Long-term physical activity and risk of obesity in Australian women using target-trial emulation: an OBCT study

Background: Physical activity is recommended for obesity prevention, but the evidence is limited by possible biases from reverse causation and confounding, and there are limited data on the long-term effects of physical activity through middle age on the development of obesity. Methods: We emulated a target trial by using 10 waves of observational data from the 1946–51 Australian Longitudinal Study on Women’s Health collected between 1996 and 2022. The target trial would determine the effect of randomizing women to either meeting or not meeting the World Health Organization’s recommended level of physical activity over an 18-year period on the 21-year incidence of obesity. Targeted maximum-likelihood estimation controlling for time-constant and time-varying confounders was used to emulate the target trial, assuming conditional exchangeability, positivity, consistency, and no interference. Results: In total, 779 women aged 47–52 years at study entry were included. The cumulative incidence of obesity was 9.9% [99.5% confidence interval (CI): 8.6%, 11.1%] in the scenario with 18-year adherence to the recommendation and 15.1% (99.5% CI: 12.8%, 17.4%) with consistent non-adherence (risk ratio: 1.5, 99.5% CI: 1.2, 1.9; risk difference: 5.3%-point, 99.5% CI: 2.3%, 8.3%). Each additional survey with non-adherence increased the risk of obesity (risk ratio: 1.1, 99.5% CI: 1.0, 1.1). There was no additional benefit of meeting a higher physical-activity target. Conclusion: The results of our analysis, designed specifically to address the key limitations of previous evidence, underscore the benefits of long-term adherence to the recommended ≥150 minutes of moderate-to-vigorous physical activity per week for obesity prevention in middle-aged women. © The Author(s) 2026. Published by Oxford University Press on behalf of the International Epidemiological Association. All rights reserved. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site—for further information please contact journals.permissions@oup.com. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/pages/standard-publication-reuse-rights)