Association between usual protein intake and muscle function in older U.S. adults: a target-trial emulation using NHANES 2011–2018

BACKGROUND: The link between long-term protein intake and muscle performance in older adults has been hard to define, partly because most studies rely on short dietary windows and are vulnerable to confounding and measurement noise. In this work, we attempted to estimate the usual protein intake and functional limitation among U.S. adults aged ≥ 60 years using a target-trial emulation framework with overlap weighting and semiparametric estimators. METHODS: Data were drawn from four NHANES survey cycles (2011-2018), including 5,736 adults aged ≥ 60 years with complete exposure, outcome, and covariate data. Usual protein intake (g/kg/day) was derived from available 24-hour recalls to approximate habitual intake. The primary outcome was PFQ-defined mobility limitation across cycles; grip strength (2011-2014) was analyzed separately as a secondary outcome. Causal contrasts across predefined intake categories (<0.8, 0.8- < 1.0, 1.0- < 1.2, ≥1.2 g/kg/day) were evaluated using covariate-balancing propensity score overlap weighting (ATO estimand) followed by marginal structural models. Doubly robust sensitivity analyses were conducted using augmented inverse probability weighting and targeted maximum likelihood estimation with generalized linear models. Simulation extrapolation (SIMEX) was applied to assess potential bias from dietary measurement error. Exploratory analyses evaluated hs-CRP as a potential mediator and tested effect modification by vitamin D status and physical activity. RESULTS: Mean usual protein intake was 0.93 g/kg/day, and approximately 42% of participants consumed at least 0.8 g/kg/day, the current Recommended Dietary Allowance (RDA) for the general adult population. In the prespecified overlap-weighted marginal structural model (ATO estimand), higher intake was associated with lower odds of mobility limitation, although the primary contrast comparing ≥ 1.2 versus < 0.8 g/kg/day was modest and not statistically significant (OR 0.89, 95% CI 0.54-1.47). A doubly robust binary contrast yielded a -6.6 percentage-point difference in predicted limitation (95% CI -25.8 to 12.7), consistent in direction but imprecise. In cycle-specific analyses, the inverse association was more pronounced in 2015-2018 (OR 0.80, 95% CI 0.65-0.98). Spline models suggested a steeper decline in predicted limitation below approximately 1.0-1.1 g/kg/day, with a flatter trajectory at higher intakes. Exploratory mediation models indicated a potential indirect component through hs-CRP, though these estimates were not overlap-weighted and should be interpreted cautiously. CONCLUSIONS: Higher usual protein intake was directionally associated with lower odds of mobility limitation among older U.S. adults within a target trial emulation framework, although the primary overlap-weighted estimates were modest and imprecise. Evidence of nonlinearity suggests that intakes near 1.0-1.1 g/kg/day may mark a range where predicted limitation declines more steeply, but uncertainty increases at higher intake levels. Given the cross-sectional design and residual potential for confounding, these findings should be interpreted cautiously. Prospective studies are needed to determine whether sustained protein intake in this range meaningfully preserves functional capacity over time.