Smoking in Midlife and the Long-Term Risk of Dementia

It is well known that smoking increases the risk of most diseases and death, but some studies suggest that smoking is also associated with a lower risk of certain neurodegenerative diseases, including Parkinson’s disease. Other studies have found a link between smoking and the risk of Alzheimer’s disease (AD), the most common form of dementia; some findings suggest that smoking reduces the risk of cognitive impairment. According to researchers, it is important to identify exposure to putative risk factors in middle age before pathologic features of AD have started to develop in the brain; however, there are few population-based cohort studies designed to study the relationship between smoking in early life and the development of dementia in later life. Both smoking and AD rates vary by race; some studies suggest that the prevalence of both dementia and smoking is higher among African Americans, creating the need for evaluating smoking and the risk of dementia in large, ethnically diverse populations. Researchers recently conducted a study to investigate the association between the amount of midlife smoking and the long-term risk of all-cause dementia, AD, and vascular dementia (VaD) 2 to 3 decades later. They reported study results in Archives of Internal Medicine [2011;171(4):333-339]. The study included a large multiethnic population of 33,108 members of a Kaiser Permanente Medical Care Program of Southern California who participated in a survey (Multiphasic Health Checkup) between 1978 and 1985 when they were 50 to 60 years of age. The current analysis cohort comprised 21,123 people who were still living and members of the health plan in 1994, when electronic medical record diagnoses of dementia became available. Diagnoses of dementia, AD, and VaD made in internal medicine, neurology, and neuropsychology were collected from January 1, 1994, to July 21, 2008. During a mean follow-up period of 23 years, 25.4% of the cohort (n=5367) were diagnosed as having dementia (including 1136 with AD and 416 with VaD). Results were adjusted for age, sex, education, race, marital status, hypertension, hyperlipidemia, body mass index (BMI), diabetes, heart disease, stroke, and alcohol use. Compared with those without dementia, those who were diagnosed with dementia were older, had fewer years of education, and were more likely to be women. Compared with whites, African Americans were more likely to have a diagnosis of dementia; Asians were less likely to have dementia. A higher percentage of divorced, widowed, or separated people and a higher percentage of never drinkers were among those with a diagnosis of dementia. Dementia was also associated with a greater likelihood of a higher mean midlife BMI and all comorbidities. There was no linear increase in the incidence rates and age-adjusted incidence rates of dementia by smoking levels; however, there was a “dramatic” increase in the incidence of dementia among participants who reported smoking ≥2 packs a day at midlife. The calculated incidence rate of dementia for those participants was 312.21 (95% confidence interval [CI], 235.11-389.30); the age-adjusted incidence rate was 786.42 (95% CI, 481.23-1091.61) per 10,000 person-years. Compared with those who never smoked, those who smoked >2 packs a day had an elevated risk of dementia (adjusted hazard ratio [HR], 2.14; 95% CI, 1.65-2.78), AD (adjusted HR, 2.57; 95% CI, 1.63-4.03), and VaD (adjusted HR, 2.72; 95% CI, 1.20-6.18). In summary, the researchers said, “in this large cohort, heavy smoking in midlife was associated with a >100% increase in risk of dementia, AD, and VaD >2 decades later. These results suggest that the brain is not immune to long-term consequences of heavy smoking.”