This study showed that obesity and diabetes mellitus affected the long-term outcomes of patients with ACS. While diabetes mellitus deteriorated the clinical outcomes, obesity ameliorated the results; the “obesity paradox” was obvious in post-ACS patients regardless of the presence of diabetes. Obesity decreased the risk of MACE driven primarily by stroke and reduced the rate of HHF exclusively in patients without diabetes mellitus. These were more evident in female and elderly patients without diabetes. However, patients with diabetes did not show such benefit. All-cause and CV mortality seemed to be lowered in overweight and obese patients with and without diabetes.
As the prevalence of diabetes mellitus has increased worldwide, the prevalence also correspondingly increased in Korea, accounting for approximately 5.1 million people with diabetes in 2016 [10]. Patients with diabetes not only have a higher risk for developing coronary heart disease, but their prognosis worsens after acute MI [11,12]. In a previous cohort study, patients with diabetes had a lower socioeconomic status than those without diabetes, which would also affect the adverse CV results [5]. The present study also showed similar findings.
Obesity increases the risk of CV disease via dysregulated metabolism sharing the common mechanism with diabetes (e.g., insulin resistance and inflammation) [13]. BMI increases the prevalence of diabetes mellitus up to 5 times depending on the race [14,15]. However, in contrast to the biological effect of obesity, overweight or obese patients show a better prognosis than normal-weight patients, which is called the “obesity paradox” [13]. In a systematic review for the general population, all-cause mortality was the lowest in the overweight BMI group [16]. In a meta-analysis study, obese patients with coronary artery disease had no increased risk for all-cause and CV death, which was most evident in BMI 25.0–29.9 kg/m2 [17]. A study including 12 million Koreans also reported the inverse relationship between BMI and all-cause mortality. The optimal BMI for lowering all-cause death was higher in men than in women and was increased with aging, which corresponded to the overweight or obese BMI categories [18]. Our study showed that overweight or obese patients did not have an increased risk of all-cause and CV mortality compared with normal-weight patients, regardless of the presence of diabetes. In the overall population, the risk of all-cause and CV mortality mortalities was significantly lower in the overweight and obese groups than in the reference group (Additional file 1: Supplementary Table 2). As observed in the present study, obese patients had a favorable outcome of MACE with lower event rates of stroke and HHF compared with the normal-weight group. Stroke was more prevalent in obese patients, but the “obesity paradox” has remained. In the REACH study, the higher BMI groups had a comparable risk of stroke in patients with established CV disease than the reference group [19]. A study of 5,202 patients with a previous history of CV disease reported a consistent finding that obesity was protective for clinical outcomes including stroke and HHF. The risks of stroke and HHF increased by 10% and 5%, respectively, with the decreased weight [20].
The association of BMI and long-term outcomes of patients with and without diabetes mellitus after CV events has not been recognized. In a study of 19,579 patients with diabetes and established CV disease, the increasing BMI enhanced the clinical outcomes during a 2-year follow-up [19]. The PROactive study revealed that the lowest mortality was seen in patients with a BMI of 30–35 kg/m2. Patients with diabetes and CV comorbidity had the risk of all-cause and CV death, which increased by 13% and 7% for each 1% of weight loss [22]. The MONICA/KORA study with 4,504 patients after MI demonstrated the relationship between obesity and CV outcomes regarding the presence of diabetes during 6 years. Higher BMI decreased the risk of long-term survival in patients without diabetes, although diabetes attenuated the protective role of obesity [23].
The present study adds to the accumulating evidence that “obesity paradox” is obvious in patients with CV disease. It is a novel finding on the relationship between BMI and diabetes mellitus, that diabetes deteriorates the prognosis of ACS patients for the long-term duration in the Korean population while obesity was protective.
The cardiometabolic consequences of obesity have a more relevant impact on CV outcomes than obesity. A study on the population with CV disease demonstrated that the cardiometabolic dysfunction increased the risk of CV morbidity and all-cause mortality. Even in comparison with normal-weight patients with cardiometabolic dysfunction, overweight and obese patients had a comparable prognosis of CV disease [24]. A propensity-matched study of 7,788 patients with heart failure also reported a consistent result: the paramount difference in mortality between obese patients with and without diabetes [25]. The impaired cardiometabolic function, such as diabetes mellitus, might offset the protective role of obesity.
Most epidemiologic studies regarding “obesity paradox” used BMI as the obesity parameter. BMI is a simple and feasible measure, but it has some pitfalls. The HORIZONS-AMI trial including patients with ST-segment elevation MI showed that obese patients have more comorbidities, vulnerable features of the coronary lesions, and premature onset of MI. However, BMI did not predict the acute and long-term mortality [26]. The ethnic difference in BMI should also be considered. The pooled analysis for post-ACS patients reported that Asians had similar BMI between people with and without diabetes (24.7 vs. 24.2 kg/m2), whereas the Caucasians with diabetes had higher BMI than those without diabetes (29.3 vs. 27.2 kg/m2) [27]. The effect of obesity and diabetes for CV outcomes might be distinguishable between the ethnicities. In contrast to BMI, central obesity (e.g., waist-to-hip ratio) was more related to the clinical outcomes, and body fat distribution or epicardial fat were considered reliable measures for predicting the prognosis in CV disease [23]. A further study with these parameters is needed to clarify the interrelationship of diabetes mellitus and obesity in patients with CV disease.
Our findings showed that the aim of treatment in post-ACS patients with and without diabetes should be to prevent or control cardiometabolic complications and not merely weight reduction. Meanwhile, lowering the weight should not be underemphasized as intentional weight loss is still associated with an improvement of comorbidities and long-term prognosis in CV disease [19].
This study has some limitations. The data obtained from the claims data could not exclude the possibility of a diagnosis code inaccuracy, disease misclassification, or the bias of the observed difference-in-difference estimate toward the null. Additionally, this study was not able to consider the duration of diabetes, the quality or grade of diabetic treatment, and the information on the treatment of overweight or obesity. To identify the impact of obesity and diabetes on the long-term results of ACS patients, a well-designed prospective study is needed.