The results of studies on the association between cigarette smoking and Alzheimer’s disease have been contradictory through the years.
Earlier case-control studies suggested an inverse relationship between smoking and Alzheimer’s disease, i.e. that smoking seemed to have a protective effect in regard to Alzheimer’s disease. A Canadian study, on the other hand, followed over 6 thousand participants, 65 years or older, for 5 years and found no significant association between smoking and Alzheimer’s disease.
An Australian study, reviewed 19 prospective studies that followed its 26 thousand participants for 2 to 30 years. The results showed that smokers had a considerably increased risk of developing Alzheimer’s disease.
Why the inconsistent results?
Several explanations for these apparent inconsistencies have been put forward. The results showing an almost therapeutic effect of smoking on Alzheimer’s disease might be the result of bias due to poorer survival of smokers. When scientists compare groups of elderly smokers and non-smokers, the mortality rates in groups of smokers are higher than in groups of non-smokers, resulting in skewed samples. Smokers are likely to have been under-represented in these studies and smokers who survive to an old age might be a more selected (i.e., healthy), group than their non-smoking peers.
The ApoE gene
There are other reasons too. The absence or presence of the ApoE4 gene seems to make a big difference when looking at the effects of smoking on the risk of Alzheimer’s disease. People who carry the ApoE4 gene have an increased risk of developing Alzheimer’s disease. ApoE4 has therefore been called a risk-factor gene as it increases considerably the risk of developing Alzheimer’s disease. Several studies have found that smokers have an increased risk of Alzheimer’s disease only if they are not carrying the ApoE4 gene. For a review of possible explanations for this resistance to the effects of smoking among the ApoE4 carriers, see here.
So if you are a smoker not carrying the ApoE4 gene and would like to decrease your risk of Alzheimer’s disease – Stop smoking!
Here is a wonderful anti-smoke campaign advertisement from YouTube:
-Anstey, K. J., von Sanden, C., Salim, A. og O’Kearney, R. (2007). Smoking as a risk factor for dementia and cognitive decline: A meta-analysis of prospective studies. American Journal of Epidemiology, 166(4), 367-378.
-Riggs, J. E. (1993). Smoking and Alzheimer’s disease: Protective effect or differential survival bias? Lancet, 342(8874), 793.
–Séverine, S., Mika, K., Meena, K., Martin, S., Archana, S-M. (2010). Effect of Apolipoprotein E ε4 on the association between health behaviors and cognitive function in late midlife. Molecular Neurodegeneration, 5(1).