The cohort consisted of 115,433 eligible women who were followed for 1,322,634 person-years. Overall, 10,594 women died during the follow-up time period. Causes of death for this study included any cancer (N=2,292); breast cancer (N=302); CVD (N=3,626); and respiratory disease (N=946). The average age at baseline was 53.1 years (standard deviation = 14.1 years).
provides the age-adjusted distribution of several participant characteristics according to BMI at baseline. Approximately 60% of participants with normal BMI (18.5–24.9 kg/m2) at age 18 still had normal BMI at cohort entry. Approximately 80% of participants who gained more than 10 kg since age 18 were overweight or obese at cohort entry. Higher BMI was observed among African-American women (and Native American women to a smaller extent), postmenopausal women (especially those who never used HT), women reporting fewer than 3 hours per week of strenuous or moderate physical activity, never drinkers of alcohol, never or former smokers, women with greater percentage of their daily calories from fat, and women with one or more co-morbidities.
Baseline characteristics of 115,433 participants from the California Teachers Study by body mass index (BMI, in kg/m2) at baseline
BMI and all-cause mortality
Risk for all-cause mortality was statistically significantly increased for underweight participants (RR = 1.33, 95% CI = 1.20–1.47) and for obese participants (RR = 1.27, 95% CI = 1.19–1.37) compared to normal weight participants after adjustment for age, race/ethnicity, weight change from age 18 to baseline, physical activity, history of co-morbidities, smoking status, alcohol consumption, HT, and percent daily calories from fat (Model 5, ). HT use (, Model 2 compared to Model 1) did not affect the association between BMI and all-cause mortality for any BMI category. Overweight participants did not significantly differ from normal weight participants with respect to all-cause mortality.
Multivariable adjusted* relative risks (RR) and 95% confidence intervals (CI) or all-cause mortality associated with body mass index (BMI) at baseline
In a restricted model, we excluded participants who reported having had a defined co-morbidity at baseline (diabetes, hypertension, heart attack, stroke, and cancer). Among the 71,144 remaining participants, the mortality risk estimate for underweight and for obese participants (RR = 1.31, 95% CI = 1.09–1.58, and RR = 1.32, 95% CI = 1.15–1.53, respectively) were only slightly attenuated (data not shown).
BMI and cause-specific mortality
Compared to normal weight participants, underweight participants were at increased risk of death due to due to respiratory disease (RR = 1.79, 95% CI = 1.36–2.34) (). Obese participants, compared to participants of normal weight, were at an increased risk of death due to any cancer (RR =1.32, 95% CI = 1.14–1.53), breast cancer (RR =1.57, 95% CI = 1.07–2.31), CVD (RR =1.38, 95% CI = 1.22–1.55), and respiratory disease (RR =1.46, 95% CI = 1.15–1.85).
Multivariable adjusted* relative risks (RR) and 95% confidence intervals (CI) for cause-specific mortality associated with body mass index (BMI) at baselinea
After stratification by smoking status (), obese never smokers had increased risk of all cause mortality (RR=1.32, 95% CI=1.21–1.45), any cancer (RR=1.48, 95% CI= 1.21–1.82), and CVD (RR=1.50, 95% CI= 1.29–1.75) as compared to normal weight never smokers; but for obese ever smokers only risk of all-cause mortality was statistically significantly increased (RR= 1.13, 95% CI= 1.02–1.26) as compared to normal weight ever smokers. Never smokers who were underweight were at increased risk for all-cause mortality (RR =1.34, 95% CI = 1.17–1.54), and for CVD mortality (RR =1.34, 95% CI = 1.07–1.68), as compared to normal weight never smokers. For smokers, being underweight was associated with risk only for all-cause mortality (RR=1.41, 95% CI = 1.21–1.65) and respiratory disease mortality (RR = 2.22, 95% CI = 1.58–3.11). The analyses for underweight smokers and non-smokers for any cancer and breast cancer were based on small numbers.
Table 4 Multivariable adjusted a relative risk (RR) and 95% confidence interval (CI) for all-cause, any cancer-specific, breast cancer-specific, cardiovascular-specific, and respiratory-specific mortality according to body mass index (BMI) at baseline, stratified (more ...)
After stratification by history of hormone therapy use (), obese never users of HT had increased risk of all cause mortality (RR=1.27, 95% CI=1.12–1.44), any cancer (RR=1.38, 95% CI= 1.03–1.84), and CVD (RR=1.41, 95% CI= 1.15–1.74) as compared to normal weight never users. Obese HT users had statistically significantly increased risk of all-cause mortality (RR= 1.19, 95% CI= 1.08–1.32), and any cancer mortality (RR=1.23, 95% CI=1.00, 1.50), as compared to normal weight ever users. Never users of HT who were underweight were at increased risk for all-cause mortality (RR =1.22, 95% CI = 1.02–1.47), and for respiratory disease (RR =1.97, 95% CI = 1.27–3.07), as compared to normal weight never users. For HT users, being underweight was associated with risk for all-cause mortality (RR=1.47, 95% CI = 1.27–1.69) and respiratory disease mortality (RR = 1.90, 95% CI = 1.28–2.82).
Table 5 Multivariable adjusted arelative risk (RR) and 95% confidence interval (CI) for all-cause, any cancer-specific, breast cancer-specific, cardiovascular-specific, and respiratory-specific mortality according to body mass index (BMI) at baseline, stratified (more ...)
Further stratification on current smoking status (former and current) or type of HT use (estrogen-only and estrogen plus progestin) revealed similar associations, however the division into more strata resulted in small numbers of events in several categories of BMI (data not shown).