Association of Long-term Exposure to Air Pollution and Dementia Risk: The Role of Homocysteine, Methionine, and Cardiovascular Burden

BACKGROUND AND OBJECTIVES: Growing evidence links air pollution with dementia risk, but the biological mechanisms are largely unknown. We investigated the role played by homocysteine (tHcy) and methionine in this association and explored whether this could be explained by cardiovascular diseases (CVDs). METHODS: Data were extracted from the ongoing Swedish National study on Aging and Care in Kungsholmen (SNAC-K), a longitudinal population-based study. At baseline, 2,512 dementia-free participants were examined up to 2013 (mean follow-up: 5.18 ± 2.96 years). Two air pollutants (particulate matter ≤2.5 μm [PM(2.5)] and nitrogen oxides [NO(x)]) were assessed yearly from 1990 until 2013 using dispersion models at residential addresses. The hazard ratio of dementia over air pollution levels was estimated using Cox models adjusted for age, sex, education, smoking, socioeconomic status, physical activity, retirement age, creatinine, year of assessment, and the use of supplements. The total effect of air pollutants on dementia was decomposed into 4 pathways involving tHcy/methionine: (1) direct effect; (2) indirect effect (mediation); (3) effect due to interaction; and (4) effect due to both mediation and interaction. To test whether the association was independent from CVDs (ischemic heart disease, atrial fibrillation, heart failure, and stroke), we repeated the analyses excluding those individuals who developed CVDs. RESULTS: The mean age of the study participants was 73.4 years (SD: 10.4), and 62.1% were female individuals. During an average period of 5 years (mean: 5.18; SD: 2.96 years), 376 cases with incident dementia were identified. There was a 70% increased hazard of dementia per unit increase of PM(2.5) during the 5 years before baseline (hazard ratio [HR]: 1.71; 95% CI 1.33–2.09). Overall, 50% (51.6%; 95% CI 9.0–94.1) of the total effect of PM(2.5) on dementia was due to mediation of tHcy (6.6%; 95% CI 1.6–11.6) and/or interaction (47.8%; 95% CI 4.9–91.7) with tHcy and 48.4% (p = 0.03) to the direct effect of PM(2.5) on dementia. High levels of methionine reduced the dementia hazard linked to PM(2.5) by 31% (HR: 0.69; 95% CI 0.56–0.85) with 24.8% attributable to the interaction with methionine and 25.9% (p = 0.001) to the direct effect of PM(2.5). No mediation effect was found through methionine. Attenuated results were obtained for NO(x). Findings for tHcy were attenuated after excluding those who developed CVDs, while remained similar for methionine. DISCUSSION: High levels of homocysteine enhanced the dementia risk attributed to air pollution, while high methionine concentrations reduced this risk. The impact of homocysteine on cardiovascular conditions partly explains this association. Alternative pathways other than cardiovascular mechanisms may be at play between methionine and dementia.