Here is the excerpt from the abstract of the paper Obesity profile in Turkey published in 2009, it does not give specifics on Istanbulites BMI but gives BMI values for Turkish citizens.
13,878 individuals (6,799 males and 7,079 females) were screened in six different regions of Turkey between 2000-2005 (Istanbul and Kastamonu from Northern part of the country, Gaziantep from South,Denizli and Kırklareli from the West and Konya from the middle part of the country). The sample was randomly selected and all participants were older than 20 years of age. Weight (kg), height (cm) and waist circumference (cm) were measured and body mass index (BMI) was calculated according to World Health Organization (WHO) methods and criteria.
Results: The mean BMI was 27.52 kg/m2, 26.80 kg/m2 in men and 28.24 kg/m2 in women.