Physical activity in the prevention and treatment of obesity and its comorbidities

On February 4–6, 1999, the American College of Sports Medicine sponsored a scientific roundtable on the role of physical activity in the prevention and treatment of obesity and its comorbidities. The purpose of the conference was to provide an evidence-based review of the current state of knowledge on physical activity as a modality for coping with the “epidemic” of obesity occurring in the United States and other nations (1,5). Participants of the conference were requested to review existing literature and to classify available data according to accepted evidence-based categories. The categories employed are those outlined in the recent Clinical Guidelines on the Identification, Evaluation, and Treatment of Obesity in Adults reported by a task force of the Obesity Education Initiative (OEI) of the National Heart Lung and Blood Institute (NHLBI) (2) (Table 1). Table 1: Categories of evidence.A particular goal of this roundtable was to review randomized clinical trials (RCT) that provide evidence in Categories A and B. Unfortunately, there are a limited number of RCT of physical activity in obese populations. On the other hand, the field is rich in observational data and studies in human physiology and behavior (Category C). A sizable portion of the Category C evidence derives from large and replicated studies and provides a considerable base upon which recommendations can be made. Such recommendations indeed are set forth in the OEI report (2), the NIH Consensus Conference Statement on Physical Activity and Cardiovascular Health (December 18–20, 1995) (3), and the Report of the Surgeon General on Physical Activity and Health (4). The current roundtable aimed to extend these previous reports by examining prior and new publications in more depth and by categorizing evidence more precisely as to type and strength. The manuscripts upon which presentations and the current panel report are based are being published concurrently in Medicine and Science in Sports and Exercise. The glossary of key terms used in the current report are those derived from the Surgeon General’s report on physical activity and health (4) (Table 2). Table 2: Glossary of terms.BACKGROUND Prevalence and trends in overweight and obesity. The most recent National Health and Nutritional Examination Survey (1988–1994) (NHANES III) (1) revealed that 54.9% of American adults are overweight or obese (see Table 3 for OEI classification of overweight and obesity). Since 1960, overweight and obesity have increased across all ages, genders, and racial/ethnic groups. Prevalence in the obese category has increased by about 10%. Not only has the overall distribution of body mass index (BMI) shifted to higher levels, but the distribution has become even more skewed toward the high end. In the recent survey (1), the highest prevalence of obesity was found among non-Hispanic black women, Mexican-American women, Mexican-American men, and among less well-educated and low-income people. Table 3: Classification of overweight and obesity by BMI.The recent OEI report (2) emphasized that waist circumference is a good indicator of abdominal obesity, which at BMI levels ≤ 35 kg·m−2 is correlated more closely with the comorbidities of overweight than is BMI (see Table 4 for OEI classification of disease risk according to waist circumference). Cross-sectional epidemiological data are not available to define the prevalence of abdominal obesity in the population; neither are data available to correlate waist circumference with comorbidities in the whole population. Such data would add significantly to our understanding of the metabolic consequences of overweight. Table 4: Disease risk* relative to normal weight and waist circumference.aPrevalence and trends in physical activity. The 1996 Behavioral Risk Factor Survey (BRFSS) reveals a high prevalence of physical inactivity among American adults and high school students (see Table 5 for the classification of physical activity used in the Surgeon General’s report on physical activity). Adolescents are more active than adults; 64% of high school students report participating in vigorous (hard or very hard) activity for at least 20 min on three or more days per week. Boys engage in physical activity more than girls, and whites in general more than in blacks or Hispanics. Physical activity declines at higher grades in school, especially among girls. Among adults, only 28% of men and women achieve moderate or vigorous levels of physical activity. Further, 27% of men and 31% of women report no regular physical activity outside of work. Educational level is a factor in exercise pattern. Less than 20% of college graduates report being inactive, whereas nearly half the population with a high school education are inactive. Black and Hispanic men and women are less active than white men and women. In spite of long-term societal trends in activity patterns, these have not been assessed quantitatively; moreover, leisure-time activity apparently has not changed much over the recent decades. Future surveys might well examine the impact of changes in activity patterns at work and at home, and in active and passive leisure time activities (i.e., less active recreation during and after school and more television viewing, computer work, and playing video games). Table 5: Classification of physical activity intensity, based on physical activity lasting up to 60 min.The economic costs of obesity and inactivity. The medical costs of obesity and physical inactivity can be estimated from the strengths of their associations with various diseases—coronary heart disease, diabetes, gallstone disease—and comorbid risk factors. The direct costs of a lack of physical activity, defined