Introduction
Cardiovascular disease is one of the major causes of death in Korea,1 and to reduce the incidence of cardiovascular disease, it is very important to know your risk factors and to prevent and/or manage them well. The concept of risk factors for cardiovascular disease began with the Framingham study in the 1960s, which noted factors of age, sex, family history, smoking, hypertension, hyperlipidemia, insulin resistance and diabetes, lack of exercise, and obesity.2 It is not clear whether obesity alone increases the risk of cardiovascular disease or whether the risk is increased by interaction of other factors related to obesity. Traditionally, however, some studies have reported that independent obesity may be linked to future cardiovascular disease.3 Treatments that lower low-density lipoprotein (LDL)-cholesterol reduce the risk of coronary artery disease, and several studies have shown that a 20%–60% reduction in LDL-cholesterol with statins reduces about 34% of coronary events over a 5-year period compared with placebo.4 In addition, patients with type 2 diabetes may experience reduced risk of cardiovascular disease when high high-density lipoprotein (HDL)-cholesterol level is maintained.5
Diabetes is a major risk factor for cardiovascular disease according to the American Heart Association, which emphasizes management of lifestyles for diabetics.6 Obesity and cholesterol are important indicators of lifestyle. Insulin resistance caused by diabetes can reduce HDL-cholesterol and triglyceride levels, which are indicative of hyperlipidemia, and consequently increase the risk of cardiovascular disease. In addition, patients with diabetes are often mildly obese, and use of regular diabetes medications except metformin contributes to weight gain.7 A study in the USA found that weight increased after the onset of diabetes, and weight control efforts were an important factor in diabetes management.8
In particular, a study on the effects of weight loss on cardiovascular disease risk in overweight and obese patients with diabetes showed that the risk of cardiovascular disease was significantly reduced by losing 5%–10% of body weight within 1 year.9 However, a Korean study found that overall risk of death and cardiovascular mortality in the increased weight group was lower than in the maintenance group.10 There is controversy over the relationship between development of cardiovascular disease and changes in weight and body mass index (BMI) in people with diabetes compared with the normal population.
The medical examination of the National Health Insurance Service (NHIS) in Korea is performed in adults and includes measurement of BMI, blood pressure, cholesterol, diabetes, hypertension, smoking, and exercise, all of which affect cardiovascular disease. Thus, analysis of these periodic and cumulative data may play an important role in predicting cardiovascular disease occurrence. If values such as BMI and cholesterol are high, or if their variabilities are rapid, we will be able to provide evidence as to whether they can affect the occurrence of cardiovascular disease.
There are some points to be aware of when observing variability in repeated measures data. In longitudinal data, the period of measurement should be considered. For example, blood pressure is high during the day and low at night. Therefore, if the blood pressure is repeatedly measured every day or night, it can be regarded as a short-term fluctuation. And since medical examination conducted by NHIS is taken every year or every 2 years, blood pressure measured at that time is a long-term change. The visit-to-visit variability refers to the degree of change in the value observed each time a patient visits a medical institution, which is also a long-term variation. In addition, it is necessary to examine whether the interval of repeated measurement points is constant or not. The number of time points for analysis is also important, but there are no guidelines on how the results of the estimates vary with the number of time points used.
The purpose of this study is to identify the pattern of changes in BMI and LDL-cholesterol by occurrence of disease in Korean patients with diabetes and the effect of variability of the two indicators on development of cardiovascular disease. In addition, we examined how biomarker change patterns affect cardiovascular disease in normal adults.