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Obesity is an excessive accumulation of energy in the form of body fat, which impairs health. Obesity increases the risk of type II diabetes, cardiovascular disease, arthritis, some forms of tumors and other various diseases(1). Drs. Must and McKeown will discuss the "burdens" associated with obesity in Chapter 2.

Obesity is part of a series of overlapping syndromes, which represent the phenotype of survival genes, in an environment of plentitude.

Figure 1.

It is possible that millions of years ago, when food and water were not easily available, individuals developed a very efficient system to utilize and store energy, salt and water.
Conversely, descendants of those same individuals, living in a new environment of plentitude, would develop obesity and potentially other characteristics of the metabolic syndrome, as illustrated to the right. The special problems associated with diabetes and obesity will be discussed in Chapter 11 by Dr. Pi-Sunyer. Dr. Reaven will cover the Metabolic Syndrome found in Chapter 9.

Figure 2.

Obesity is part of a group of diseases called "complex diseases". The phenotype of complex diseases reflects the multifactorial effect of all contributing genes (polygenic) and all environmental factors. Currently, none of the genes contributing to the common form of human obesity are known. However, the genes contributing to several monogenic forms of obesity have been identified in the last five years. Lessons from monogenic and polygenic obesity in rodents are discussed in Chapter 6 by Drs. Leibel and McMinn, and the genetics of obesity in humans is presented in Chapter 8 by Drs. O'Rahilly and Farooqi.

It is important, however, to emphasize that although these survival genes predispose humans to develop obesity, it is the change in the environment that is responsible for this health problem we are facing today.

Obesity is not a recent phenomenon. The historical roots of obesity can be traced back to 25,000 years ago. Stone age artifacts of corpulent women have been found in several sites across Europe. However, it is most likely that the artist who created the Venus of Willendorf(2) to the left was representing a form of monogenic obesity and her disease was not the result of the interaction of genes with the environment.

Figure 3.

The artist that best emphasizes the importance of the environment for obesity was Botero, below. As it can be seen, mother and father are obese, children are obese, but…. it is not all in the genes as the dog is obese as well. The environmental contributions to obesity will be discussed in Chapter 7 by Drs. Hill and Donahoo.

Although obesity is not a recent phenomenon, the epidemic of obesity is. Unfortunately, current programs to prevent or reverse obesity have been unsuccessful. In the Healthy People 2000 report published in 1991, the U.S. government set forth a goal that no more than 20% of adults in the U.S. aged 20 and older, and not more than 15% of adolescents aged 12 to 19, should be overweight by the turn of the twenty first century(5).

Figure 4.

The map below, taken from the CDC, Behavior Risk Factor Surveillance System 2002 (BRFSS), illustrates the changes in the prevalence of obesity during this period among U.S. adults. BRFSS uses self-reported height and weight to calculate obesity and therefore self-reported data may underestimate obesity prevalence(6). Full reports of the data can be obtained at www.CDC.gov(6). In 2000, the prevalence of obesity was 20% among U.S. adults, a 61% increase since 1991. Thus, approximately 40 million people in the U.S. are obese, of which 20 million are women and 20 million are men.

Figure 5.

In the Healthy People 2000 report, the goals were to not have more than 20% adults overweight. In 2000, 56.4% of U.S. adults (65.5% of men and 47.6% of women) were overweight compared with 45% in 1991. Also, in 1992, 13% of children and adolescents were overweight. Today, there are nearly twice as many overweight children and almost three times as many overweight adolescents than in 1980.

The goals of Healthy People 2000 were not met, but not because appropriate funds were not spent on obesity. The direct cost of obesity and physical inactivity accounts for approximately 9.4% of U.S. health care expenditures(7). What should the goal of the Healthy People 2010 be? Rather than propose unrealistic goals, the Healthy People 2010 includes a Call To Action To Prevent and Decrease Overweight and Obesity based on five principles:

• Promote the recognition of overweight and obesity as major public health problems.
• Assist Americans in balancing healthful eating with regular physical activity to achieve and maintain a healthy or healthier body weight.
• Identify effective and culturally appropriate intervention to prevent and treat overweight and obesity.
• Encourage environmental changes that help prevent overweight and obesity.
• Develop and enhance public-private partnerships to help implement this vision.

It is expected that the current section of www.Endotext.com will contribute to facilitate the implementation of several of these principles. Additional federal programs and resources can be obtained at www.health.gov/healthpeople.com, www.health.gov/healthypeople/state/toolkit and www.health.gov/healthypeople/publications/healthycommunities2001. It is imperative that the best management strategies are implemented for the prevention and treatment of obesity. Dr. Atkinson presents clinical guidelines on identification, evaluation and pharmacological treatment of obesity in Chapter 15. Management of obesity requires a multidisciplinary approach. Dr. Wing in Chapter 17 will present the behavioral treatment of obesity and Drs. Dwyer and Melanson, in Chapter 18, will discuss the dietary treatment of obesity. Drs. Dohm and Fushiki will cover the exercise treatments of obesity in Chapter 19.

