Relative Risk of Mortality in the Physically Inactive Is Underestimated Because of Real Changes in Exposure Level during Follow-up

• Published in: American journal of epidemiology Vol. 160; no. 2; pp. 189 - 195
• Main Author: Andersen, L. B.
• Format: Journal Article
• Language: English
• Published: Cary, NC Oxford University Press 15.07.2004; Oxford Publishing Limited (England)
• Subjects: Abbreviation: SD; Adult; Aged; Aged, 80 and over; Analysis. Health state; Behavior; Biological and medical sciences; Denmark - epidemiology; Epidemiologic Factors; Epidemiology; Exercise; Female; follow-up studies; General aspects; Health risk assessment; Humans; Leisure Activities; Life Style; Male; Medical sciences; Middle Aged; Mortality; Mortality risk; Physical fitness; Prospective Studies; Public health. Hygiene; Public health. Hygiene-occupational medicine; Risk; Risk taking; standard deviation; Studies; Denmark; Copenhagen Denmark; Denmark, Copenhagen; Physical exercise; Human; follow-up studies; Follow up study; Mortality; exercise; Exposure; Behavior; Epidemiology; Public health; Relative risk
• ISSN: 0002-9262; 1476-6256
• DOI: 10.1093/aje/kwh195

Relative risk among exposure groups in prospective cohort studies is based on the assumption that all subjects are exposed at the level recorded at baseline throughout the study. Changes in risk behavior during follow-up will dilute the relative risk. This prospective cohort study in Copenhagen, Denmark, between 1964 and 1994 included 30,640 men and women; 19,149 were examined twice, with an interval of 6.7 (standard deviation, 3.4) years. Relative risks calculated from baseline measurements for moderately active and sedentary groups compared with the highly active group were 1.11 (95% confidence interval: 1.05, 1.18) and 1.64 (95% confidence interval: 1.53, 1.75), respectively. The relative risk between the highly active group and the sedentary group decreased with increasing follow-up time. When intraindividual changes in physical activity level during follow-up were taken into account, the relative risk of physical inactivity was 24–59% higher compared with the relative risk estimated from baseline measurements. The risk of a sedentary lifestyle is underestimated when it is calculated from one baseline measurement in prospective studies, because subjects change behavior during follow-up.