Type 2 diabetes
NHSII study participants received a supplemental questionnaire if they reported having been diagnosed with diabetes
mellitus on the biennial questionnaires between 1991 and 2005. These supplemental questionnaires collected
information to distinguish between type 1 and type 2 diabetes mellitus on the basis of diabetes diagnosis and
treatment. The validation of this method of case confirmation has been reported previously (18). The 6% of
respondents who reported having diabetes on the baseline 1989 questionnaire were not sent the supplemental
questionnaire and, consequently, had details only on date of diagnosis, without information on whether the diabetes
was type 1 or type 2. To restrict the analysis to cases likely to be T2DM, we included women who reported having
diabetes mellitus on the 1989 questionnaire only if they were age 30 years or older at the time of diagnosis.