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There were 2090 individuals in our cohort study. The characteristics for the whole population and for different BMI categories were shown in Table 1. The mean age was 69.25±8.58 years old, and the mean BMI was 23.25±3.98 kg/m2 at baseline. In total, 11.39% of the population was underweight, 45.55% were normal weight, 31.63% were overweight and 11.43% were obese. Participants who were male and who reported smoking and drinking were expected to have a lower BMI; and BMI was expected to decline with advancing age. At baseline, a higher BMI was associated with a higher prevalence of pre-existing diseases, including hypertension, coronary heart disease, rheumatic heart disease and COPD.
Characteristic All, n (%) BMI (kg/m2), n (%) P Value < 18.5 18.5-23.9 24.0-27.9 ≥28 Gender Female 1059 (50.70) 119 (50.00) 418 (43.91) 369 (55.82) 153 (64.02) < 0.01 Male 1031 (49.30) 119 (50.00) 534 (56.09) 292 (44.18) 86 (35.98) Age (y) 55-64 719 (34.00 53 (22.27) 278 (29.20) 267 (40.39) 121 (50.63) < 0.01 65-74 745 (36.00) 73 (30.67) 338 (35.50) 250 (37.82) 84 (35.15) 75-95 626 (30.00) 112 (47.06) 336 (35.30) 144 (21.79) 34 (14.22) Smoking No 1469 (70.00) 138 (57.98) 629 (66.07) 511 (77.31) 191 (79.92) < 0.01 Yes 621 (30.00) 100 (42.02) 323 (33.93) 150 (22.69) 48 (20.08) Drinking No 447 (21.00) 30 (12.61) 217 (22.79) 151 (22.84) 49 (20.50) < 0.01 Yes 1643 (79.00) 208 (87.39) 735 (77.21) 510 (77.16) 190 (79.50) Disease# No 1485 (71.00) 215 (90.34) 728 (76.47) 401 (60.67) 141 (59.00) < 0.01 Yes 605 (29.00) 23 (9.66) 224 (23.53) 260 (39.33) 98 (41.00) Note. #Disease: pre-existing diseases, including hypertension, coronary heart disease, rheumatic heart disease and COPD. Table 1. Basic Characteristic of Participants by BMI Category
The outcomes of the participants are shown in Table 2. During the 20-year follow-up, 1164 deaths, accounting for 55.96% of the total population, were recorded. Among the causes of death, the top three were cardiovascular disease, cerebrovascular disease and tumor. The prevalence of all-cause mortality was 71.4% in the underweight group, 60.3% for the normal weight group, 45.7% for the overweight group, and 49.4% for the obese group.
Outcomes All BMI (kg/m2) < 18.5 18.5-23.9 24.0-27.9 ≥28 n (%) n (%) n (%) n (%) n (%) Survival 926 44.31 68 28.57 378 39.71 359 54.31 121 50.63 Cardiovascular disease 253 12.11 40 16.81 123 12.92 65 9.83 25 10.46 Cerebrovascular disease 240 11.48 30 12.61 118 12.39 60 9.08 32 13.39 Tumor 129 6.17 17 7.14 57 5.99 39 5.90 16 6.69 Respiratory disease 101 4.83 20 8.40 57 5.99 18 2.72 6 2.51 Digestive disease 27 1.29 1 0.42 11 1.16 13 1.97 2 0.84 Harm & poisoning 26 1.24 2 0.84 17 1.79 2 0.30 5 2.09 Kidney disease 14 0.67 1 0.42 4 0.42 5 0.76 4 1.67 Diabetes mellitus 13 0.62 1 0.42 3 0.32 8 1.21 1 0.42 Other reason 133 6.36 13 5.46 66 6.93 38 5.75 16 6.69 Natural death 183 8.76 40 16.81 93 9.77 42 6.35 8 3.35 Total 2090 100.00 238 100.00 952 100.00 661 100.00 239 100.00 Table 2. Description of Outcomes Including Survival and Specific Disease Mortality by BMI Category
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To determine the relationship between BMI and mortality, a BMI-mortality curve was drawn and is presented in Figure 1, showing that people with a BMI of approximately 25 kg/m2had the lowest mortality. The figure also indicated that, for lower ranges of BMI, the mortality of men was higher than that of women; but for higher BMI ranges, the difference narrowed. BMI-mortality curves stratified by age are shown in Figure 2; the ranges of BMI associated with the lowest mortality in different age groups were closed to each other. Furthermore, as age increased, the mortality increased, and there is much higher mortality in the range of 75-95 years old, regardless of age, an underweight status was associated with a high mortality. Overall, the slope of the BMI-mortality curvesincreased with advancing age.
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Cox model and HRs for the overall population are shown in Table 3. The normal weight group was regarded as the comparison group; HRs were calculated using different Cox models, in which different confounding factors were adjusted. The HR in the unadjusted model, i.e., model 1, was 1.45 (95% CI: 1.20-1.72) for the underweight group; and 0.65 (95% CI: 0.57-0.75) for the overweight group.
