Obesity is one of the most common disorders in clinical practice. Defined (by the National Institute of Health – NIH) as a body weight 20% or more above “desirable” weight.
Obesity is an excess of body fat; overweight is an excess of body weight, including all components of body composition (muscle, bone, water and fat). In clinical practice, the two are used interchangeably to refer to excess body fat. The two most commonly used terms to quantify obesity are relative weight (RW) and body mass index (BMI). The RW is the actual weight divided by the “desirable weight” (derived from “acceptable weight” tables). The BMI, or Quetelet index, is the actual body weight divided by the height squared (kg per m2). This index more closely corresponds to measurements of body fat and better differentiates “overweight” due to an increase in muscle mass from true obesity.
A recent National Institute of Health Consensus Conference defined obesity (somewhat arbitrarily) as a RW of greater than 120% (BMI > 27 kg/m2). “Morbid” obesity is commonly defined as a RW greater than 200% (BMI >40 kg/m2).
OBESITY IN AMERICANS
Over one third of adult Americans are overweight. Perched at the center of chronic disease risk and psychosocial disability for millions of Americans, successful management of obesity offers unique patient care and public health opportunities. If all Americans were to achieve a normal body weight, it has been estimated that there would be a 3 year increase in life expectancy, 25% less coronary heart disease, and 35% less congestive heart failure and stroke.
OBESITY IN ASIANS
Currently, about 15 per cent of the Indian population falls under the category of obese. If the parameters are revised, the figure would swell to 25-30 per cent. Researchers in Hong Kong found that as people surpassed a BMI of 23, there was a very sharp rise in the risk of heart disease.
Asian and Caucasian may be the same weight and height, but the Asian is at greater risk for fat-related illnesses such as heart disease and diabetes.
Studies in India found that city dwellers have an average BMI of 24.5, and have diabetes rates four times higher than their rural counterparts, hypertension 2 1/2 times higher, and heart disease two times higher.
Unfortunately, obesity is also one of the most difficult and frustrating disorders to manage successfully. Primary care providers and patients with little benefit expend considerable effort. Using standard treatments in university settings, only 20% of patients lose 20 pounds at two-year follow-up while only 5% of patients lose 40 pounds. This lack of clinical success has created a never-ending demand for new weight loss treatments. Approximately 50% of women and 25% of men are dieting at any one time. Americans spent over $30 billion last year on diet books, diet meals, weight-loss classes, diet drugs, exercise tapes, “fat farms,” and other weight loss aids!
The challenge for health care providers is to identify those patients with obesity who are most likely to medically benefit from treatment and most likely to maintain weight loss, and to provide them with sound advice, skills for long-term lifestyle change and support. The purpose of this presentation is to review new developments in our understanding of obesity and its treatment to assist in this difficult but important task.
Health Consequences of Obesity
The relationship between body weight and mortality is curvilinear, similar to other cardiovascular risk factors. Most studies have demonstrated a J-shaped or U-shaped relationship, suggesting that the thinnest portion of the population also have an excess mortality. This is thought to be primarily due to the higher rate of cigarette smoking in the thinnest group.
The relationship of body weight to mortality is also effected by age. The body weights associated with the lowest mortality increase with age, and newer weight tables take this into account. In addition, as age increases to over 65, the relationship of body weight and mortality takes on a more striking U-shape.
This suggests that although obesity remains an important risk factor in the elderly, under nutrition is also extremely important.
The increase in total mortality related to obesity results predominantly from Coronary Heart Disease (CHD). Evidence is mixed whether obesity is an independent risk factor for coronary heart disease. For example, the 1993 cholesterol treatment guidelines omit obesity an a risk factor for CHD, while the previous edition in 1988 included obesity. Nonetheless, obesity is clearly an important risk factor for the development of many other CHD risk factors. Obese individuals age 20-44, for example, have a 3.8 times greater risk of type II diabetes, 5.6 times greater risk of hypertension, and 2.1 times greater risk of hypercholesterolemia. As a result, type II diabetes and stroke also contribute to the increase in obesity-related mortality. The obese also have an increase risk of certain cancers including colon, rectum, and prostate in men and uterus, biliary tract, breast, and ovary in women.
