The threshold effect of a body shape index on phenotypic age acceleration in U.S. adults: Insights from NHANES 1999 to 2010

This study examines the relationship between a body shape index (ABSI) and accelerated aging in a nationally representative U.S. population. Data from the 1999 to 2010 National Health and Nutrition Examination Survey were analyzed. ABSI numerically equals to waist circumference divided by the square of height and two-thirds power of body mass index. Phenotypic age (PhenoAge) is a biological age estimation based on a combination of clinical markers, while phenotypic age acceleration (PhenoAgeAccel) is calculated as the difference between 1's PhenoAge and chronological age. Multivariable linear regression models were used to assess the association between ABSI and PhenoAgeAccel. Smoothed curve fitting and threshold effect analysis were performed to explore the nonlinear relationship between these variables. Subgroup analyses were conducted to evaluate the consistency of the association across different demographic and clinical strata. A total of 23,045 participants were included, with a mean age of 49.15 years and 49.1% male representation. After adjusting for multiple covariates, the multivariable linear regression model showed a positive correlation between ABSI and PhenoAgeAccel (β: 1.05, 95% CI: 0.77-1.34). An L-shaped relationship was observed between ABSI and PhenoAgeAccel (P for nonlinearity < .001), with an inflection point at 7.97. Below this inflection point, no significant association was observed, while above it, ABSI was positively associated with PhenoAgeAccel (β: 2.63, 95% CI: 2.28-2.97, P < .001). Subgroup analyses indicated that the association between ABSI and PhenoAgeAccel was more pronounced in individuals aged 20 to 39 years compared with those aged ≥ 40 years, in men compared with women, and in those without hypertension or diabetes compared with those with these conditions. This study demonstrates a positive L-shaped correlation between ABSI and PhenoAgeAccel in U.S. adults.