The measurement of physical activity in free-living humans and the effect of seasonal and short-term changes in physical activity on cardiovascular disease risk factors.

摘要

Recently published federal public health initiatives have called for research to "develop better methods for analysis and quantification of (physical) activity" behaviors and to better understand the most important "features of physical activity (total amount, intensity, duration, frequency, pattern, or type) that confers health benefit". Accordingly, this dissertation examined measurement issues related to the sources of variance in daily physical activity, as well as the types and features (e.g., dose) of physical activity that were associated with the cardiovascular disease risk factors body mass and blood lipids. Three investigations from within the Seasonal Variation of Blood Cholesterol Study (Seasons) cohort were conducted. Seasons was an observational study of 641 healthy adults that collected longitudinal assessments of body mass, blood lipids, physical activity, and dietary intake over 12 months of follow-up.;The first investigation found that day-to-day variability was the major source of variance (65--80%) in activity, and that seasonal and day-of-the-week effects were minimal (2%). Between-subject differences accounted for 20--35% of the variance observed, and 14 to 21 days of assessment were required to reliably measure non-occupational activity. The second investigation found that men increased (0.5 kg, p 0.05) and women maintained (--0.2 kg, p > 0.05) their body mass levels over follow-up, and that both sexes had lower body mass (0.4 to 0.6 kg, p 0.09) and higher activity levels (0.7 to 1.1MET-hrs·d--1, p 0.05) in spring/summer compared to winter months. Total, occupational, and leisure activities were associated with lower cross-sectional body mass levels, and household and leisure activities were associated with longitudinal reductions in body mass. The final investigation revealed that individuals who were below recommended levels of moderate-vigorous physical activity ( ∼ 2 MET-hrs·d--1) had lower high-density lipoprotein cholesterol and higher triglyceride levels, placing them at increased risk of coronary heart disease. Additionally, total activity and duration of activity, but not intensity, were associated with lower levels of, or changes in blood lipids, particularly among men. These findings have important implications for the measurement of physical activity in epidemiologic studies, and they underscore the notion that naturally occurring changes in physical activity levels are directly related to disease risk.