Scientists at the University of California San Francisco say a majority of published studies analyzing the relationship between Alzheimer's disease and smoking indicates that the habit is a significant risk factor for the disease. Researchers also found that studies funded by the tobacco industry tended to conclude that smoking protects against the development of AD, while independent studies showed that smoking increased the risk of developing the disease. The findings were published online today in the January issue of the Journal of Alzheimer's Disease.

The UCSF team reviewed 43 published studies from 1984 to 2007, and authors of one-fourth of the studies had an affiliation with the tobacco industry. The average risk of a smoker developing AD, based on studies without tobacco industry affiliation, was estimated to be 1.72, meaning that smoking nearly doubled the risk of AD.

In contrast, the team found that studies authored by individuals with tobacco industry affiliations, showed a risk factor of .86 (less than one), suggesting that smoking protects against AD. When all studies were considered together, the risk factor for developing AD from smoking was essentially neutral at a statistically insignificant 1.05.

Previous reviews of the association between smoking and AD have not controlled for study design and author affiliation with the tobacco industry, according to Cataldo. To determine if study authors had connections to the tobacco industry, the UCSF team analyzed 877 previously secret tobacco industry documents.

Researchers used an inclusive definition of "tobacco industry affiliation" and examined authors' current or past funding, employment, paid consultation, and collaboration or co-authorship on a study with someone who had current or previous tobacco industry funding within 10 years of publication.

"We know that industry-sponsored research is more likely to reach conclusions favorable to the sponsor," said Stanton A. Glantz, PhD, of the UCSF Department of Medicine and a study co-author. "Our findings point to the ongoing corrosive nature of tobacco industry funding and point to the need for academic institutions to decline tobacco industry funding to protect the research process."

Citation: Janine K. Cataldo, Judith J. Prochaska, Stanton A. Glantz, 'Cigarette Smoking is a Risk Factor for Alzheimer's Disease: An Analysis Controlling for Tobacco Industry Affiliation', Journal of Alzheimer's Disease, November 2010, 19(2); doi:10.3233/JAD-2010-1240