As a result of these conditions, relative weights of 130% are associated with an excess mortality of 35%. Relative weights of 150% have a greater than two-fold excess death rate. Patients with “morbid” obesity (relative weights greater than 200%), have a greater than 10-fold increase in death rates.
Obesity is also associated with a variety of other medical disorders including degenerative joint disease of both weight bearing and non-weight bearing joints, diseases of the digestive tract (gallstones, reflux esophagitis), thromboembolic disorders, heart failure (both systolic and diastolic), respiratory impairment, and skin disorders. Obese patients also have a greater incidence of surgical and obstetric complications and are more prone to accidents. Although obesity is not associated with an increased risk of major psychiatric disorders, obese patients are at increased risk of psychological disorders and social discrimination.
Regional fat distribution: Recent investigations suggest that the location of the excess body fat (regional fat distribution) is a major determinant of the degree of excess morbidity and mortality due to obesity. At least three components of body fat are associated with obesity-related adverse health outcomes. These are the total amount of body fat (expressed as a percentage of body weight), the amount of subcutaneous truncal or abdominal fat (upper body fat), and the amount of visceral fat located in the abdominal cavity. These three are partly correlated with each other but exhibit a fairly high degree of independence. Each of these components of body fat is associated with varying degrees of metabolic abnormalities and independently predict adverse health outcomes. In each of 6 prospective epidemiologic studies, increased abdominal obesity was associated with increased cardiovascular and total mortality.
Body fat distribution can be assessed by a number of measurement techniques. Measurements of skinfolds (subscapular and triceps) reflect subcutaneous fat. Measurement of circumferences (waist and hip) reflect both abdominal and visceral fat. CT and MR scans measure subcutaneous and visceral fat. Clinically, measurement of the waist and hip circumference is most useful. The waist is measured at the umbilicus and the hips at the greater trochanter. A waist to hip ratio of 1.0 and 0.8 are considered normal in men and women, respectively. Ratios above these values reflect abdominal or visceral obesity and a greater risk of obesity-related disorders.
Etiology of Obesity
Numerous line of evidence suggest strong genetic influences on the development of obesity. Most convincing are genetic studies of adoptees and twins. In a study of 800 Danish adoptees, there was no relationship between the body weight of adoptees and their adopting parents but a close correlation with the body weights of their biological parents. In a study of approximately 4000 twins, a much closer correlation between body weights was found in monozygotic than dizygotic twins. In this study, genetic factors accounted for approximately two thirds of the variation in weights. More recent studies of twins reared apart and the response of twins to overfeeding showed similar results.
The exact mechanisms by which such genetic factors result in obesity are considerably less clear. Differences in both energy intake and energy expenditure have been investigated. Genetic influences on control of appetite and eating behavior have long been considered. Animal studies have demonstrated the influence of dozens of factors on eating behavior, and it is likely that similar factors are at work in humans. Observational studies of eating behavior have suggested that the obese both eat more food and do so more rapidly than the non-obese.
Differences in energy expenditure are also likely to be at least partially determined by genetic influences. Differences in the resting metabolic expenditure (RME), for example, could easily result in considerable differences in body weight since RME accounts for approximately 60-75% of total energy expenditure. The RME can vary by as much as 20% between individuals of the same age, sex, and body build; such differences could account for approximately 400 kcals of energy expenditure per day! Recent evidence suggests that the metabolic rate is similar in family members, and as expected, individuals with lower metabolic rates are more likely to gain weight. Differences in the thermic effect of food, the amount of energy expended following a meal, may also contribute to obesity. Although some investigators have shown a decreased thermic effect of food in the obese, others have not.
Environmental factors are also clearly important in the development of obesity. Decreased physical activity and food choices that result in increased energy intake also clearly contribute to the development of obesity. Medical illness and some medications can also result in obesity, but such instances account for less than 1% of cases. Hypothyroidism and Cushing’s syndrome are the most common. Diseases of the hypothalamus can also result in obesity but these are quite rare. Major depression, which usually results in weight loss, can occasionally present with weight gain. Consideration of these causes is particularly important when evaluating unexplained, recent weight gain.
Thus, the etiology of obesity is multifactorial, and almost certainly under both genetic and environmental influences.