“People who give
up smoking are prone to developing diabetes because they gain weight,” TheTimes reported. It said a study has found
that quitters are twice as likely as smokers, and 70% more likely than
non-smokers, to have type 2 diabetes.
This study found that smokers and recent
quitters had a greater risk of diabetes compared to those who had never smoked,
but that three years after quitting this risk had reduced. The suggestion that
this is because quitters are more likely to gain weight is logical, but it
cannot be proven by this cohort study.
The results of this study do not mean
that smoking is protective to health. Smokers and
former smokers were at greater risk of diabetes than those who had never
smoked, and the benefits of giving up far outweigh any temporary increase in
risk. Instead, these findings emphasise the importance of an active lifestyle
and a healthy balanced diet, and demonstrate the
importance of providing quitters with education and support to achieve this.
Where did the story come from?
This research was carried out by
Hsin-Chieh Yeh and colleagues from Johns
Hopkins University,
Baltimore; the Federal University of Rio Grande
do Sul, Brazil
and the University of North Carolina, Chapel Hill.
The study was funded by the National Heart, Lung, and Blood Institute and the
National Institute of Diabetes and Digestive and Kidney Disease. It was published in the Annals
of Internal Medicine.
What kind of research was this?
This cohort study enrolled a large group
of middle-aged people who were free of diabetes, and followed them up over nine
years to assess whether quitting smoking affected the risk of diabetes.
Where a randomised controlled trial
(RCT) would be unethical, a cohort study is the best alternative for examining
whether a particular exposure, in this case quitting smoking, increases the
risk of a certain disease developing over time. The research needs to ensure
that people are free from the disease at the beginning of the study, and take
into account other confounding factors that could influence any observed
associations.
What did the research involve?
The data for this study were obtained
from a previous study on atherosclerosis called the Atherosclerosis Risk in
Communities (ARIC) study, which recruited middle-aged people from several sites
in the US.
The ARIC recruits visited a clinic between 1987 and 1989, and then had three
follow-up visits scheduled at about three-year intervals from 1990 to 1998.
From this point onwards until 2004, they were only contacted by phone. Smoking
status and number of cigarettes smoked was assessed at every follow-up. The
development of diabetes up to the last clinic visit in 1998 was determined by
fasting blood glucose levels, and from 1998 up to 2004 by self-report of a
doctor’s diagnosis of diabetes or use of diabetes medications.
For this particular study, the 17-year
follow-up information from the ARIC study was used for the 9,398 middle-aged
adults who were free from diabetes when ARIC began and in the first three years
of follow-up, and who had information on smoking status at each point during
follow-up. For all participants, physical examination, various other medical
data and information on other lifestyle factors were collected during
follow-up, and various analyses carried out.
People were grouped according to how
much they smoked at the beginning of the study. This was calculated as
pack-years smoked (average number of cigarettes per day multiplied by the years
of smoking divided by 20). People who were lifelong non-smokers formed the
control group. For each category the incidence of diabetes during follow-up was
calculated.
To assess the effect of quitting smoking
on diabetes risk, the researchers looked at the effect of a change in smoking
status from the start of the study to the first three-year follow-up, and risk
of diabetes at the three and nine-year follow-ups. They also looked at the
change in smoking status and the effects on various metabolic variables, such
as weight, waist and hip circumference, blood pressure and cholesterol. A
number of other statistical analyses were then carried out, including an
assessment of how various measures at the beginning of the study could affect
weight-gain risk, how various other factors affected diabetes risk, and
analyses using only self-reported data.
That multiple statistical tests were
carried out is a slight drawback to the study. There are also likely to
have been some unavoidable inaccuracies in self-reported measures such as
duration of smoking, number of cigarettes smoked and time since quitting.
What were the basic results?
The study found that smoking increased
the risk of diabetes, and that there was a dose-response relationship, meaning
that the more packs smoked the greater the diabetes risk. Quitting smoking was
also associated with increased risk compared to never smoking. New quitters at
three-year follow up (380 of them) were 1.73 times more likely than those who
had never smoked to develop diabetes. However, when the analysis was adjusted
for weight change, white blood cell count at the beginning of the study and all
other known risk factors for diabetes (including gender, BMI, waist
circumference, physical activity, triglyceride level, cholesterol, blood
pressure), the incidence was 1.24 times greater in quitters than those who had
never smoked, but this was no longer significant.
The highest risk of diabetes for
quitters occurred in the first three years, but gradually reduced to zero at 12
years. Former smokers who had smoked more than three years ago did not have a
significantly increased risk of diabetes.
How did the researchers interpret the results?
The researchers concluded that cigarette
smoking increases the risk of developing type 2 diabetes, but that quitting
smoking also increases the risk in the short term. They advise that smokers who
have other risk factors for diabetes receive smoking cessation care coupled with strategies for
diabetes prevention and early detection.
Conclusion
Smoking is associated with an increased
risk of diabetes, and the current study confirms this. However, the effect of
quitting smoking on diabetes risk has been unclear until now. This study found
that quitting is associated with an increased risk of developing diabetes in
the short term, but that this risk decreases over time. When the researchers
adjusted their analyses for weight change since quitting this affected the
risk.
This was a well conducted study
that carried out extensive follow-up on a large number of participants. However,
there are some points to consider:
·
As
the authors acknowledge, though they adjusted for various established diabetes
risk factors, there is still the possibility of residual confounding from
unmeasured factors.
·
Several
of the self-reported measures, most notably smoking status, frequency of
smoking and time since quitting, are likely to involve some degree of
inaccuracy.
·
Multiple
statistical tests were carried out, and this is a slight drawback to this
research, as it increases the risk that the findings are due to chance only.
However, this likelihood is reduced by the fact that the study specified its
research hypothesis prior to the start of the study.
·
The
theory is that, while quitting smoking may reduce inflammation in the body and
so reduce diabetes risk, the weight gain that quitters often experience could
adversely influence this risk. Although this pattern may be suggested by these
results, no firm conclusions can be made. Adjusting for weight change did
reduce the strength of the association between quitting and diabetes risk but
the risk remained significant indicating that there are other factors involved.
In addition, the reasons for the person’s weight gain have not been examined.
The recommendation of the
researchers seems sensible. Smokers who quit should receive advice about
avoiding weight gain, diabetes prevention and how to spot the early signs of
the disease.
