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      We appreciate the letter by Bassett and Lee contrasting the leisure-time physical activity data in the National Health and Nutrition Examination Survey (NHANES) datasets and the Behavioral Risk Factor Surveillance System (BRFSS).
      First, we would like to clarify that, in our article,
      • Ladabaum U.
      • Mannalithara A.
      • Myer P.A.
      • Singh G.
      Obesity, abdominal obesity, physical activity, and caloric intake in U.S. adults: 1988-2010.
      we were careful not to make claims regarding what fraction of the obesity problem can be attributed to energy intake or energy expenditure. We reported that we did not detect substantial increases over time in self-reported energy intake, but that, in contrast, the trends in obesity were associated with increases in the self-reported prevalence of leisure time inactivity. We did take the opportunity to stress the importance of physical activity to health in the “Discussion” section of our article.
      In regard to the physical activity trends, we acknowledge that research methods for NHANES have changed over time, and therefore the data must be analyzed with caution. We agree with Bassett and Lee that the trends in physical inactivity might be explained in part by changes in survey methods. We stated in the “Discussion” section: “Changes in survey methodology over time could have affected physical activity estimates.”
      • Ladabaum U.
      • Mannalithara A.
      • Myer P.A.
      • Singh G.
      Obesity, abdominal obesity, physical activity, and caloric intake in U.S. adults: 1988-2010.
      It is difficult to know precisely how much of the trends can be explained by changes in survey methods.
      We agree with Bassett and Lee that in trying to ascertain true trends in the population, examining several data sources is useful. The fact that the BRFSS includes a question that has remained relatively constant over time is a strength. However, we wonder whether it might be easier to answer a casual “Yes” to a question that essentially asks “Did you do anything” (as in the BRFSS) than to have to provide some actual support for what physical activity one did (naming the activity, frequency, or time spent, as in NHANES). Both carry the risk of recall bias or misreporting. Although we appreciate the concerns over the change in NHANES methods over time, the potential limitation of the single question in the BRFSS must be acknowledged.
      We welcome the discussion that our article has engendered, and we thank Bassett and Lee for contributing to the discussion. They make an important point that some evidence exists that energy expenditure from combined leisure time, occupational, housework, and transport-based physical activities has declined, which is something we did not explore in detail in our study.
      Although there is no clear answer at this time regarding the relative contribution of energy intake or physical activity (or other variables including dietary components, patterns of activity, and environmental factors, including the gut microbiome) to the public health problem of obesity, we believe that public health messages should continue to emphasize the importance of both a healthy diet and remaining physically active throughout life.

      Reference

        • Ladabaum U.
        • Mannalithara A.
        • Myer P.A.
        • Singh G.
        Obesity, abdominal obesity, physical activity, and caloric intake in U.S. adults: 1988-2010.
        Am J Med. 2014; 127: 717-727

      Linked Article

      • Trends in Physical Inactivity
        The American Journal of MedicineVol. 128Issue 5
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          A recent article by Ladabaum et al1 raised an important question regarding the leading drivers for the obesity epidemic in US adults. We appreciate that the authors called attention to physical inactivity as an important public health problem and attempted to relate the rise in obesity to trends in leisure-time physical activity and energy intake. We agree that the prevalence of adult obesity has increased during this period. This is consistent with previous studies using the National Health and Nutrition Examination Survey (NHANES), which defined obesity according to waist circumference2 or body mass index.
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