Physical activity may benefit oral health, but causal evidence is lacking. This study estimated the effect of modifying physical activity on self-rated oral health and masticatory function impairment in older adults using longitudinal modified treatment policies.

Data from three waves (2016–2024) of THE SIGNS Study, a nationally representative cohort of community-dwelling adults aged 60 years and above in Singapore ( N = 3981), were analysed. Ten physical activity scenarios were emulated, including upward, downward, and stochastic shifts in weekly metabolic equivalent (MET) minutes (WHO guideline: ≥600MET-min/week). Causal parameters were estimated using targeted maximum likelihood estimation with Super Learner.

For poor self-rated oral health, raising all participants to at least 450 MET-min/week (equivalent to 112.5 min per week of moderate-intensity physical activity) produced the largest risk reduction (RR: 0.88, 95% CI: 0.82–0.95). Even raising participants below 150 MET-min/week toward 150 MET-min/week reduced risk by 7% (RR: 0.93, 95% CI: 0.88-0.99). For masticatory function impairment, reducing physical activity increased risk in a dose-response manner: eliminating all activity increased risk by 21% (RR: 1.21, 95% CI: 1.10-1.33), while raising 50% of inactive participants toward 600 MET-min/week reduced risk by 6% (RR: 0.94, 95% CI: 0.90–0.99). A stricter masticatory function impairment definition produced consistent but stronger effects. Estimates were robust across estimators and under extreme attrition assumptions. Increasing physical activity may reduce the risk of poor self-rated oral health, while reducing physical activity may increase masticatory function impairment.

These findings support integrating physical activity promotion within oral health strategies for ageing populations.