The treatment of obesity in children represents a special challenge and opportunity. This is the focus of Chapter 16 by Dr. Rosenbaum. When obesity is extreme, surgical treatment is one of the few effective interventions, which is the theme of Chapter 20 by Dr. Pories.

Why is the treatment of obesity so difficult? Firstly, until recently, obesity had not been considered a disease and therefore the impetus to treat it was absent. Today, the first of the five overarching principles by the Healthy People 2010 is to "Promote the recognition of overweight and obesity as a major public health problem". Secondly, the development of obesity is a process that takes years and yet, it is expected that the reverse will occur in weeks. Unrealistic expectations will make any management plan fail. Thirdly, the etiology and some aspects of the pathophysiology of obesity are poorly understood and therefore treatments that are not based on solid scientific foundations are to have an unexpected result. It is only a few years ago(10) that leptin was discovered and closed the loop of body weight regulation.

Figure 6.

In the most simplistic way, as illustrated to the right, there are three systems that are part of the closed loop that regulates body weight(10): 1) the messenger system in the periphery, informs the brain of the amount of food intake and body fat; 2) the translation system in the brain, receives the information from the periphery and compares it with an internal standard of body fat, the set point of body weight or lipostat; and 3) the effector system regulates energy balance. Dr. Considine will discuss the regulation of energy intake in Chapter 3, focusing on the messenger system. Dr. Lowell in Chapter 4 will discuss the regulation of energy expenditure, concentrating on the effector system. Finally, the neuroendocrine integration of body weight regulation (translation system) will be presented by Drs. Tschop and Horvath in Chapter 5. The sum of this knowledge has generated a cadre of new molecular targets for the treatment of obesity, which is the focus of Chapter 21 by Dr. Tartaglia. Clearly, the most cost effective way to improve the major public health problem of obesity is to prevent it. This is articulated in three of the five priorities in the Healthy People 2010 by the Surgeon General. Also, obesity prevention is the topic of discussion in Chapter 22 by Dr. Burguera.

DEFINITION AND CLASSIFICATION OF OBESITY

Obesity is an excessive accumulation of energy in the form of body fat which impairs health. The degree of health impairment is determined by three factors: 1) the amount of fat 2) the distribution of fat and 3) the presence of other risk factors.

An expert panel organized by the National Institutes of Health in 1998 recommended the use of the Body Mass Index (BMI) to define and classify obesity(11). The panel's recommendations have been adopted widely and although BMI represents a crude measurement, it correlates well with the amount of body fat in the majority of individuals.

Figure 7.

The calculation of BMI requires only the measurement of an individual's weight and height as shown here. However, BMI has several limitations. It can overestimate body fat in people that are very muscular and it can underestimate body fat in people that have lost body muscle, such as many elderly people with chronic illness or extremely sedentary individuals. BMI is only an indirect correlation of the amount of body fat. In a 70 kg normal adult male, 42 kg is water; 12 kg is protein storing 42 Mcal; 12 kg is fat storing 108 Mcal; 0.5 kg is glycogen storing 2 Mcal; and the rest is 3.5 kg storing no calories. If the objective is to directly measure fat mass, other techniques are available that are more expensive and complicated (although not completely accurate) than the measurement of BMI(12). For the present time, most of these techniques are reserved for the research laboratory.

Today, over 50 scientific and medical organizations have endorsed the NIH clinical guidelines supporting the use of BMI to define and classify obesity (Table 1).

It is important to use a common definition and classification of obesity so that meaningful comparisons of weight status within and between populations can be performed. In addition, if a prevention program is to be successful, it is necessary to be able to identify those individuals that are pre-obese and those that are of increased risk for morbidity and mortality. Finally, clear definitions will provide a firm basis for evaluating interventions(8).

A diversity of tables have been published to rapidly calculate the BMI of a subject and to assign a weight classification. An example is shown here(6).

Figure 8.

For an adult population, the BMI values utilized for the definition and classification of obesity is age and gender independent. However, in children and adolescents, overweight has been defined as a gender and age specific BMI at or above the 95th percentile based on revised data from the Center for Disease Control and Prevention(6), below.

Figure 9.

In addition to describing the classification of obesity, Table 1 provides an assessment of the health risks associated with the amount of body fat. However, body fat distribution provides an additional risk that is not given by simply the amount of body fat. This is particularly important since abdominal fat mass can vary significantly within a narrow range of total body fat or BMI. Furthermore, within a given BMI, men have on average twice the amount of abdominal fat than that of pre-menopausal women(1).

There are several methods(13) to accurately localize body fat distribution in humans (computer tomography, ultrasound and magnetic resource imaging), but two methods are practical, inexpensive and have a great degree of epidemiological correlates: A) the waist to hip ratio (WHR). A WHR of >1.0 in men and >0.85 in women identify subjects with abdominal fat accumulation and B) waist circumference. The waist circumference is the most convenient and simple measurement that correlates well with BMI, WHR and most importantly with risk factors for cardiovascular disease. It is measured in centimeters at the midpoint between the lower border of the rib cage and the upper border of the pelvis. A waist circumference of > 102 cm (~ 40 inches) in men and >88 cm (~35 inch) in women is consistent with abdominal obesity and provides a substantial increased risk for metabolic complications.