Introduction
Several population based studies have shown that type 2 diabetes
increases the risk of dementia.1–5 Cognitive decline is an intermediate
stage between normal ageing and dementia.6 As dementia
may be most effectively delayed in its initial stages, identifying
diabetes as a modifiable risk factor for early cognitive decline
could be of major public health importance. Estimates in the
United States indicate that delaying onset of dementia by one
year could lead to 800 000 fewer cases after 50 years.7
Though many investigations have examined diabetes in relation
to early cognitive decline,5 8–19 only one large prospective
study has focused on women.8 Type 2 diabetes disproportionately
affects older women and is a stronger risk factor for cardiovascular
disease in women than in men.20 As cardiovascular
disease is an independent risk factor for cognitive decline, we
need to determine the impact of diabetes on cognition in
women.20 Moreover, few studies have evaluated the influence of
different treatments for diabetes on the association between type
2 diabetes and cognition.
We assessed the associations between type 2 diabetes, different
treatments for diabetes, and cognitive function in more than
16 000 women.
Methods
The nurses’ health study is a prospective cohort of 121 700 US
female registered nurses, who were aged 30-55 years in 1976,
when the study began. Participants’ health information has been
updated with biennial mailed questionnaires. Over 90% of the
original cohort have been followed up to date.
From 1995-2001, participants aged 70 years and older who
had not had a stroke were given baseline cognitive assessments
by telephone. Overall, 93% completed the interview. Interviewers
were blinded to participants’ health status (including diabetes).
For the baseline analyses of cognitive function, we included
18 999 women with complete information on education and
without type 1 diabetes, gestational diabetes, or unconfirmed
diabetes (see below).
The follow up cognitive assessment began about two years
after the baseline interview. After the exclusion of the 3% who
died, calls have been attempted for 98% to date. Of these, 92%
(n = 16 596) completed the interview, 5% (n = 967) refused, 3%
(n = 526) were unreachable. For analyses of cognitive decline, we
included 16 596 participants who completed both assessments
and excluded women in whom diabetes had been newly
diagnosed between the baseline and second interviews.