A study has found that the risk of Alzheimer's disease decreases in a step-wise manner as the duration of smoking cessation increases.

A research team from the Department of Occupational and Environmental Medicine at Kyung Hee University Medical Center announced these findings after tracking 1,403,636 adults for an average of 10.5 years, based on data from the National Health Insurance Service collected between 2002 and 2023.

Alzheimer's disease is the most common form of dementia, accounting for approximately 80% of all cases.

The risk of developing the disease increases with age, and it is a primary cause of senile dementia.

The research team categorized participants into non-smokers, sustained quitters, and current smokers based on smoking status repeatedly confirmed during health checkups.

Individuals who had temporarily quit but resumed smoking, or those with irregular smoking patterns, were excluded from the sustained quitter group.

The analysis showed that a total of 58,519 new cases of Alzheimer's disease occurred during the follow-up period, and the risk of Alzheimer's for current smokers was 24.6% higher than that for non-smokers.

Smoking cessation showed a clear pattern of reducing the risk of Alzheimer's disease as the period of abstinence lengthened.

By duration of cessation, individuals who had quit for less than 2 years had a 10.1% lower risk of Alzheimer's compared to current smokers.

Subsequently, the risk decreased by 23.4% for the 2–3 year cessation group, 17.6% for the 4–5 year group, and 29.0% for the 6–7 year group.

For those who maintained long-term smoking cessation for 8 years or more, the risk of Alzheimer's disease dropped by as much as 41.8%.

The research team analyzed that these results go beyond a simple statistical correlation and are deeply related to the biological damage smoking inflicts on the brain.

Previous research indicates that smoking increases oxidative stress in the body, triggers chronic inflammatory responses, and causes deterioration in cerebrovascular function and damage to the blood-brain barrier.

These changes are known to promote the accumulation of amyloid-beta and abnormalities in tau protein, which are the core pathologies of Alzheimer's disease.

Conversely, the research team interprets that quitting smoking may gradually alleviate such damage, potentially leading to a slow reduction in the risk of Alzheimer's disease.

In particular, the team noted that signs of a reduced risk of Alzheimer's appeared immediately after quitting smoking.

In fact, while individuals who had quit for less than 2 years still had a 13.6% higher risk of Alzheimer's compared to lifetime non-smokers, their risk was lower than that of current smokers.

When smoking cessation lasted for 2 years or more, the risk level showed an effect approaching that of non-smokers.

This is interpreted to mean that while some cerebrovascular function and inflammatory responses begin to recover immediately after quitting, a certain amount of time is required for the neurodegenerative damage accumulated through smoking to be sufficiently mitigated.

The research team attached significance to the fact that, unlike previous studies, this research analyzed Alzheimer's disease separately rather than all types of dementia combined.

Most previous studies have analyzed total dementia, which combines vascular dementia and Alzheimer's disease. The research team noted that since smoking causes both vascular damage and neurodegeneration, it is necessary to distinguish between the differences in each disease.

The research team stated, "Since Alzheimer's disease is a condition where pathological changes progress slowly for decades before onset, the effects of smoking cessation are more pronounced when maintained over the long term rather than the short term," adding, "The most distinct reduction in risk confirmed in the group that quit for 8 years or more proves this."

The results of this study were published in the latest issue of the international journal, Alzheimer's Research & Therapy.

(Photo: Yonhap News)