conservatively as absence of leisure-time physical activity, are approximately 24 billion dollars, or 2.4% of the U.S. health care expenditures. Direct costs for obesity defined as a BMI greater than 30 kg·m−2, in 1995 dollars, totals 70 billion dollars. The costs of inactivity are independent of obesity, and the costs of obesity are independent of those due to a lack of physical activity. Overall, the direct costs of inactivity and obesity are estimated to consume some 9.4% of national health care expenditures in the United States. Determinants of overweight and obesity. The high and increasing prevalence of overweight and obesity must be due in large part to environmental factors. Development of obesity requires that energy intake exceed energy expenditure; maintenance of obesity demands a higher energy input or a lower energy expenditure, or both, than needed for a healthier weight. Factors affecting both intake and expenditure of energy probably play a role in the causation and maintenance of obesity. Societal trends go against reducing energy intake as well as against increasing energy expenditure. A relatively high intake of energy is driven by a food supply that contains readily available, energy-dense foods, served in large portions. Energy expenditure is lowered by progressively lesser amounts of physical activity required at work and at leisure. Most energy expenditure is obligatory, determined by resting metabolic rate and the thermic response to food intake, but physical activity must not be discounted in the equation for total energy expenditure. Even in the absence of occupations or recreations that consume large amounts of energy, regular physical activity offers a means to lessen the severity of overweight and obesity in the population. A sustained increment in energy expenditure of 200 kcal·d−1 through increased physical activity would reduce body weight by about 5 kg over a period of 6 months to 1 yr, assuming no increase in food consumption. Unfortunately, any weight loss achieved by moderate physical activity can be easily reversed by small compensatory increases in food intake. Measurements of body habitus and energy parameters. If the role of physical activity in the causation or treatment of obesity is to be placed on a quantitative basis, accurate measurements must be made of body composition, energy intake and expenditure, and levels of physical activity. Methods for measurements in each of these areas are improving but still have significant limitations. Significant strides have been made in measurements of body composition—total body fat, body fat distribution, lean body mass, and muscle mass. Several of these techniques are increasingly being used in epidemiological studies, but the most sophisticated measurements can be costly. Despite the advantage of being safe enough for use in children and pregnancy, the latter are too laborious and costly for use in large populations. Methods for measurement of nutrient and energy intake leave much to be desired. Questionnaires and diaries of food intake are plagued by under reporting. Precise methods for estimating energy intake, as would be required to define differences in intake responsible for weight gain, are not available. The most objective and accurate method for assessing the level of physical activity and energy expenditure of activity is average daily metabolic rate, determined by the doubly labeled water, minus the basal metabolic rate. This method, however, is limited to studies in small numbers of subjects. More applicable to population studies are motion sensors, specifically, accelerometers. These instruments reliably assess patterns of physical activity. Accelerometers offer the advantage of measuring motion from nonexercise (i.e., lifestyle) activity as well as exercise activity. Another way to estimate “integrated” physical activity is to measure cardiorespiratory fitness. This measure is represented by maximal oxygen consumption determined under exercise conditions. This measurement carries the advantage of being quantitative and available for epidemiological studies. It is not, however, a direct measure of physical activity but only a reflection of it. Moreover, cardiorespiratory fitness is largely an indication of recent intensive exercise; it may not be a useful tool to assess the effects of moderate exercise, and it is also influenced by inherited characteristics. PHYSICAL ACTIVITY IN THE ETIOLOGY AND TREATMENT OF OBESITY Sedentary life habits and inactivity in the etiology of overweight and obesity. No RCT are available that address whether sedentary life habits and inactivity contribute to the development of obesity in populations. Available data therefore are restricted to observational studies (Evidence Category C). Studies of ecological trends in populations provide suggestive evidence that declining amounts of physical activity have contributed importantly to a rising prevalence of overweight and obesity. Some studies have reported secular decreases in energy intake concurrently with increases in body weight, both in children and adults; these decreases infer a corresponding even greater in energy expenditure, which would be a for weight studies, each on about lower levels of activity being correlated with higher levels of body In these studies, not be of with less education and lesser greater body that was with more of television and other sedentary In used to about food intake and exercise on large of women and weight correlated with high exercise in men and with both high exercise and in women. the high consumption of and and with body weight. This a etiology for increased body weight. A review of studies revealed some levels of physical activity and risk of obesity. 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