Hypertension (HTN) is defined as a repeatedly elevated blood pressure (BP) exceeding a systolic blood pressure (SBP) over 140 mmHg and/or a diastolic blood pressure (DBP) over 90 mmHg [1]. It is clear that HTN has become a worldwide epidemic and an important risk factor of cardiovascular disease (CVD), which might cause fatal results in an individual such as stroke, myocardial infarction, ischemic heart disease, kidney disease, and peripheral vascular disease [1-4]. Revised reports have been published every four or five years since the Joint National Committee (JNC) on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure was first established in 1977 [5]. In the recent JNC-7 report, a new category known as prehypertension (PreHTN) was added. In this category, the normal and high-normal BP of the JNC-6 criteria are combined because even normal BP (SBP 120 to 129 mmHg or DBP 80 to 84 mmHg) are potentially related to future CVD. Several studies have investigated the short-term and long-term progression rates from the non-hypertensive category to HTN [6-12]. According to the Framingham Heart Study, at the age of 55 yr, prehypertensive people had a two times higher risk of progression to HTN than normotensive people [8]. Many studies have suggested that lifestyle modifications might help people to manage HTN [6, 9, 10, 13]. Researchers have been interested in behavioral changes such as exercise, a healthy diet, smoking cessation, etc. For example, a follow-up of the data from the British Health and Lifestyle Surveys for 7 yr showed that the relative risk (RR) (95% confidence intervals, CI) of subjects who were overweight at follow-up but not at baseline was 1.65 (1.33-2.14) and that the RR of subjects who were overweight at baseline but not at follow-up was 0.58 (0.34-1.01) compared with those who were not overweight at baseline or follow-up. Those who were smokers at follow-up had a higher risk (RR, 95% CI: 1.12, 0.94-1.34) of HTN than those who were non-smokers at baseline or follow-up [9]. However, there have been few studies on the association between CVD risk factors and the progression of HTN among people with PreHTN in Korea. The purpose of this study was thus to examine factors influencing progression using a prospective cohort dataset, the Korean Cancer Prevention Study (KCPS) from the National Health Insurance Corporation (NHIC).

To our knowledge, this is the first investigation of the relationship between changes in CVD risk factors and the rate of progression of HTN using national longitudinal data in Korea. These longitudinal data showed that improvements in CVD risk factors diminished the rate of progression of HTN in prehypertensive men and women. The periodic screening of BP in adults has been recommended to detect the onset of HTN [3, 16] so that appropriate measures can be taken to prevent the morbidity and mortality associated with raised BP [17]. The European Task Force on Prevention of Coronary Disease recommended a follow-up interval of up to 5 yr [18]. Current recommendations for follow-up BP screening of individuals without HTN are empirical rather than evidence-based, and vary widely [16, 18].

It has previously been reported that older age, higher BMI, and elevated baseline BP are independent risk factors for future HTN [3, 19, 20]. Recent reports from the Framingham Heart Study suggested that persons with normal BP at 55 years of age had a 90% lifetime risk of developing HTN [3, 21]. An increasing incidence of HTN with increasing age was found, which was in line with other results [11, 22]. Cigarette smoking is well established as a causal factor in coronary heart disease and stroke. According to the Korea Medical Insurance Corporation (KMIC) study, smoking is also a major independent risk factor for ischemic heart disease (IHD), CVD, and atherosclerotic cardiovascular disease (ASCVD). The RR from current smoking was reported to be 3.3 (95% CI: 1.7-6.2) for IHD and 1.6 (95% CI: 1.2-2.3) for CVD [23]. In the Cox proportional hazard model, the HR (95% CI) of ex-smokers was 0.756 (0.696-0.821) in men, compared with continuous smokers among the current smokers at baseline. Heavy alcohol consumption may cause other cardiac disorders and is associated with an increased risk of stroke [24]. The HR (95% CI) of non-drinkers was 0.669 (0.607-0.738) in men and 0.829 (0.613-1.121) in women, respectively, compared with continuous drinkers. In general, regular exercise reduces the risk of coronary artery disesase (CHD). Regular exercise has BP-lowering effects [25]. It was reported that individuals participating in 20 min of light or moderate regular exercise every day had about a 30% lower risk of death from CHD than sedentary individuals [26]. In this study, in those not taking regular exercise at baseline, the HR (95% CI) of subjects who later started regular exercise was 0.653 (0.593-0.718) in men and 0.534 (0.426-0.669) in women, compared with those not participating in regular exercise at all. Obesity contributes to elevated BP [17]. It has consistently been proven through clinical trials that reducing weight can lower SBP and DBP [27]. The risk of HTN is higher among obese people (BMI ≥25 kg/m²). The BMI is significantly and positively correlated with both SBP and DBP [28]. Jee et al. [15] found that underweight, overweight and obese men and women had higher rates of death than normal weight men and women. Several previous studies have reported that weight loss might play a protective role in the progression to HTN [9, 29, 30]. In this study, the HR (95% CI) of the weight loss group (BMI <25 kg/m²) was 0.715 (0.593-0.861) in men and 0.634 (0.401-1.001) in women, compared with the continuously obese group (BMI ≥25 kg/m²). Kim et al. [11] reported that diabetes mellitus (DM) was an independent risk factor for HTN in women, but not men. In this study, the baseline FSG level was associated with the progression of HTN in both men and women. However, lowering the FSG did not play a protective role in the progression to HTN in either men or women. In previous studies, a higher TC was a predictor of progression to HTN [30]. In another study, the direct and progressive relationship between TC and CVD was reported [31]. In this study, TC was a strong predictor of the progression to HTN. Compared to the consistently high TC group (TC ≥240 mg/dL), the 'back to normal' TC group (TC <240 mg/dL) lowered their risk by 1.27 times in men, controlling for other CVD risk factors.

This study has several potential limitations. First, there was no information on antihypertensive medications in this population, which could be a confounding factor in the association between the CVD risk factors and BP. The absence of medication information could have resulted in a conservative estimate of the incidence of PreHTN and HTN. Second, misclassification bias could have arisen from measurement error because the biennial health examinations were carried out at various hospitals throughout the country and the laboratory techniques varied. However, the data collected were subject to internal and external quality control [32, 33]. Third, we categorized the individuals based on the average BP in 1994 and 1995 obtained from a single BP measurement, as in the recent study by Hansen et al. [34], while other studies used two readings [2]. Lastly, since we used data from biennial BP measurements, we had no information on possible changes during the intervening period. It is possible that individuals with HTN changed their unhealthy lifestyles. For example, when a current smoker was diagnosed with PreHTN at the first examination, she/he may have subsequently quit smoking. After stopping smoking, BP could have returned to normal. We are not in a position to know whether smoking cessation alone contributed to decreasing BP.

Despite several limitations, this study has a few strengths, including the extensive coverage of South Koreans with repeated measures of BP, FSG, TC, weight, and height through the KCPS data. This large sample size and the prospective design had sufficient statistical power in general, even in subgroup and dose-response analyses [23, 35].

In conclusion, this study confirmed the findings of previous studies demonstrating a relationship between changes in CVD risk factors and the progression from PreHTN to HTN. With regards public health implications in Korea, the prehypertensive population should be targeted for specific health intervention to change their unhealthy lifestyles. Large intervention studies should be established urgently in order to confirm the results of this study.