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Purpose: We investigated time trends of the obesity-mortality association, accounting for age, sex, and cause-specific deaths.

Methods: We analysed pooled nationwide data in Sweden for 3,472,310 individuals aged 17–39 years at baseline in 1963–2016. Cox regression and flexible parametric survival models investigated BMI-mortality associations in sub-groups of sex and baseline calendar years (men: <1975, 1975–1985, ≥1985 and women: <1985, 1985–1994, ≥1995).

Results: Comparing men with obesity vs. normal weight, all-cause and “other-cause” mortality associations decreased over periods; HR (95% CI) 1.92 (1.83–2.01) and 1.70 (1.58–1.82) for all-cause and 1.72 (1.58–1.87) and 1.40 (1.28–1.53) for “other-cause” mortality in <1975 and ≥1985, but increased for CVD mortality; HR 2.71 (2.51–2.94) and 3.91 (3.37–4.53). Higher age at death before 1975 coincided with more obesity-related deaths at higher ages. Furthermore, the all-cause mortality association for different ages in men showed no clear differences between periods (p-interaction=0.09), suggesting no calendar effect after accounting for attained age. Similar, but less pronounced, results were observed in women. Associations with cancer mortality showed no clear trends in men or in women.

Conclusions: Accounting for differences in age and death causes between calendar periods when investigating BMI-mortality time trends may avoid misinterpreting the risks associated with obesity over time.