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Accounting for time-varying exposures and covariates in the relationship between obesity and diabetes: analysis using parametric g-formula
  1. Boyoung Park1,2,
  2. Junghyun Yoon1,
  3. Thi Xuan Mai Tran1
  1. 1 Department of Preventive Medicine, Hanyang University College of Medicine, Seoul, Korea (the Republic of)
  2. 2 Hanyang Institute of Bioscience and Biotechnology, Hanyang University, Seoul, Korea (the Republic of)
  1. Correspondence to Dr Boyoung Park; hayejine{at}hanmail.net

Abstract

Background Previous studies investigating the association between obesity and diabetes often did not consider the role of time-varying covariates affected by previous obesity status. This study quantified the association between obesity and diabetes using parametric g-formula.

Methods We included 8924 participants without diabetes from the Korean Genome and Epidemiology Study—Ansan and Ansung study(2001–2002)—with up to the seventh biennial follow-up data from 2015 to 2016. Obesity status was categorised as normal (body mass index (BMI) <23.5 kg/m2), overweight (23.5–24.9 kg/m2), obese 1 (25.0–27.4 kg/m2) and obese 2 (≥27.5 kg/m2). Hazard ratios (HRs) comparing baseline or time-varying obesity status were estimated using Cox models, whereas risk ratio (RR) was estimated using g-formula.

Results The Cox model for baseline obesity status demonstrated an increased risk of diabetes in overweight (HR 1.85; 95% CI=1.48–2.31), obese 1 (2.40; 1.97–2.93) and obese 2 (3.65; 2.98–4.47) statuses than that in normal weight status. Obesity as a time-varying exposure with time-varying covariates had HRs of 1.31 (1.07–1.60), 1.55 (1.29–1.86) and 2.58 (2.14–3.12) for overweight, obese 1 and obese 2 statuses. Parametric g-formula comparing if everyone had been in each obesity category versus normal over 15 years showed increased associations of RRs of 1.37 (1.34–1.40), 1.78 (1.76–1.80) and 2.42 (2.34–2.50).

Conclusions Higher BMI classification category was associated with increased risk of diabetes after accounting for time-varying covariates using g-formula. The results from g-formula were smaller than when considering baseline obesity status only but comparable with the results from time-varying Cox model.

  • OBESITY
  • DIABETES MELLITUS
  • EPIDEMIOLOGY

Data availability statement

Data are available upon reasonable request. The data that support the findings of this study are available on the website of the Korea Disease Control and Prevention Agency (https://is.kdca.go.kr/). We accessed the database after submitting the study protocol, the IRB approval document, and the reviewed request form from the relevant committee. More information is available from the corresponding author on request.

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Data availability statement

Data are available upon reasonable request. The data that support the findings of this study are available on the website of the Korea Disease Control and Prevention Agency (https://is.kdca.go.kr/). We accessed the database after submitting the study protocol, the IRB approval document, and the reviewed request form from the relevant committee. More information is available from the corresponding author on request.

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Footnotes

  • Contributors BP, JY and TXMT were involved in the conception, design and conduct of the study and the analysis and interpretation of the results. BP wrote the first draft of the manuscript, and all authors edited, reviewed and approved the final version of the manuscript. BP is the guarantor of this work and, as such, had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

  • Funding This work was supported by a National Research Foundation of Korea grant funded by the Korean government (MSIT) (grant no. 2021R1A2C1011958).

  • Competing interests None declared.

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.