Obesity Epidemiology

304 EPIDEMIOLOGIC STUDIES OF DETERMINANTS OF OBESITYincreased 8.6% among women (from 43.0% to 46.7%) and 3.5% among men (from 48.0%to 49.7%). The increase was greatest for non-Hispanic black women (a 15.0% increase; from31.4% to 36.1%) and non-Hispanic black men (a 12.4% increase; from 40.3% to 45.3%). 7According to data from BRFSS, the proportion of the U.S. population that reported noLTPA (such as running, calisthenics, golf, gardening, or walking) in the previous monthdecreased from about 31% in 1989 to about 25% in 2005 (http://www.cdc.gov/nccdphp/dupa/physical/stats/index.htm).As discussed in Chapter 7, LTPA accounts for only a small part of total physicalactivity energy expenditure. Thus, it is important to examine time trends of other typesof physical activity, including occupational, household, and transportation activity.Estimates—based on data from the Bureau of Labor Statistics, the U.S. Census Bureau,and job classifications in a study by King et al. 8 —show that in the past 50 years, thepercentage of the labor force in high-activity occupations (e.g., farm workers, waiters andwaitresses, construction laborers, and cleaning-service workers) declined from approximately30% in 1950 to 23% in 2000. 7 Conversely, the percentage of the labor force inlow-activity occupations (e.g., executives, administrators, managers, teachers, researchers,clerks, and motor vehicle operators) rose from 23% in 1950 to 42% in 2000.The shift in job categories, combined with increasing automation and computer use,has led to a substantial decline in occupational physical activity energy expenditure.Another major contributor to the decline in overall physical activity is the dramaticincrease in the use of automobiles for both work- and nonwork-related travels, a trendaccompanied by a decline in walking or use of public transportation. The proportion ofthe population living in suburbs or “urban sprawl” has more than doubled in the pastseveral decades to nearly 50% in 2000, contributing to increased driving and decreasedwalking or use of other means of transportation. 6Concurrent with the declines in overall physical activity energy expenditure, sedentarybehavior has increased dramatically. TV watching is a major sedentary behavior in theUnited States; an adult male spends an average of 29 hours/week watching TV and anadult female, 34 hours/week. 9 In parallel with increasing obesity, the past five decades haveseen a steady increase in the number of hours spent watching TV and using computers.To sum up, although time trend data show that LTPA has not decreased, and may haveeven increased moderately, this increase has not compensated for the substantial declinesin occupational, household, and transportation activities. Thus, overall physical activityin the population has decreased considerably.Ecological and Cross-Sectional Studies ofPhysical Activity and ObesityPrentice and Jebb 10 examined the relationship between physical activity and time trends ofobesity in Britain. Similar to the secular trends in the United States, increased car ownershipand TV viewing were closely related to the growing prevalence of obesity over time.A strong inverse association between social class and prevalence of obesity appeared to beexplained by patterns of physical inactivity rather than by changes in dietary fat. Such analyses,however, are affected by simultaneous changes in other dietary and lifestyle factorsand should, therefore, be kept in perspective. Nonetheless, the secular decline in physicalactivity that coincides with increasing obesity rates has been observed in many societies.Ewing et al. 11 conducted an ecological analysis of the relationship between urban sprawland physical activity and obesity using data from BRFSS (including 448 counties and83 metropolitan areas). Sprawl indices derived through principal components analysis from