Table I. Examination of the Age Comparability for the DM and Non-DM Groups

APPENDIX

Detailed data description

Table I. Examination of the age comparability for the DM and Non-DM groups

cohort / DM status / N / Data points / Age at baseline
Mean SD / Age in 2007
MeanSD
TLSA 1989-2007 / DM group / 43 / 258 / 65.093.58 / 83.09
Non-DM group / 942 / 5,652 / 65.414.07 / 83.41
TLSA 1996-2007 / DM group / 94 / 376 / 58.13 / 69.13
Non-DM group / 1,405 / 5,620 / 58.22 / 69.22
TLSA 2003-2007 / DM group / 63 / 126 / 53.73 / 57.73
Non-DM group / 1,229 / 2,458 / 53.30 / 57.30

Table II. Time axis for the DM and Non-DM groups in three cohorts

cohort / DM status / 1989 / 1993 / 1996 / 1999 / 2003 / 2007
TLSA 1989-2007 / DM group / -18 / -14 / -11 / -8 / -4 / 0
Non-DM group / -18 / -14 / -11 / -8 / -4 / 0
TLSA 1996-2007 / DM group / -11 / -8 / -4 / 0
Non-DM group / -11 / -8 / -4 / 0
TLSA 2003-2007 / DM group / -4 / 0
Non-DM group / -4 / 0

Calculation of the turning point

In the trajectory modeling, we centered time to -18. The intercept (time=0) thus represents 18 years before diabetes diagnosis. Based on model fit, a quadratic model can best characterize the BMI change before self-reported physician diagnosed diabetes. The quadratic trajectory means we can find a turning point of BMI change becausethe curve’s vertex could be calculated as per the parabolic formula. We assumed the trajectory to be BMI= a+ b*time + c*time2. The turning point was located at time=-b/2c, which is 18-(-b/2c) years before self-reported diabetes.

Note1. -(βtime+βdiabetes*time)/2(βtime2+βdiabetes*time2)=11.5; 18-11.5=6.5 yrs

Note 2. -(βtime+βdiabetes*time)/2(βtime2+βdiabetes*time2)=12.35; 18-5.35= 5.65 yrs

Note 3. βdiabetes*time=0.209, p=0.0358, βdiabetes*time2=-0.009, p=0.0194;-(βtime+βdiabetes*time)/2(βtime2+βdiabetes*time2)= 9.5; 18-9.5=8.5 yrs

Example calculation

If an individual has a height of 1.7 meters and has not developed diabetes, he/she will gain 2.29kg in 18 years. The 2.29 kg was calculated based on the coefficient estimated for the Diabetes Free group: BMI = 23.073 + 0.080 * time - 0.002 * time2

The change in BMI for this diabetes free individual within 18 years is estimated to be: 0.08*18-0.002*182=0.792 kg/m2 ; weight change (kg)= (1.7m)2*0.79 kg/m2=2.29 kg.

If an individual develops diabetes, he/she will gain a total of 3.82kg during 18- to 6.5-year before physician-diagnosed diabetes, and then lose a total of 1.25kg within 6.5 years preceding the diabetes diagnosis. The weight changes of 3.82 kg and 1.25 kg during the 18- to 6.5-year and the 6.5-year period before diabetes diagnosis were calculated based on the coefficients estimated for the Developing Diabetes group: BMI = (23.073 + 1.294) + (0.080 + 0.150) * time-(0.002+0.008) * time2 = 24.367+0.23* time-0.01*time2

The change in BMI for this individual developing diabetes during the 18- to 6.5-year periods before physician-diagnosed diabetes is estimated to be: 0.23*11.5-0.01*(11.5)2=1.3225 kg/m2 ; weight change (kg)= (1.7m)2*1.3225 kg/m2=3.82 kg.

The change in BMI for this individual developing diabetes during the 6.5-year period before physician-diagnosed diabetes is estimated to be: 0.23*18-0.01*182=0.89 kg/m2 ; weight change (kg)= (1.7m)2*0.89 kg/m2=2.5721 kg; 3.82kg-2.57kg= 1.25 kg.