the global score (mean difference in decline − 0.01, − 0.04 to
0.03). In addition, qualitative relations with longer duration
diabetes and use of medication were generally similar to those
observed with baseline cognitive function.
Discussion
In this large prospective study of women aged 70-81 years with
type 2 diabetes who were living in the community we found that
they had marginally worse baseline cognitive performance and
greater cognitive decline than women without diabetes. Longer
duration of diabetes resulted in larger associations. However,
women who said they were on hypoglycaemic treatment seemed
to have a similar likelihood of poor cognition as women without
diabetes, while women not taking medication for diabetes or
those taking insulin had worse performance.
A major strength of our study is the large sample size for
assessing the relations between type 2 diabetes, duration,
treatment, and cognition. Other strengths are the prospective
assessment of diabetes and potential confounders over 25 years
of follow up and the relative homogeneity of the sample in terms
of education and access to health care, which should minimise
confounding.
Limitations
Limitations should be considered. Firstly, as we relied on the
women reporting their own diabetes status, we may have
included some women with undiagnosed diabetes in the
reference group, which could have led to underestimation of the
true associations. However, undiagnosed diabetes was probably
rare in these nurses. Among a random sample of those with no
reported diabetes, plasma samples indicated just 2% had
diagnostic signs of type 2 diabetes. Secondly, as in all studies of
cognitive decline, there is regression to the mean on the repeat
cognitive assessment. As women with type 2 diabetes had worse
cognitive performance at baseline, regression to the mean would
probably have attenuated the true magnitude of cognitive
decline associated with diabetes.
In addition, there are important issues to consider in
interpreting our findings regarding pharmaceutical treatment of
diabetes. Participants who were not taking any treatment for
diabetes probably included a heterogeneous group of women
with untreated diabetes and diabetes controlled through diet.
Diabetes that can be controlled through diet may not be associated
with poor cognition.14 Thus, we have probably underestimated
the effect of untreated diabetes. However, the increased
odds of poor cognition associated with no treatment was similar
across those with shorter and longer duration of diabetes (and