Longitudinal associations of habitual and within-person protein intake with BMI in Chinese adults

China has undergone rapid nutrition and urbanization transitions, yet the longitudinal association between dietary protein composition and body mass index (BMI) remains unclear. The analysis included 62,980 observations from 22,634 adults participating in seven China Health and Nutrition Survey waves (1993–2011). Three-level nutrient-density models distinguished habitual between-person intake from within-person deviations. BMI was 0.361 kg/m 2 higher for each 5-percentage-point increase in habitual protein share of energy (95% CI 0.275, 0.447; p < 0.001), while the within-person estimate was not statistically significant in the primary nutrient-density model ( β = 0.028, 95% CI − 0.003, 0.059; p = 0.090). The between-person association was stronger in men ( β = 0.618) than women ( β = 0.148; interaction p < 0.001) and varied across provinces and long-term urbanization contexts (both p < 0.001). Alternative nutrient-density and energy-residual models retained the positive between-person association. Within-person estimates remained non-significant in the primary and alternative nutrient-density models, whereas the small association observed in the energy-residual analysis was sensitive to the inclusion of gender interaction terms. Additional adjustment for physical activity and leisure sedentary time attenuated gender-specific estimates by 5.8% in women and 1.3% in men. Overall, the protein–BMI association was most evident for habitual between-person differences in protein intake and varied by gender and geographical context. These findings support obesity-prevention strategies focused on sustained dietary quality and population-specific contexts rather than uniform protein restriction.