BMI Model 1 Model 2 Model 3 Model 4 HR (95% CI) HR (95% CI) HR (95% CI) HR (95% CI) 18.5-23.9 1 1 1 1 < 18.5 1.45*** (1.20-1.72) 1.41*** (1.19-1.67) 1.37*** (1.15-1.63) 1.39*** (1.16-1.65) 24.0-27.9 0.65*** (0.57-0.75) 0.76*** (0.66-0.88) 0.77*** (0.67-0.88) 0.76*** (0.66-0.88) ≥28 0.78*** (0.61-0.91) 1.01 (0.82-1.23) 1.02 (0.83-1.25) 1.01 (0.82-1.24) Note. ***P < 0.01; model 1 no adjusted; model 2 adjusted for gender and age; model 3 additionally adjusted for smoking status and drinking status; model 4 additionally adjusted for pre-existing disease like hypertension, coronary heart disease, rheumatic heart disease and COPD. Table 3. HRs of All-cause Mortality for BMI Categories in the Overall Population
In model 2, which adjusted for gender and age, the HR was 1.41 (95% CI: 1.19-1.67) for the underweight group and 0.76 (95% CI: 0.66-0.88) for the overweight group; the HR for the group with obesity was not statistically significant. When further adjusted for smoking status, drinking status, and pre-existing diseases such as hypertension, coronary heart disease, rheumatic heart disease and COPD, the HRs were closer to the results in model 2, as shown in Table 3.
The HRs of all-cause mortality for different BMI groups, stratified by gender, age, smoking status, drinking status and pre-existing disease, are shown in Table 4; the HRs for the same BMI group were different in different variable stratifications. Being underweight seemed to be a risk factor in the population aged over 75 years but not in the younger population. Overweight seemed to be a strong protective factor in men but not in women.
Characteristic BMI (x±s) HR (95% CI) < 18.5 24.0-27.9 ≥28 Gender Female 23.71±4.22 1.30** (1.00-1.68) 0.93 (0.76-1.14) 1.18# (0.90-1.54) Male 22.76±3.66 1.38*** (1.09-1.74) 0.64*** (0.52-0.80) 0.94## (0.68-1.29) Age (y) 55-64 24.25±3.93 1.10## (0.71-1.72) 0.67*** (0.50-0.90) 0.86 (0.59-1.27) 65-74 23.42±3.93 1.12## (0.82-1.53) 0.74*** (0.59-0.92) 1.14## (0.84-1.54) 75-95 21.87±3.71 1.55*** (1.22-1.98) 0.83 (0.66-1.05) 1.04 (0.68-1.58) Smoking No 23.69±3.98 1.31** (1.05-1.64) 0.78*** (0.66-0.92) 0.99## (0.78-1.26) Yes 22.19±3.80 1.38** (1.05-1.82) 0.75** (0.57-0.98) 1.33## (0.88-1.99) Drinking No 23.61±3.88 1.84** (1.15-1.95) 0.81 (0.59-1.12) 0.85 (0.50-1.45) Yes 23.14±4.01 1.27** (1.05-1.53) 0.76*** (0.64-0.89) 1.09### (0.87-1.36) Disease No 22.65±3.94 1.48*** (1.17-1.70) 0.82** (0.69-0.97) 1.09## (0.84-1.41) Yes 24.71±3.70 1.04 (0.60-1.81) 0.67*** (0.58-0.86) 0.96## (0.69-1.36) Note. **P < 0.05; ***P < 0.01 (compared with the normal weight group). #P < 0.1; ##P < 0.05;###P < 0.01 (significant in overweight group comparison but not in normal weight group comparison). Table 4. BMI and HRs of All-cause Mortality for BMI Categoriesin Subgroups
The HRs of all-cause mortality for BMI (1 kg/m2 increase) differed in each BMI group. In the underweight group, the HR for a 1 kg/m2increase was 0.817 (95% CI: 0.709-0.940); in the normal weight group, the protective effect weakened, with an HR of 0.904 (95% CI: 0.857-0.955); and in the overweight group the corresponding HR was 1.050 (95% CI: 0.948-1.162). When BMI reached the obese range, the HR was 1.150 (95% CI: 1.054-1.255), which turns out to be a risk factor.
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Until the year 2000, there were 694 deaths and 1, 396 surviving individuals. Among the 1, 396 survivors, 1, 128 individuals took the second measurement. Missing values were filled according to a linear fitting in which the existing second BMI measurement values were regarded as dependent variables. Other factors, such as the first BMI measurement values, gender, age, smoking status, and drinking status were regarded as independent variables. The mean of the second BMI measurement was 23.81±3.64 kg/m2. BMI change per year was defined as (BMI2-BMI1)/8; BMI2 was the BMI in 2000 and BMI1 was the BMI in 1992. BMI change per year was divided into three intervals: a BMI change per year of less than-0.05 kg/m2was regarded as a BMI drop, a BMI change per year of more than 0.05 kg/m2 was regarded as a BMI rise, a BMI change per year between-0.05 kg/m2and 0.05 kg/m2 was regarded as stable BMI. In the overall population model after adjusting for BMI, gender, age, smoking status, drinking status, and pre-existing diseases, the HR for a BMI drop was 1.576 (95% CI: 1.209-2.054), for a BMI rise was 1.700 (95% CI: 1.311-2.205). The HRs for a BMI drop and rise in specific weight groups are shown in Table 5.
Changes HR (95% CI) BMI Drop BMI Stable BMI Rise Underweight 3.245* (0.824-12.772) 1 1.531 (0.631-3.714) Normal weight 1.892*** (1.291-2.773) 1 1.795*** (1.243-2.591) Overweight 1.434 (0.904-2.277) 1 1.962*** (1.202-3.203) Obese 0.525 (0.211-1.306) 1 0.402 (0.125-1.289) Note. *P < 0.1, *** P < 0.01. Table 5. HRs for BMI Change in Different Weight Groups
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To further study the impact of BMI on factors related to mortality, we used a Cox model in each BMI group to calculate the HRs for factors such as gender, age, smoking, drinking and pre-existing diseases. The results were shown in Figure 3. The HRs for smoking and drinking showed an increasing trend in the four BMI groups, while the HR for the males declined as BMI increased, and the HR for age (1-year increase) seemed to be unchanged in each BMI group.