In this study, we established the NHANES longitudinal cohort using open-source data from the NCHS. By employing multivariate Cox regression models and RCS, we discovered a nonlinear relationship between BMI changes and the all-cause mortality in CVD patients. Patients with a BMI of 19.61 at age 25 and 26.55 after age 50 exhibited the lowest long-term all-cause mortality risk. The BMI change from young adulthood to middle-old age displayed a U-shaped correlation with all-cause mortality risk. Our research indicates that both excessive weight gain and weight loss can increase the risk of all-cause mortality among middle-aged and elderly CVD patients. Therefore, when formulating weight management strategies for this population, individualization should be achieved rather than blindly emphasizing weight loss.
We observed a U-shaped association between BMI changes over a long lifespan and the risk of all-cause mortality among CVD patients, which appears to contradict the conventional understanding that simply reducing BMI leads to benefits. However, our findings align with numerous past studies. A study[9] conducted in South Korea among adults with cardiac arrest arrived at nearly identical conclusions to ours, revealing a U-shaped correlation between BMI changes in the four years before cardiac arrest, with excessive weight loss increasing the risk of cardiac arrest, particularly prominent among overweight individuals. A decrease in BMI by more than 15% was associated with approximately a 3.1-fold increase in the risk of cardiac arrest. Another study[10] among cancer patients in Japan suggested that maintaining a stable BMI might be a superior strategy for reducing the incidence of coronary heart disease, heart failure, and atrial fibrillation, even among overweight individuals. This is because both excessive reduction and excessive increase in BMI were found to elevate the risk of CVD events, with the CVD risk associated with excessive weight loss far exceeding that of excessive weight gain (16% vs. 10%). Studies conducted in China have also unveiled a nonlinear relationship between BMI changes and blood pressure[11, 12]. While most previous studies were conducted in Asia or Europe, ours is among the first to be undertaken in a nationally representative sample of the American population. This contributes significantly to the mutual validation and generalization of research findings.
We further quantified the dose-response relationship between BMI changes and all-cause mortality risk. Our study revealed that 50–70% of participants had a BMI within the range of 18.5–25 at age 25, whereas this proportion declined to 17%-24% after the age of 50, aligning with the previous understanding that weight gain is a natural process in life[13]. We discovered that an excessive emphasis on weight loss could paradoxically increase the risk of all-cause mortality. This phenomenon is known as the "obesity paradox"[14], and several theories have been proposed to explain it. Overweight and obese individuals tend to have higher levels of cardiorespiratory fitness, which may counteract the adverse effects of overweight and other cardiovascular risk factors[15–17]. However, excessive weight loss can disrupt this balance between cardiorespiratory fitness and overweight. Additionally, another explanation posits that excessive weight loss can lead to alterations in body composition. Moderate weight loss results in a reduction in visceral adipose tissue, whereas excessive weight loss depletes muscle mass, leading to a frail state associated with poor outcomes in various diseases[18–20].
Our conclusions can provide evidence for individual long-term BMI management to reduce the risk of death among CVD patients, which is of great significance for improving the healthy life expectancy of the population. For instance, based on our findings, we recommend maintaining BMI within the normal range during young and middle adulthood by controlling weight, as BMI during this period is roughly positively correlated with all-cause mortality risk, while BMI in middle and old age exhibits a U-shaped correlation with all-cause mortality risk, where excessive weight loss and gain can both increase mortality risk. When assessing the all-cause mortality risk of CVD patients using BMI changes over a long life cycle, it is essential to recognize the nonlinear U-shaped relationship between them.
There are several limitations in this study. Firstly, we have merely considered BMI at two time points: 25 years old and after 50 years old, without examining the intermediate changes between these two points. The BMI at 25 years old and the diagnosis of CVD were obtained through questionnaire surveys. Although NHANES questionnaire survey data have been extensively used in clinical studies, the potential for information bias still exists. Secondly, we have not investigated the causes that led to changes in BMI. Lastly, due to the limitation of sample size, we have not conducted a subgroup analysis to explore the population specificity of the above results.