Physical activity from the epidemiological perspective : measurement issues and health effects

Abstract

The aim of this thesis was to increase our understanding of links between physical activity and health. The relationships between physical activity and breast cancer risk and between current physical activity and overall mortality were examined. Additionally, a novel method was developed to measure physical activity in epidemiological studies. Besides validation studies, a large cohort was enrolled for assessment of physical activity with this method.

Although an association between physical activity during adolescence and breast cancer is biologically plausible, results from individual studies have been inconclusive. This led us to conduct a quantitative summary analysis of published studies. Nineteen case-control and four cohort studies were included in our meta-analysis. Comparing the highest to the lowest category of physical activity, the summary relative risk (RR) of breast cancer was 0.81 (95 % confidence interval [CI] 0.73 0.89). This risk reduction of almost 20 % was fairly consistent across different strata. The analysis identified an important source of heterogeneity within the literature; the various methods used to assess physical activity, a complex exposure.

In a population-based cohort study of 99,099 Norwegian and Swedish women, we found that current rather than past physical activity substantially reduces mortality. This was observed even at low levels of physical activity, and was accentuated with increased physical activity. During an average 11.4 years of follow-up, risk of death decreased monotonically over five categories of physical activity at the time of enrollment (p for trend <0.0001) and was reduced by half in the highest compared with the lowest category (RR = 0.46; 95 % CI 0.33 0.65). Physical activity was assessed on a 5-point scale, which makes it difficult to translate the results to a quantifiable public health message.

A prerequisite for advancements in our understanding of health effects of physical activity is better assessment methods. We developed an instrument of self-reported time spent on different intensity levels of physical activity (and inactivity) during a typical day, allowing for estimation of total energy expenditure on an interval scale. In a first validation study, we tested if 80 volunteers using our instrument could correctly estimate MET (Metabolic Energy Turnover) values during concurrent work on a bicycle ergometer. The Pearson correlation coefficient between true and estimated METs was 0.89 (range 0.81 - 0.95). In a retest reliability assessment of 20 subjects, intraclass correlation coefficient was 0.99.

In a second validation study we addressed the ability to correctly remember and integrate 'usual' energy expenditure over time. A population-based sample of 418 Swedish men and women, aged 20 59 years, completed a questionnaire containing the new instrument. For validation, three 24-hour recalls by phone served as the gold standard. Reproducibility was assessed through administering the instrument another three times over 6 months. Pearson correlation between usual daily energy expenditure measured by the instrument and the 24-hour mean recall was 0.73. Reproducibility showed an intraclass correlation of 0.55. We concluded that the instrument provides reasonably valid estimates of total energy expenditure.

We established a general population cohort based on 43,876 subjects who took part in The National March, a fund-raising event for the Swedish Cancer Society. All participants were invited to fill out a 36-page questionnaire, of which three pages were devoted to physical activity. We cross-sectionally compared measures of physical activity obtained through different means of inquiry. Household and leisure time physical activity represented no more than 17 % of total physical activity assessed by the new instrument. The Spearman correlation coefficient between total physical activity obtained with the new instrument and the sum of household and leisure time physical activity was 0.26. The correlation was even lower when comparing total activity obtained with the instrument to self-rated fitness and self-rated total activity judged relative to peers, indicating that the estimated physical activity level in an epidemiological study is contingent on the mode of inquiry.