Korean researchers who analyzed National Health Insurance big data covering more than 1.4 million people have found that the longer a person stays off cigarettes, the more their risk of developing Alzheimer's disease declines in stages. The risk reduction became clear after two years of quitting, and those who maintained abstinence for eight years or more had a 41.8% lower Alzheimer's risk than current smokers.
According to Yonhap News on Thursday, Alzheimer's disease is the most common type of dementia, accounting for about 80% of all cases. It is a representative neurodegenerative disease whose risk increases with age. While smoking was known to be a risk factor, few large-scale studies had analyzed the risk-reduction effect according to the length of smoking cessation.
A research team from the Department of Occupational and Environmental Medicine at Kyung Hee University Hospital used data from the National Health Insurance Service from 2002 to 2023 to track 1,403,636 adults over an average of 10.5 years. Based on smoking status repeatedly confirmed during health checkups, the subjects were classified as non-smokers, sustained quitters, and current smokers. People who repeatedly quit and resumed smoking or whose smoking status was inconsistent were excluded from the sustained-quitter group.
During the follow-up period, a total of 58,519 people were newly diagnosed with Alzheimer's disease. The analysis confirmed that current smokers had a 24.6% higher risk of developing Alzheimer's disease than non-smokers.
By contrast, the longer the cessation period, the more gradually the risk fell. Compared with current smokers, those who had quit for less than two years had a 10.1% lower Alzheimer's risk, while the reductions were 23.4% for two to three years, 17.6% for four to five years, and 29.0% for six to seven years. For those who maintained long-term abstinence of eight years or more, the risk fell by as much as 41.8%.
The research team also noted that the benefits of quitting appeared relatively early. Those who had quit for less than two years had a 13.6% higher Alzheimer's risk than lifelong non-smokers, but a lower risk than current smokers. After the cessation period exceeded two years, the risk level tended to approach that of non-smokers.
The researchers explained that these results are related to the biological effects of smoking on the brain. Smoking increases oxidative stress and triggers chronic inflammatory responses, as well as causing cerebrovascular dysfunction and damage to the blood-brain barrier. Such changes are known to promote the accumulation of amyloid beta and abnormalities in tau protein, which are considered major pathologies of Alzheimer's disease.
Conversely, when a person quits smoking, cerebrovascular function and inflammatory responses gradually recover, potentially lowering the risk of Alzheimer's disease over time, according to the team's analysis. However, they emphasized that long-term cessation is important, as a certain period is needed for the neurodegenerative damage accumulated through smoking to be sufficiently alleviated.
This study is also significant in that it separately analyzed Alzheimer's disease rather than all dementia. Most previous studies analyzed all dementia, including vascular dementia and Alzheimer's disease, but the team explained that since smoking affects vascular damage and neurodegeneration respectively, it is necessary to examine the diseases separately.
"Alzheimer's disease is a condition in which pathological changes progress slowly for decades before onset, so the effects of quitting smoking also appear greater when maintained over the long term rather than the short term," the research team said. "The fact that the greatest risk-reduction effect was confirmed in the group that had quit for eight years or more demonstrates this."
Meanwhile, the findings were published in the latest issue of the international journal Alzheimer's Research & Therapy.
Smoking "a pack a day for more than 20 years" has a 98.2% impact on small cell lung cancer.
