[Distribution characteristics and long-term change trend of body mass index in Chinese older adults aged 65 years and above].

<b>Objective:</b> To describe the body mass index (BMI) level and long-term trends of Chinese older adults aged 65 and above. <b>Methods:</b> Older adults aged 65 and above from six waves (2002-2018) of the China Longitudinal Healthy Longevity Survey were selected as the study population. Multiple cross-sectional design with six survey waves conducted in 2002, 2005, 2008, 2011, 2014, and 2018 was adopted, enrolling 15 647, 15 358, 15 622, 9 166, 6 302, and 12 417 participants, respectively. Additionally, a total of 13, 755 participants were included in the cohort study design. Relevant information was collected through questionnaires and physical examinations. The <i>χ</i>² trend test was used to compare the changes in the rates of underweight and overweight/obesity over the years, and the linear mixed-effects model (LMM) was used to fit trajectory curves of BMI changes with advancing age in older adults. <b>Results:</b> The baseline ages of the participants included in 2002, 2005, 2008, 2011, 2014, and 2018 were (85.16±11.26), (84.23±11.83), (84.99±12.16), (81.10±11.86), (78.89±11.30), and (83.08±12.42) years, respectively, with a relatively high proportion of females and rural residents. In the cohort study, the 13 755 participants had a median (<i>Q</i>₁<i>, Q</i>₃) follow-up time of 6.5 (5.2, 10.0) years, with a cumulative follow-up duration of 109 041 person-years. In each wave, males had higher BMI than females, urban residents had higher BMI than rural residents, and BMI gradually decreased with increasing age (all <i>P</i><0.001). The mean BMI of older adults in China increased from (19.37±3.80) kg/m² in 2002 to (22.04±4.01) kg/m² in 2018 (<i>P</i><0.001). Across all survey years, the prevalence of underweight was consistently higher in women than in men and in rural areas than in urban areas, with an upward trend as age increased (all <i>P</i><0.001). In 2018, the underweight rates in the 65-79, 80-89, 90-99, and ≥100-year-old age groups were 8.0%, 16.7%, 26.2%, and 35.5%, respectively. Meanwhile, the prevalence of overweight/obesity was higher in men than in women and in urban areas than in rural areas, showing a declining trend with advancing age (all <i>P</i><0.001). The prevalence of underweight among the older adults decreased significantly from 45.2% in 2002 to 18.9% in 2018 (<i>P</i><0.001), while the prevalence of overweight/obesity increased from 11.0% in 1998 to 29.6% in 2018 (<i>P</i><0.001). The trajectory curves fitted by the LMM model showed that individuals born in later decades had higher BMI levels at the same age compared to earlier cohorts. <b>Conclusion:</b> From 2002 to 2018, the BMI level among Chinese older adults showed an increasing trend. The prevalence of underweight showed a declining trend, while the rates of obesity and overweight increased. However, the underweight rate remained notably high among the oldest old.