Most people know smoking can damage their heart and lungs, but the brain can also suffer from cigarettes, with smoking linked to an increased risk of cognitive decline. Now there's hope: quitting appears to help protect memory and thinking skills as we age.

That's what a study published recently in the journal Neurology found: older adults who quit smoking are less likely to develop dementia than those who continue to smoke. The research also revealed an important nuance: how much weight a person gains after quitting seems to affect some of that protective effect.

The longer participants remained smoke-free, the greater the apparent benefit. After roughly seven years, dementia risk among former smokers approached that of people who had never smoked.

Health data from nearly 33,000 middle-aged and older adults participating in a nationally representative study of aging in the United States, the long-running Health and Retirement Study, were analyzed. At its start, none of the people in the study had dementia; their average age was 61; and about 20 percent were current smokers, 36 percent were former smokers, and 41 percent had never smoked.

To track changes over time, participants were interviewed every two years about their smoking habits, body weight and overall health. Researchers from Zhejiang University School of Medicine in Hangzhou, China also used memory and thinking tests to assess their cognitive function and gathered information from people familiar with the participants' daily functioning and behavior.

The investigators followed participants for years, ultimately documenting 5,868 cases of dementia.

After accounting for factors that can influence dementia risk, including age, physical activity, cardiovascular health, heart disease and stroke, the researchers found that people who quit smoking had a 15 percent lower risk of dementia than those who continued smoking.

Even more encouraging, the longer participants remained smoke-free, the greater the apparent benefit. After roughly seven years, dementia risk among former smokers approached that of people who had never smoked.

The findings add to previous evidence linking smoking to cognitive decline and dementia. Scientists believe smoking may contribute to damage in blood vessels that supply the brain with oxygen and nutrients. Smoking is also associated with chronic inflammation, which has increasingly been implicated in neurodegenerative diseases such as Alzheimer's disease.

Some research suggests smoking may even accelerate the build-up of beta-amyloid and tau, proteins that form the hallmark plaques and tangles found in Alzheimer's disease.

“What we found is that quitting is still associated with better brain outcomes, but maintaining your weight may help preserve those benefits.”

The most interesting finding may be what happened after participants quit. Weight gain is a common concern among people trying to stop smoking. Researchers reported that those who gained little or moderate amounts of weight (up to about 11 pounds), continued to experience lower dementia risk and slower cognitive decline. However, participants who gained 22 pounds or more after quitting did not appear to receive the same cognitive advantage.

That doesn't mean that weight gain cancels out the many health benefits of quitting smoking. Rather the findings suggest that smoking cessation and healthy weight management may work best together.

“People often worry about what happens after they quit smoking — including weight gain and associated metabolic changes,” lead author, Hui Chen, PhD, explained in a press release. “What we found is that quitting is still associated with better brain outcomes, but maintaining your weight may help preserve those benefits.”

The researchers emphasize that this was an observational study. In other words, it found an association but cannot prove that quitting smoking directly prevents dementia. Other factors may also play a role. Smoking habits and weight were also self-reported, raising the possibility of reporting errors.

Even so, the results provide one more compelling reason to quit. While many people quit to avoid cancer, heart disease and stroke, this study suggests that giving up cigarettes may also help preserve memory and thinking abilities later in life, especially when accompanied by attention to overall health and weight management.