Smoking increases the risk of lung cancer, heart disease, stroke, emphysema and infertility and now, a new study says it could also increase the risk of developing Alzheimer's disease.

Heavy smoking at least two packs a day in middle age increases the risk of later developing Alzheimer's disease by 157 percent, and vascular dementia by 172 percent, according to the study. Vascular dementia is dementia caused by reduction or loss of blood supply to the brain.

The findings add to previous work linking smoking and development of dementia. However, this study is different in that the researchers followed a large group of study participants over a long period of time which was necessary for finding a link to an age-related disease like Alzheimer's , said study researcher Rachel Whitmer, a research scientist at the Kaiser Permanente Division of Research in California.

"You need a study of a large group of people with several decades of follow-up, and enough smokers who make it to an age where they are at risk for dementia," Whitmer told MyHealthNewsDaily.

Researchers still aren't completely sure why smoking affects brain function. But smoking is known to cause inflammation and oxidative stress cell damage from toxic free radicals in the body conditions that are associated with the onset of Alzheimer's, she said.

"If you are an elderly person who smokes, and you're lucky enough to not have cardiovascular disease, respiratory disease or cancer, you're still at a greater risk of dementia throughout late life," Whitmer said.

The new study was published online today (Oct. 25) in the journal Archives of Internal Medicine.

Up in smoke

In the study, 21,123 California residents ages 50 to 60 participated in a health survey from 1978 to 1985, in which they indicated whether they had never never smoked, or were former or current smokers.

Researchers followed up with them about 23 years later, between 1994 and 2008, and found that 5,367 of the people, or 25.4 percent, had been diagnosed with dementia. Of those , 1,136 were diagnosed with Alzheimer's disease and 416 were diagnosed with vascular dementia.

Researchers found that people who smoked more than two packs of cigarettes a day had a higher risk of developing either type of dementia than nonsmokers.

Former smokers, and people who smoked less than half of a pack a day, did not seem to have an increased risk of dementia than nonsmokers, the study said.

Conflicting evidence

Past studies have found a link between smoking and dementia. However, not all of them report the same relationship.

A 2003 study of 218 middle-aged Japanese-American men in the journal Neurobiology of Aging found the more cigarettes a man smoked, the greater his risk of developing Alzheimer's disease .

A 10-year study of 1,436 elderly Taiwanese people, for example, found past and current smokers were one-third less likely to be cognitively impaired than people who had never smoked. That finding, published in September in the journal Archives of Gerontology and Geriatrics, suggested smoking is a protective factor in cognitive function.

That seemingly protective effect may come from nicotine's action as a stimulant, prompting smokers to perform better on cognitive tests, Whitmer said.

However, that effect was not found in the new study, and was not found among long-term smokers with Alzheimer's and dementia particularly, she said.

Earlier studies linking smoking with a decreased risk in Alzheimer's suffered from flaws in their methodology, said Suzanne Tyas, an epidemiologist at the University of Waterloo in Canada, who was not associated with the new findings.

People with Alzheimer's disease can't report their own smoking history in studies, whereas people without Alzheimer's can, she said. And when asking the daughter or son of someone with Alzheimer's to report their father's or mother's history, they may not be aware that their parents used to smoke.

The fact that Alzheimer's is an age-related disease provides an inherent flaw in studies trying to compare older smokers with and without the disease, she said.

"Smokers die earlier, and Alzheimer's is age-related, so there's a survival bias," Tyas told MyHealthNewsDaily.

Tyas was one of the lead researchers in a 2003 study of 218 middle-aged Japanese-American men in the journal Neurobiology of Aging, which found the more cigarettes a man smoked, the greater his risk of developing Alzheimer's disease.