Obesity is an important determinant of mortality, morbidity, and diminished quality of life and physical functioning. The increase in the prevalence of obesity around the world represents a significant global public health problem and an economic burden. Appropriate management of the ever-increasing number of obese patients is an important challenge. From a public health point of view, prevention of obesity is even more important. It is not easy to handle the consequences of the increasing sedentary ways of life amid abundant and affordable foods with a high energy density. This requires individual action supported by structural changes in our physical, sociocultural, and economic environments (10).