Researchers have surprised the medical world with a study concluding that smoking does not shorten telomeres. The study suggests that telomere length in adults should be regarded as a static biomarker that does not significantly change during adult life.
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Telomeres are the nucleotide sequences found at the end of chromosomes that prevent chromosomes from deteriorating and ensure DNA replication is correctly performed.
Study author Melissa Bateson from Newcastle University and colleagues focused on smoking because more data has been published on the associations between smoking and telomere length than for any other behavior that is considered unhealthy.
Some epidemiological studies have shown an association between smoking and shorter telomeres, while others have not.
For example, in 2016, Professor Brandon Pierce from the University of Chicago and assistant graduate student Chenan Zhang decided to take a look at how smoking affects telomeres in older adults. They used a large set of longitudinal data available for more than 5,600 participants in the Health and Retirement Study who were asked about their current smoking status every two years between 1992 and 2008.
In 2008, saliva samples were taken from the participants and used to measure telomere length.
During each two-year assessment, the participants were grouped according to whether they had never smoked, were former smokers or were current smokers, with current smokers asked exactly how many cigarettes they smoked per day.
Many previous studies that had not shown any association between smoking and telomere length were cross-sectional analyses that had only compared current smokers versus never smokers, without including previous smokers who had quit before the studies.
The length of the participants’ telomeres was measured over a follow-up period of about 8.6 years using blood samples taken at baseline and at the end of the study.
After adjusting for factors such as the differences between men and women, how long they smoked, and when they chose to quit, Pierce and Zhang showed that smoking is indeed linked to shorter telomere length in both men and women.
We don't know whether telomere length is somehow a causal actor in that story of how smoking affects health. Smoking affects humans through a variety of mechanisms, and perhaps telomere shortening is one of them.
So we're not claiming that telomere length is an important mediator of smoking's effects on health. But it appears to be an important biomarker of the damage smoking can do to the human genome.”
Professor Brandan Pierce, Senior Author
Telomere length is not affected by smoking
As reported in the Royal Society journal Open Science, the current study found that while smokers do indeed have shorter telomeres, there is no evidence that telomeres become shorter more quickly in smokers than they do in non-smokers, as would be the case if smoking causes the shortening. The findings suggest that it is not smoking that causes telomeres to shorten in adult smokers.
For the study, Bateson and team performed a meta-analysis of 18 datasets available from longitudinal studies conducted across ten countries and four continents. The study was made possible by all of the authors of these studies collaborating and agreeing to make their data available for the meta-analysis. The dataset is the largest of its kind and combines information available for 12,579 adults (aged 26 to 80 years), 4,678 of whom were current smokers and 7,901 were non-smokers.
Bateson says the significance of the study is that it forces scientists to re-evaluate the value of telomere length as a read-out of how our current lifestyles are affecting our bodies:
We don't dispute the abundant evidence that smoking is bad for you, but merely the evidence that telomere length is a good way of assessing the biological damage done by smoking and possibly, by extension, other unhealthy behaviors.”
Melissa Bateson, First Author
This leads to the question of exactly why telomeres are indeed shorter among smokers. The international team of researchers says it is possible that the association is connected to a third variable – exposure to forms of adversity early in life such as emotional or physical abuse – a possibility that the team from Newcastle University are currently looking into.
The study findings contradict the scientific community’s previous understanding of telomere length as a biomarker for morbidity and shortened lifespan. Telomere length had previously been thought of as responding dynamically to adult activities, shortening in response to unhealthy behaviors and maybe lengthening in response to healthier activities.
However, the current study suggests that telomere length in adults should now be considered a static biomarker that changes relatively little during adult life.
Bateson says that for the scientific community, it means that measuring changes in adult telomere length may be less useful than previously thought for identifying behavior that is harmful and for monitoring the consequences of changes in behavior.
Commenting on the study more generally, Bateson says: “the findings underline the need for caution when interpreting correlational data. Just because two variables are correlated does not mean that one variable causes the other."
Bateson, M., et al. (2019). Smoking does not accelerate leucocyte telomere attrition: a meta-analysis of 18 longitudinal cohorts. Open Science. https://doi.org/10.1098/rsos.190420