Cumulative Internet Use, Depression, and Incident Dementia in the United States, 2002–2021: Mediation Analysis and Population Health Projection

ABSTRACT Background Internet use is linked to cognitive health, yet long-term US evidence remains scarce. Whether cumulative Internet use relates to incident dementia, the extent to which depressive symptoms mediate this, and the associated population health projections, including preventable cases and averted care costs, remain uncertain. Methods We included 7,068 US Health and Retirement Study participants aged ≥50 years. Cumulative Internet use was assessed across 2002–2010, and depressive symptoms in 2012. Participants were followed for incident dementia between 2014 and 2021. Cox proportional hazards models, causal mediation analyses, and Potential Impact Fractions were used to assess risk, mediation, and project preventable cases and averted care costs. Results Greater cumulative Internet use was associated with lower dementia risk. Compared with never users, hazard ratios were 0.85 (95% CI: 0.66, 1.10) for 1 time point, 0.71 (95% CI: 0.51, 0.99) for 2, and 0.60 (95% CI: 0.48, 0.74) for ≥3. Mediation analyses suggested a modest potential role for depressive symptoms. Hypothetical projections indicate increasing Internet use could prevent 0.29–0.92 million US dementia cases during 2014–2021, averting US$9.94–$31.63 billion in related care costs. Conclusions In US middle-aged and older adults, more frequent Internet use is associated with a lower dementia hazard, while mediation analyses suggested a modest potential role for depressive symptoms. These findings highlight digital engagement as a target for healthy cognitive aging, while underscoring the need for causal and intervention research.