For several years, smokers could point to a faint silver lining to their vice: studies suggesting a lower risk of developing Alzheimer's. But now that appears to be a false consolation. After tracking the health of several thousand people, scientists report in tomorrow's Lancet that smokers are twice as likely to develop Alzheimer's as people who have never lighted up. The study also found that smokers show signs of the disease up to 8 years earlier than lifelong nonsmokers.
Because past studies have focused on the smoking habits of people who already had dementia, they failed to take into account that smokers tend to die earlier than nonsmokers, says Monique Breteler from Erasmus Medical School, The Netherlands. Thus people who otherwise would have developed dementia will die before exhibiting symptoms. To avoid this confounding influence, Breteler and her colleagues followed 6780 people living in a Rotterdam suburb, all of whom were 55 or older and did not have dementia when they agreed to participate.
After 2 years, smokers were twice as likely to be diagnosed with Alzheimer's compared to lifelong nonsmokers. Former smokers had a 30% increased risk of developing the disease. Curiously, the researchers found, smokers who carry the APOE4 gene--a risk factor for Alzheimer's--were just as likely as nonsmokers to get the disease, while those lacking the gene had a fourfold increased risk. While that finding is surprising, says Breteler, it may be explained by the fact that APOE4 also increases the risk of cardiovascular disease--thus smokers with the gene may be dying off earlier.
"It is really important to see this result" from a prospective study, says Jean-Françoise Dartigues of the University of Bordeaux, France, who is heading similar work. He agrees that the APOE4 effect could have arisen from a selection bias. "I will be very interested to see whether other studies confirm the APOE4 finding," adds Breteler, whose group is continuing its study.