There is no doubt that smoking
damages health and increases the risk of many diseases and premature death.
However, a study that followed hundreds of older adults for more than 10 years
found no link between tobacco smoking and raised dementia risk.
Dementia is the
general name for conditions that diminish the capacity to think, remember,
reason, and interact with others. These symptoms can progress to the point that
people are no longer able to carry out their daily activities and take care of
themselves.
The
recent finding contradicts many earlier studies that have tied smoking to
higher risk of dementia. This could be because, for the new investigation, researchers
from the University of Kentucky in Lexington analyzed the data in a different
way.
They
used a statistical method called "competing risk analysis" to allow
for the strong effect that smoking has on risk of death.
In
a paper that
now features in the Journal of
Alzheimer's Disease, first study author Erin L. Abner Ph.D., an
associate professor in the university's Sanders-Brown Center on Aging, and her
colleagues describe their approach and findings.
Dr.
Abner explains that while previous dementia studies had used "solid"
data, they had not taken "into account the idea of competing risk of
mortality."
Dementia is a growing global health issue
According
to the World Health Organization (WHO), around 50 million
people were living with dementia worldwide in 2015. The WHO
expect this number to triple to 152 million by 2050.
Although
dementia arises mainly in older people and is a main cause of disability for
them, health experts do not consider it to be a normal consequence of aging.
Alzheimer's disease is the most common cause of
dementia and likely accounts for 60–70 percent of cases, globally.
In
the United States, estimates from the Centers for Disease Control and
Prevention (CDC) estimate that the number of people aged 65 years and older
with Alzheimer's disease is
set to grow from 4.7 million in 2010 to 13.8 million in 2050. Over this
period, the biggest growth will be in those who are 85 years of age and older.
A
burgeoning cost of care accompanies these figures. The CDC calculated that the
total cost of health, long-term, and hospice care for people with dementia was
$277 billion in 2018.
Need to account for competing risk
Dr.
Abner remarks that competing risk analysis is a recognized investigative tool,
and scientists have used it successfully in many other fields.
Yet,
even though the competing risk of death is "an important
consideration" when studying diseases in older adults, dementia
studies rarely use it.
Dr.
Abner gives the example of how to investigate ties between smoking and cancer deaths. How should the analysis
deal with deaths from another condition, such as heart disease?
"In the case of our study," she adds,
"if smoking kills someone before they show signs of dementia, how can you
accurately count that person?"
So,
the team sought to address this in its own analysis of data from a University
of Kentucky Alzheimer's Disease Center study that followed 531 older people for
an average of 11.5 years.
'Smoking doesn't appear to cause dementia'
At
baseline, the researchers recorded none of the participants to be cognitively
impaired. At this time, 49 of the subjects reported being current smokers, and
231 said they were former smokers.
Over
the follow-up, 111 of the cohort had a diagnosis of dementia while another 242
died without dementia.
The
analysis revealed a link between smoking and early death. However, the authors
note that after they adjusted the data for "the competing risk of death
without dementia, smoking was not associated with incident dementia."
Dr.
Abner says that the finding is consistent with pathological studies of brain
tissue that have found that hallmarks of Alzheimer's disease are not more
common in people with a history of smoking.
However,
she points out that because the study only looked at people from one research
center, it does not necessarily represent other populations.
She
and her colleagues maintain that the most important aspect of their
investigation is that it shows how competing risk analysis can impact research
into dementia. They wish, she adds, "to advocate for its adoption in the
appropriate areas of study."
"To be clear, we are absolutely
not promoting smoking in any way. We're saying that smoking doesn't appear to cause
dementia in this population."
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