Daily smoking linked to brain shrinkage in massive study

Daily cigarette smoking reduces the size of your brain, a new study of more than 28,000 people suggests.

Scientists have previously found that people who smoke tend to have smaller brains compared to non-smokers, in terms of volume, but it was unclear whether smoking causes the brain to shrink or if people with smaller brains are more likely to start smoking. Now, researchers provide strong evidence that smoking causally shrinks the brain in a new report, which was posted April 28 on the preprint database medRxiv and has yet to be peer-reviewed.

"This is a very important study," Dajiang Liu, who studies the genetics of smoking risk at the Penn State College of Medicine and was not involved in the study, told Live Science in an email. "The work is rigorously conducted and the result is important from a public health perspective."

The scientists analyzed brain imaging data from the U.K. Biobank, a massive repository of genetic and health data from U.K.-based participants. In addition to brain scans, the team analyzed participants' self-reported smoking habits, as collected in surveys. Participants took these surveys twice, once between 2006 to 2010 and again between 2012 and 2013. In the second time window, participants' brains were also imaged using a method called magnetic resonance imaging (MRI).

Related: Quitting smoking by age 35 brings your risk of death in line with 'never smokers'

The scientists found that, compared to people who'd never smoked daily, participants who smoked on a daily basis at some point prior to having their brains imaged had brain volumes that were 0.4 cubic inches (7.1 cubic centimeters) smaller, on average. 

This difference in brain volume included a 0.3 cubic inch (5.5 cc) decrease in the gray matter of the brain, which contains the bulky bodies of brain cells, or neurons. Smoking daily at some point in the past was also linked to a 0.1 cubic inch (1.6 cc) decrease in the white matter of the brain, which includes the long insulated wires that connect neurons to each other.

Next, the researchers found that, among the past daily smokers, participants who smoked more heavily showed even larger differences in gray matter volume. Each additional "pack year" smoked — a measure equivalent to smoking one pack a day for one year — was linked to a roughly 0.01-cubic-inch (0.15 cc) decrease in gray matter volume, on average. This "dose-response" relationship supports the idea that smoking causally reduces brain volumes, wrote the researchers in their paper. 

In contrast, how heavily people smoked did not significantly impact their white matter volume.

Further analyses revealed that people who had quit smoking for longer had slightly more gray matter in their brains, compared with those who quit more recently. This suggests that stopping smoking can slightly reverse the decline in brain volume. For example, quitting smoking an extra year earlier was linked to a further 0.005 cubic inch (0.09 cc) increase in gray matter volume among the past daily smokers.

The scientists also examined participants' genetic data to see if gene variants that influence smoking risk might be linked to differences in gray matter volume. They found that people with a higher genetic risk were more likely to have smoked in the past, but their genetics, in isolation, were not linked to gray matter volume. Instead, a history of daily smoking was strongly tied to gray matter volume, supporting the idea that smoking drives the changes.    

As brain shrinkage has been associated with neurological diseases, such as Alzheimer’s disease, establishing a causal link between smoking and brain volume decline furthers our understanding of whether smoking directly drives these diseases through this mechanism, wrote Liu. Further experiments will help to confirm the causal relationship between smoking and brain size, and the effectiveness of drugs that could potentially help prevent brain tissue loss, he added.