Association between Longitudinal Changes in Body Fat Percentage across Adulthood and Mortality: A Prospective Study

Our study aimed to investigate the associations between body fat percentage (BF%) at different life stages, patterns of BF% change, trajectory groups across adulthood, and subsequent mortality risk. Using data from the National Health and Nutrition Examination Survey 1999-2018, we analyzed 25,620 participants (50% male, mean age 59.75 ± 12.50 years). BF% was calculated using the CUN-BAE formula for three life stages: young adulthood (age 25), middle age (ten years prior to baseline), and late adulthood (baseline). Group trajectory models were applied to identify patterns of BF% changes over time. Cox proportional hazards models assessed associations between BF% at different life stages, patterns of BF% change, trajectory groups, and mortality risk. Restricted cubic spline analyses were used to examine the dose–response relationship between BF% and mortality. During a median follow-up of 9.36 years, 5,694 deaths occurred. The relationship between BF% and all-cause mortality varied by life stage: positive linear association in young adulthood, J-shaped in middle age, and inverse J-shaped in late adulthood. Compared with participants who maintained normal BF%, those with persistent obesity from young to middle adulthood showed a 32% increased risk of all-cause mortality (hazard ratio 1.32, 95% confidence interval 1.16-1.49), while the same obesity persistence from middle to late adulthood was associated with a 16% risk reduction (0.84, 0.76-0.93). In middle to late adulthood, both transitioning from non-obesity to obesity (0.77, 0.70-0.86) and maintaining obese (0.84, 0.76-0.93) were associated with reduced all-cause mortality, whereas transitioning from obesity to non-obesity (1.19, 1.05-1.36) was linked to increased risk of all-cause mortality. Maintaining the highest overweight category throughout adulthood consistently reduced risk of all-cause mortality across all time intervals. Latent class trajectory modeling identified four distinct BF% patterns: low-slow increase-stable (reference), lower-slow increase-stable (1.07, 1.00-1.15), high-rapid increase-stable (1.12, 1.01-1.24), and very high-rapid increase-decrease (1.52, 1.30-1.77). Similar patterns were observed for cardiovascular mortality, with no significant associations for cancer mortality. Our results suggest that maintaining a lower BF% throughout adulthood, particularly in early adulthood, is crucial for reducing mortality.