Is there an association between smoking and a reduced risk of developing dementia and Parkinson's disease?

From the Guidelines

Smokers do not have a reduced risk of developing dementia and Parkinson's disease, and in fact, smoking is associated with an increased risk of vascular dementia and Alzheimer's disease. While some studies may suggest a complex relationship between smoking and these conditions, the majority of evidence indicates that smoking causes significant damage to blood vessels and increases inflammation, which can accelerate cognitive decline and contribute to dementia 1. The toxic chemicals in cigarette smoke cause oxidative stress and damage to brain cells, counteracting any potential benefits from nicotine.

Key Points to Consider

• Smoking is a major risk factor for numerous health problems, including cancer, heart disease, and stroke, which far outweigh any theoretical neuroprotective effects 1.
• The apparent protective effect of smoking for Parkinson's disease is likely due to selection bias or the pharmacological effects of nicotine on dopamine systems, not because smoking is beneficial.
• Dementia is a significant public health concern, with over 50 million people living with the condition globally, and this figure is projected to increase to 152 million by 2050 1.
• Established protective factors for brain health include regular exercise, a healthy diet, mental stimulation, and controlling cardiovascular risk factors.

Recommendations for Patients

• Avoid smoking completely to reduce the risk of developing dementia and other smoking-related health problems.
• Focus on established protective factors for brain health, such as regular exercise, a healthy diet, mental stimulation, and controlling cardiovascular risk factors.
• Patients with a history of smoking should be advised to quit smoking and offered smoking cessation interventions to reduce the risks of atherosclerosis progression and stroke 1.

From the Research

Association between Smoking and Neurodegenerative Diseases

• There is evidence to suggest an association between smoking and a reduced risk of developing Parkinson's disease (PD) 2, 3, 4, 5.
• Studies have found that cigarette smoking is associated with decreased risk of PD, with some suggesting a neuroprotective effect of smoking on PD 2, 3, 4.
• The relationship between smoking and Alzheimer's disease (AD) is less clear, with some studies finding an inconsistent association 2, 3.
• One study found that heavy lifetime cigarette smoking was associated with significantly reduced relative risk for Lewy body accumulation, a hallmark of PD, but not AD-type pathologic changes 2.

Potential Mechanisms

• Nicotine has been proposed as a potential molecule that may mediate the protective effect of smoking on PD, but randomized controlled clinical trials have failed to demonstrate benefit on motor endpoints 4.
• Other molecules and biochemical cascades triggered by smoking, such as monoamine oxidase inhibitors, gut microbiome disruption, and antioxidant response induction, may also play a role in the potential protective effect of smoking on PD 4.

Vascular Risk Factors and Cognition

• Vascular risk factors, including smoking, have been associated with cognitive deficits and incident dementia in the general population, but their role in cognitive dysfunction in PD is still unclear 6.
• One study found that heart disease was the only vascular factor significantly more prevalent in PD patients with dementia compared to those with normal cognition 6.